Journal articles on the topic 'Operoni'
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1
Nguyen, Huy N., Ashish Jain, Oliver Eulenstein, and Iddo Friedberg. "Tracing the ancestry of operons in bacteria." Bioinformatics 35, no. 17 (January 24, 2019): 2998–3004. http://dx.doi.org/10.1093/bioinformatics/btz053.
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Abstract Motivation Complexity is a fundamental attribute of life. Complex systems are made of parts that together perform functions that a single component, or subsets of components, cannot. Examples of complex molecular systems include protein structures such as the F1Fo-ATPase, the ribosome, or the flagellar motor: each one of these structures requires most or all of its components to function properly. Given the ubiquity of complex systems in the biosphere, understanding the evolution of complexity is central to biology. At the molecular level, operons are classic examples of a complex system. An operon’s genes are co-transcribed under the control of a single promoter to a polycistronic mRNA molecule, and the operon’s gene products often form molecular complexes or metabolic pathways. With the large number of complete bacterial genomes available, we now have the opportunity to explore the evolution of these complex entities, by identifying possible intermediate states of operons. Results In this work, we developed a maximum parsimony algorithm to reconstruct ancestral operon states, and show a simple vertical evolution model of how operons may evolve from the individual component genes. We describe several ancestral states that are plausible functional intermediate forms leading to the full operon. We also offer Reconstruction of Ancestral Gene blocks Using Events or ROAGUE as a software tool for those interested in exploring gene block and operon evolution. Availability and implementation The software accompanying this paper is available under GPLv3 license on: https://github.com/nguyenngochuy91/Ancestral-Blocks-Reconstruction. Supplementary information Supplementary data are available at Bioinformatics online.
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Brandis, Gerrit, Sha Cao, and Diarmaid Hughes. "Operon Concatenation Is an Ancient Feature That Restricts the Potential to Rearrange Bacterial Chromosomes." Molecular Biology and Evolution 36, no. 9 (May 27, 2019): 1990–2000. http://dx.doi.org/10.1093/molbev/msz129.
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Abstract The last common ancestor of the Gammaproteobacteria carried an important 40-kb chromosome section encoding 51 proteins of the transcriptional and translational machinery. These genes were organized into eight contiguous operons (rrnB-tufB-secE-rpoBC-str-S10-spc-alpha). Over 2 Gy of evolution, in different lineages, some of the operons became separated by multigene insertions. Surprisingly, in many Enterobacteriaceae, much of the ancient organization is conserved, indicating a strong selective force on the operons to remain colinear. Here, we show for one operon pair, tufB-secE in Salmonella, that an interruption of contiguity significantly reduces growth rate. Our data show that the tufB-secE operons are concatenated by an interoperon terminator–promoter overlap that plays a significant role regulating gene expression. Interrupting operon contiguity interferes with this regulation, reducing cellular fitness. Six operons of the ancestral chromosome section remain contiguous in Salmonella (tufB-secE-rpoBC and S10-spc-alpha) and, strikingly, each of these operon pairs is also connected by an interoperon terminator–promoter overlap. Accordingly, we propose that operon concatenation is an ancient feature that restricts the potential to rearrange bacterial chromosomes and can select for the maintenance of a colinear operon organization over billions of years.
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Ming, Li, and Anna Boiko. "Recipes of the Russian opera tradition in the Chinese opera art of the 20th century." OOO "Zhurnal "Voprosy Istorii" 2020, no. 11-1 (November 1, 2020): 170–78. http://dx.doi.org/10.31166/voprosyistorii202011statyi07.
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The publication is devoted to the consideration of the peculiarities of the reception of Russian opera tradition in the Chinese opera of 20th century. The significant role of the historical and the cultural events that affected the spread Russian opera in China is analyzed. Particular attention is paid to the study of the activities of representatives the Russian emigration in the field of opera art. It was emphasized that the influence of Russian and foreign opera contributed to the formation of the genre the Chinese opera of the European type.
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Acinas, Silvia G., Luisa A. Marcelino, Vanja Klepac-Ceraj, and Martin F. Polz. "Divergence and Redundancy of 16S rRNA Sequences in Genomes with Multiple rrn Operons." Journal of Bacteriology 186, no. 9 (May 1, 2004): 2629–35. http://dx.doi.org/10.1128/jb.186.9.2629-2635.2004.
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ABSTRACT The level of sequence heterogeneity among rrn operons within genomes determines the accuracy of diversity estimation by 16S rRNA-based methods. Furthermore, the occurrence of widespread horizontal gene transfer (HGT) between distantly related rrn operons casts doubt on reconstructions of phylogenetic relationships. For this study, patterns of distribution of rrn copy numbers, interoperonic divergence, and redundancy of 16S rRNA sequences were evaluated. Bacterial genomes display up to 15 operons and operon numbers up to 7 are commonly found, but ∼40% of the organisms analyzed have either one or two operons. Among the Archaea, a single operon appears to dominate and the highest number of operons is five. About 40% of sequences among 380 operons in 76 bacterial genomes with multiple operons were identical to at least one other 16S rRNA sequence in the same genome, and in 38% of the genomes all 16S rRNAs were invariant. For Archaea, the number of identical operons was only 25%, but only five genomes with 21 operons are currently available. These considerations suggest an upper bound of roughly threefold overestimation of bacterial diversity resulting from cloning and sequencing of 16S rRNA genes from the environment; however, the inclusion of genomes with a single rrn operon may lower this correction factor to ∼2.5. Divergence among operons appears to be small overall for both Bacteria and Archaea, with the vast majority of 16S rRNA sequences showing <1% nucleotide differences. Only five genomes with operons with a higher level of nucleotide divergence were detected, and Thermoanaerobacter tengcongensis exhibited the highest level of divergence (11.6%) noted to date. Overall, four of the five extreme cases of operon differences occurred among thermophilic bacteria, suggesting a much higher incidence of HGT in these bacteria than in other groups.
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Yap, Wai Ho, Zhenshui Zhang, and Yue Wang. "Distinct Types of rRNA Operons Exist in the Genome of the Actinomycete Thermomonospora chromogena and Evidence for Horizontal Transfer of an Entire rRNA Operon." Journal of Bacteriology 181, no. 17 (September 1, 1999): 5201–9. http://dx.doi.org/10.1128/jb.181.17.5201-5209.1999.
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ABSTRACT We describe here the presence of two distinct types of rRNA operons in the genome of a thermophilic actinomycete Thermomonospora chromogena. The genome of T. chromogena contains six rRNA operons (rrn), of which four complete and two incomplete ones were cloned and sequenced. Comparative analysis revealed that the operon rrnB exhibits high levels of sequence variations to the other five nearly identical ones throughout the entire length of the operon. The coding sequences for the 16S and 23S rRNA genes differ by approximately 6 and 10%, respectively, between the two types of operons. Normal functionality ofrrnB is concluded on the basis of the nonrandom distribution of nucleotide substitutions, the presence of compensating nucleotide covariations, the preservation of secondary and tertiary rRNA structures, and the detection of correctly processed rRNAs in the cell. Comparative sequence analysis also revealed a close evolutionary relationship between rrnB operon of T. chromogena and rrnA operon of another thermophilic actinomycete Thermobispora bispora. We propose thatT. chromogena acquired rrnB operon fromT. bispora or a related organism via horizontal gene transfer.
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Dennis, Patrick P., Sonia Ziesche, and Shanthini Mylvaganam. "Transcription Analysis of Two Disparate rRNA Operons in the Halophilic Archaeon Haloarcula marismortui." Journal of Bacteriology 180, no. 18 (September 15, 1998): 4804–13. http://dx.doi.org/10.1128/jb.180.18.4804-4813.1998.
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ABSTRACT The genome of the halophilic archaeon Haloarcula marismortui contains two rRNA operons designated rrnAand rrnB. Genomic clones of the two operons and their flanking regions have been sequenced, and primary transcripts and processing intermediates derived from each operon have been characterized. The 16S, 23S, and 5S genes from the two operons were found to differ at 74 of 1,472 positions, 39 of 2,922 positions, and 2 of 122 positions, respectively. This degree of sequence divergence for multicopy (paralogous) rRNA genes was 10- to 50-fold or more higher than anticipated. The two operons exhibit other profound differences that include (i) the presence in rrnA and the absence inrrnB of tRNAAla and tRNACys genes in the intergenic and distal regions, respectively, (ii) divergent 5′ flanking sequences, and (iii) distinct pathways for processing and maturation of 16S rRNA. Processing and maturation of 16S and 23S rRNA from rrnA operon transcripts and of 23S rRNA fromrrnB operon transcripts follow the canonical halophilic pathway, whereas maturation of 16S rRNA from rrnB operon transcripts follows an unusual and different pathway that is apparently devoid of any 5′ processing intermediate.
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Reksoprawiro, Ricky, Gabriella Scarlett, Alexander Joseph Ibnu Wibowo, and Novi Amelia. "Consumer Culture Theory: Hubungan Timbal Balik antar Social Operant Resources dan Operand Resources dalam Studi Empiris McDonald’s Indonesia." Kajian Branding Indonesia 2, no. 1 (January 13, 2020): 132–61. http://dx.doi.org/10.21632/kbi.2.1.132-161.
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Penelitian ini bertujuan untuk menguji secara empiris secara garis besar untuk mengetahui hubungan antara konsep social operant resources terhadap operand resources yang dilandasi oleh consumer culture theory. Secara spesifik, studi ini ingin menguji sebagai berikut: (i) pengaruh social operant resources terhadap economic operand resources; (ii) pengaruh social operant resources terhadap material operand resources; (iii) pengaruh economic operand resources terhadap social operant resources; dan (iv) pengaruh material operand resources terhadap social operant resources; (v) hubungan timbal balik antara social operant resources dan economic operand resources; (vi) hubungan timbal balik antara social operant resources dan material operand resources. Permasalahan dalam studi ini ditelaah dengan berbasis pada konsep consumer culture theory (CCT). Pengumpulan data primer dilakukan menggunakan kuesioner melalui survei online. Seluruh indikator diukur menggunakan skala Likert tujuh poin. Penelitian ini berhasil memperoleh data responden sebanyak 358 orang konsumen restoran McDonald’s. Teknik analisis faktor eksploratori exploratory factor analysis (EFA) diterapkan untuk melakukan uji validitas konstruk. Selanjutnya, keenam hipotesis diuji menggunakan teknik structural equation modeling (SEM). Hasil studi ini berhasil membuktikan bahwa keenam hipotesis yang diajukan tidak ditolak. Secara ringkas, social operant resources memengaruhi economic operand resources, social operant resources memengaruhi material operand resources, economic operand resources memengaruhi social operant resources, material operand resources memengaruhi social operant resources, ada hubungan timbal balik antara social operant resources dan economic material resources, dan ada hubungan timbal balik antara social operant resources dan material operand resources.
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Menendez, M. C., M. J. Garcia, M. C. Navarro, J. A. Gonzalez-y-Merchand, S. Rivera-Gutierrez, L. Garcia-Sanchez, and R. A. Cox. "Characterization of an rRNA Operon (rrnB) of Mycobacterium fortuitum and Other Mycobacterial Species: Implications for the Classification of Mycobacteria." Journal of Bacteriology 184, no. 4 (February 15, 2002): 1078–88. http://dx.doi.org/10.1128/jb.184.4.1078-1088.2002.
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ABSTRACT Mycobacteria are thought to have either one or two rRNA operons per genome. All mycobacteria investigated to date have an operon, designated rrnA, located downstream from the murA gene. We report that Mycobacteriun fortuitum has a second rrn operon, designated rrnB, which is located downstream from the tyrS gene; tyrS is very close to the 3" end of a gene (3-mag) coding for 3-methylpurine-DNA-glycosylase. The second rrn operon of Mycobacterium smegmatis was shown to have a similar organization, namely, 5" 3-mag-tyrS-rrnB 3". The rrnB operon of M. fortuitum was found to have a single dedicated promoter. During exponential growth in a rich medium, the rrnB and rrnA operons were the major and minor contributors, respectively, to pre-rRNA synthesis. Genomic DNA was isolated from eight other fast-growing mycobacterial species. Samples were investigated by Southern blot analysis using probes for murA, tyrS, and 16S rRNA sequences. The results revealed that both rrnA and rrnB operons were present in each species. The results form the basis for a proposed new scheme for the classification of mycobacteria. The approach, which is phylogenetic in concept, is based on particular properties of the rrn operons of a cell, namely, the number per genome and a feature of 16S rRNA gene sequences.
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Asai, Tsuneaki, Ciarán Condon, Justina Voulgaris, Dmitry Zaporojets, Binghua Shen, Michaal Al-Omar, Craig Squires, and Catherine L. Squires. "Construction and Initial Characterization of Escherichia coli Strains with Few or No Intact Chromosomal rRNA Operons." Journal of Bacteriology 181, no. 12 (June 15, 1999): 3803–9. http://dx.doi.org/10.1128/jb.181.12.3803-3809.1999.
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ABSTRACT The Escherichia coli genome carries seven rRNA (rrn) operons, each containing three rRNA genes. The presence of multiple operons has been an obstacle to many studies of rRNA because the effect of mutations in one operon is diluted by the six remaining wild-type copies. To create a tool useful for manipulating rRNA, we sequentially inactivated from one to all seven of these operons with deletions spanning the 16S and 23S rRNA genes. In the final strain, carrying no intact rRNA operon on the chromosome, rRNA molecules were expressed from a multicopy plasmid containing a single rRNA operon (prrn). Characterization of these rrndeletion strains revealed that deletion of two operons was required to observe a reduction in the growth rate and rRNA/protein ratio. When the number of deletions was extended from three to six, the decrease in the growth rate was slightly more than the decrease in the rRNA/protein ratio, suggesting that ribosome efficiency was reduced. This reduction was most pronounced in the Δ7 prrn strain, in which the growth rate, unlike the rRNA/protein ratio, was not completely restored to wild-type levels by a cloned rRNA operon. The decreases in growth rate and rRNA/protein ratio were surprisingly moderate in the rrndeletion strains; the presence of even a single operon on the chromosome was able to produce as much as 56% of wild-type levels of rRNA. We discuss possible applications of these strains in rRNA studies.
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Lakhova, Tatiana N., Fedor V. Kazantsev, Aleksey M. Mukhin, Nikolay A. Kolchanov, Yury G. Matushkin, and Sergey A. Lashin. "Algorithm for the Reconstruction of Mathematical Frame Models of Bacterial Transcription Regulation." Mathematics 10, no. 23 (November 28, 2022): 4480. http://dx.doi.org/10.3390/math10234480.
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Transcription regulation plays an important role in bacterial activity. The operon concept coined by François Jacob and Jacques Monod has had a considerable effect on investigations into gene expression regulation, including modeling. However, most such studies have considered the regulation models devised manually for one or several operons. For that reason, the objective of the present study was automated genome model reconstruction for different bacteria. The suggested algorithm accounted for all possible interactions of transcription factors and their binding sites in an operon’s promoter region. Transcription factor enumeration was performed using the deep-first search technique. The obtained models are of interest for those involved in the research of transcription factor regulatory effects on bacterial gene expression in microbiology and biotechnology.
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Townsend, Stacy M., Naomi E. Kramer, Robert Edwards, Stephen Baker, Nancy Hamlin, Mark Simmonds, Kim Stevens, et al. "Salmonella enterica Serovar Typhi Possesses a Unique Repertoire of Fimbrial Gene Sequences." Infection and Immunity 69, no. 5 (May 1, 2001): 2894–901. http://dx.doi.org/10.1128/iai.69.5.2894-2901.2001.
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ABSTRACT Salmonella enterica serotype Typhi differs from nontyphoidal Salmonella serotypes by its strict host adaptation to humans and higher primates. Since fimbriae have been implicated in host adaptation, we investigated whether the serotype Typhi genome contains fimbrial operons which are unique to this pathogen or restricted to typhoidal Salmonella serotypes. This study established for the first time the total number of fimbrial operons present in an individual Salmonella serotype. The serotype Typhi CT18 genome, which has been sequenced by the Typhi Sequencing Group at the Sanger Centre, contained a type IV fimbrial operon, an orthologue of the agf operon, and 12 putative fimbrial operons of the chaperone-usher assembly class. In addition tosef, fim, saf, and tcf, which had been described previously in serotype Typhi, we identified eight new putative chaperone-usher-dependent fimbrial operons, which were termedbcf, sta, stb, ste, std, stc, stg, and sth. Hybridization analysis performed with 16 strains ofSalmonella reference collection C and 22 strains ofSalmonella reference collection B showed that all eight putative fimbrial operons of serotype Typhi were also present in a number of nontyphoidal Salmonella serotypes. Thus, a simple correlation between host range and the presence of a single fimbrial operon seems at present unlikely. However, the serotype Typhi genome differed from that of all other Salmonella serotypes investigated in that it contained a unique combination of putative fimbrial operons.
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Seitzer, Phillip, Andrew I. Yao, Ariana Cisneros, and Marc T. Facciotti. "The Exploration of Novel Regulatory Relationships Drives Haloarchaeal Operon-Like Structural Dynamics over Short Evolutionary Distances." Microorganisms 8, no. 12 (November 30, 2020): 1900. http://dx.doi.org/10.3390/microorganisms8121900.
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Operons are a dominant feature of bacterial and archaeal genome organization. Numerous investigations have related aspects of operon structure to operon function, making operons exemplars for studies aimed at deciphering Nature’s design principles for genomic organization at a local scale. We consider this understanding to be both fundamentally important and ultimately useful in the de novo design of increasingly complex synthetic circuits. Here we analyze the evolution of the genomic context of operon-like structures in a set of 76 sequenced and annotated species of halophilic archaea. The phylogenetic depth and breadth of this dataset allows insight into changes in operon-like structures over shorter evolutionary time scales than have been studied in previous cross-species analysis of operon evolution. Our analysis, implemented in the updated software package JContextExplorer finds that operon-like context as measured by changes in structure frequently differs from a sequence divergence model of whole-species phylogeny and that changes seem to be dominated by the exploration of novel regulatory relationships.
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Miguel-Arribas, Andrés, Ling Juan Wu, Claudia Michaelis, Ken-ichi Yoshida, Elisabeth Grohmann, and Wilfried J. J. Meijer. "Conjugation Operons in Gram-Positive Bacteria with and without Antitermination Systems." Microorganisms 10, no. 3 (March 8, 2022): 587. http://dx.doi.org/10.3390/microorganisms10030587.
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Genes involved in the same cellular process are often clustered together in an operon whose expression is controlled by an upstream promoter. Generally, the activity of the promoter is strictly controlled. However, spurious transcription undermines this strict regulation, particularly affecting large operons. The negative effects of spurious transcription can be mitigated by the presence of multiple terminators inside the operon, in combination with an antitermination system. Antitermination systems modify the transcription elongation complexes and enable them to bypass terminators. Bacterial conjugation is the process by which a conjugative DNA element is transferred from a donor to a recipient cell. Conjugation involves many genes that are mostly organized in one or a few large operons. It has recently been shown that many conjugation operons present on plasmids replicating in Gram-positive bacteria possess a bipartite antitermination system that allows not only many terminators inside the conjugation operon to be bypassed, but also the differential expression of a subset of genes. Here, we show that some conjugation operons on plasmids belonging to the Inc18 family of Gram-positive broad host-range plasmids do not possess an antitermination system, suggesting that the absence of an antitermination system may have advantages. The possible (dis)advantages of conjugation operons possessing (or not) an antitermination system are discussed.
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Xie, Gary, Nemat O. Keyhani, Carol A. Bonner, and Roy A. Jensen. "Ancient Origin of the Tryptophan Operon and the Dynamics of Evolutionary Change." Microbiology and Molecular Biology Reviews 67, no. 3 (September 2003): 303–42. http://dx.doi.org/10.1128/mmbr.67.3.303-342.2003.
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SUMMARY The seven conserved enzymatic domains required for tryptophan (Trp) biosynthesis are encoded in seven genetic regions that are organized differently (whole-pathway operons, multiple partial-pathway operons, and dispersed genes) in prokaryotes. A comparative bioinformatics evaluation of the conservation and organization of the genes of Trp biosynthesis in prokaryotic operons should serve as an excellent model for assessing the feasibility of predicting the evolutionary histories of genes and operons associated with other biochemical pathways. These comparisons should provide a better understanding of possible explanations for differences in operon organization in different organisms at a genomics level. These analyses may also permit identification of some of the prevailing forces that dictated specific gene rearrangements during the course of evolution. Operons concerned with Trp biosynthesis in prokaryotes have been in a dynamic state of flux. Analysis of closely related organisms among the Bacteria at various phylogenetic nodes reveals many examples of operon scission, gene dispersal, gene fusion, gene scrambling, and gene loss from which the direction of evolutionary events can be deduced. Two milestone evolutionary events have been mapped to the 16S rRNA tree of Bacteria, one splitting the operon in two, and the other rejoining it by gene fusion. The Archaea, though less resolved due to a lesser genome representation, appear to exhibit more gene scrambling than the Bacteria. The trp operon appears to have been an ancient innovation; it was already present in the common ancestor of Bacteria and Archaea. Although the operon has been subjected, even in recent times, to dynamic changes in gene rearrangement, the ancestral gene order can be deduced with confidence. The evolutionary history of the genes of the pathway is discernible in rough outline as a vertical line of descent, with events of lateral gene transfer or paralogy enriching the analysis as interesting features that can be distinguished. As additional genomes are thoroughly analyzed, an increasingly refined resolution of the sequential evolutionary steps is clearly possible. These comparisons suggest that present-day trp operons that possess finely tuned regulatory features are under strong positive selection and are able to resist the disruptive evolutionary events that may be experienced by simpler, poorly regulated operons.
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Kumar, Ashwani, Mridula Bose, and Vani Brahmachari. "Analysis of Expression Profile of Mammalian Cell Entry (mce) Operons of Mycobacterium tuberculosis." Infection and Immunity 71, no. 10 (October 2003): 6083–87. http://dx.doi.org/10.1128/iai.71.10.6083-6087.2003.
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ABSTRACT The sequencing of the complete genome of M. tuberculosis H37Rv has resulted in the recognition of four mce operons in its genome by in silico analysis. In an attempt to understand the significance of the redundancy of mce operons, we analyzed the expression profile of mce operons after different periods of growth in culture as well as during in vivo infection. Our results strongly suggest that mce1 is expressed as a polycistronic message. In culture from day 8 to day 12, expression of only mce1 was observed, but as the cultures progress towards stationary phase the expression profile of mce operons was altered; the transcripts of the mce1 operon were barely detected while those of the mce4 operon were prominent. In an analysis of the expression of mce operons in tubercle material collected from infected animal tissues, we detected the expression of mce1, -3 and -4. Our results imply that mce operons other than mce1 are also expressed during infection and that it is necessary to examine their role in pathogenesis.
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Monshupanee, Tanakarn, Sirirat Fa-aroonsawat, and Wipa Chungjatupornchai. "A cyanobacterial strain with all chromosomal rRNA operons inactivated: a single nucleotide mutation of 23S rRNA confers temperature-sensitive phenotypes." Microbiology 152, no. 5 (May 1, 2006): 1417–25. http://dx.doi.org/10.1099/mic.0.28691-0.
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The presence of a multicopy chromosome, with each copy containing two rRNA operons (rrnA and rrnB), has been an obstacle to analysing mutated rRNA in Synechococcus PCC 7942. To create a system for expressing homogeneous mutated rRNA, the chromosomal rrn operons were sequentially inactivated and a final strain was successfully obtained with all the chromosomal rrn operons inactivated but carrying a replaceable multicopy plasmid containing a single rrn operon. The lag time required for growth response on dark/light shift of mutant strains with chromosomal rrnA or rrnB inactivated was increased 50 % over that of the wild-type strain; however, the presence of the plasmid-borne rrn operon restored the lag time. The doubling time of mutant strains carrying only a functional rrnB operon, but not strains carrying only a functional rrnA operon, was significantly longer than that of the wild-type strain. A strain in which essentially all the cellular 23S rRNA contained the mutation C2588A was temperature sensitive at 16 °C and 45 °C. Position C2588 is equivalent to C2611 of the peptidyltransferase centre in domain V of Escherichia coli 23S rRNA.
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Cooper, Robert A. "Teaching the Big Ideas of Biology with Operon Models." American Biology Teacher 77, no. 1 (January 1, 2015): 30–39. http://dx.doi.org/10.1525/abt.2015.77.1.5.
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This paper presents an activity that engages students in model-based reasoning, requiring them to predict the behavior of the trp and lac operons under different environmental conditions. Students are presented six scenarios for the trp operon and five for the lac operon. In most of the scenarios, specific mutations have occurred in genetic elements of the system that alter the behavior from the norm. Students are also challenged to relate their understanding of operon behavior to the “Big Ideas” of homeostasis, evolution, information, interactions, and emergent properties. By using operons to teach students to reason with models of complex systems and understand broad themes, we equip them with powerful skills and ideas that form a solid foundation for their future learning in biology.
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Gonzalez-y-Merchand, J. A., M. J. Colston, and R. A. Cox. "Roles of Multiple Promoters in Transcription of Ribosomal DNA: Effects of Growth Conditions on Precursor rRNA Synthesis in Mycobacteria." Journal of Bacteriology 180, no. 21 (November 1, 1998): 5756–61. http://dx.doi.org/10.1128/jb.180.21.5756-5761.1998.
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ABSTRACT The roles of multiple promoters in the synthesis of rRNA under different conditions of growth were investigated, using two mycobacterial species as model organisms. When Mycobacterium smegmatis was grown under optimal conditions, its two rRNA operons contributed equally, with two promoters, one from each operon, being responsible for most transcripts. In stationary-phase growth or balanced growth under carbon starvation conditions, one operon (rrnA f) dominated and its three promoters contributed more equally to the generation of transcripts.Mycobacterium tuberculosis has a single operon with two promoters, one of which generated 80% of transcripts, at all stages of growth. We infer that each promoter functions independently according to its intrinsic strength when cells are growing slowly so that one operon with three promoters is roughly equivalent to three operons with one promoter; at high growth rates, occlusion effects reduce the efficiency of multiple promoters to that of a single promoter.
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Arnvig, Kristine B., B. Gopal, K. G. Papavinasasundaram, Robert A. Cox, and M. Joseph Colston. "The mechanism of upstream activation in the rrnB operon of Mycobacterium smegmatis is different from the Escherichia coli paradigm." Microbiology 151, no. 2 (February 1, 2005): 467–73. http://dx.doi.org/10.1099/mic.0.27597-0.
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Mycobacteria are slow-growing bacteria with a generation time of from 2–3 h up to several weeks. Consistent with the low growth rate, mycobacterial species have a maximum of two rRNA operons, rrnA and rrnB. The rrnA operon is present in all mycobacteria and has between two and five promoters, depending on species, whereas the rrnB operon, with a single promoter, is only found in some of the faster-growing species. The promoter region of the rrnB operon of a typical fast grower, Mycobacterium smegmatis, was investigated. By using lacZ reporter gene fusions it was demonstrated that the rrnB operon contains a highly activating region upstream of the core promoter, comparable to other bacterial rrn operons. However, the results suggest that, unlike the situation in, for example, Escherichia coli, the activating mechanism is solely factor dependent, and that no UP element is involved.
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Choi, Soon-Yong, Dindo Reyes, Montira Leelakriangsak, and Peter Zuber. "The Global Regulator Spx Functions in the Control of Organosulfur Metabolism in Bacillus subtilis." Journal of Bacteriology 188, no. 16 (August 15, 2006): 5741–51. http://dx.doi.org/10.1128/jb.00443-06.
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ABSTRACT Spx is a global transcriptional regulator of the oxidative stress response in Bacillus subtilis. Its target is RNA polymerase, where it contacts the α subunit C-terminal domain. Recently, evidence was presented that Spx participates in sulfate-dependent control of organosulfur utilization operons, including the ytmI, yxeI, ssu, and yrrT operons. The yrrT operon includes the genes that function in cysteine synthesis from S-adenosylmethionine through intermediates S-adenosylhomocysteine, ribosylhomocysteine, homocysteine, and cystathionine. These operons are also negatively controlled by CymR, the repressor of cysteine biosynthesis operons. All of the operons are repressed in media containing cysteine or sulfate but are derepressed in medium containing the alternative sulfur source, methionine. Spx was found to negatively control the expression of these operons in sulfate medium, in part, by stimulating the expression of the cymR gene. In addition, microarray analysis, monitoring of yrrT-lacZ fusion expression, and in vitro transcription studies indicate that Spx directly activates yrrT operon expression during growth in medium containing methionine as sole sulfur source. These experiments have uncovered additional roles for Spx in the control of gene expression during unperturbed, steady-state growth.
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White-Ziegler, Christine A., Anuradha Villapakkam, Karla Ronaszeki, and Sarah Young. "H-NS Controls pap and daaFimbrial Transcription in Escherichia coli in Response to Multiple Environmental Cues." Journal of Bacteriology 182, no. 22 (November 15, 2000): 6391–400. http://dx.doi.org/10.1128/jb.182.22.6391-6400.2000.
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ABSTRACT A comparative study was completed to determine the influence of various environmental stimuli on the transcription of three different fimbrial operons in Escherichia coli and to determine the role of the histone-like protein H-NS in this environmental regulation. The fimbrial operons studied included the pap operon, which encodes pyelonephritis-associated pili (P pili), the daaoperon, which encodes F1845 fimbriae, and the fan operon, which encodes K99 fimbriae. Using lacZYA transcriptional fusions within each of the fimbrial operons, we tested temperature, osmolarity, carbon source, rich medium, oxygen levels, pH, amino acids, solid medium, and iron concentration for their effects on fimbrial gene expression. Low temperature, high osmolarity, glucose as a carbon source, and rich medium repressed transcription of all three operons. High iron did not alter transcription of any of the operons tested, whereas the remaining stimuli had effects on individual operons. For the pap and daa operons, introduction of thehns651 mutation relieved the repression, either fully or partially, due to low temperature, glucose as a carbon source, rich medium, and high osmolarity. Taken together, these data indicate that there are common environmental cues that regulate fimbrial transcription in E. coli and that H-NS is an important environmental regulator for fimbrial transcription in response to several stimuli.
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Ahn, Hyeonju, Donghyeok Seol, Seoae Cho, Heebal Kim, and Woori Kwak. "Enhanced Symbiotic Characteristics in Bacterial Genomes with the Disruption of rRNA Operon." Biology 9, no. 12 (December 3, 2020): 440. http://dx.doi.org/10.3390/biology9120440.
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Ribosomal RNA is an indispensable molecule in living organisms that plays an essential role in protein synthesis. Especially in bacteria, 16S, 23S, and 5S rRNAs are usually co-transcribed as operons. Despite the positive effects of rRNA co-transcription on growth and reproduction rate, a recent study revealed that bacteria with unlinked rRNA operons are more widespread than expected. However, it is still unclear why the rRNA operon is broken. Here, we explored rRNA operon linkage status in 15,898 bacterial genomes and investigated whether they have common features or lifestyles; 574 genomes were found to have unlinked rRNA operons and tended to be phylogenetically conserved. Most of them were symbionts and showed enhanced symbiotic genomic features such as reduced genome size and high adenine–thymine (AT) content. In an eggNOG-mapper analysis, they were also found to have significantly fewer genes than rRNA operon-linked bacteria in the “transcription” and “energy production and conversion in metabolism” categories. These genomes also tend to decrease RNases related to the synthesis of ribosomes and tRNA processing. Based on these results, the disruption of the rRNA operon seems to be one of the tendencies associated with the characteristics of bacteria requiring a low dynamic range.
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Esparza, M., B. Bowien, Eugenia Jedlicki, and David S. Holmes. "Gene Organization and CO2-Responsive Expression of Four cbb Operons in the Biomining Bacterium Acidithiobacillus Ferrooxidans." Advanced Materials Research 71-73 (May 2009): 207–10. http://dx.doi.org/10.4028/www.scientific.net/amr.71-73.207.
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Acidithiobacillus ferrooxidans is an obligately chemolithoautotrophic, -proteobacterium that fixes CO2 by the Calvin-Benson-Bassham (CBB) reductive pentose phosphate cycle. Our objective is to identify genes potentially involved in CO2 fixation and to advance our understanding of how they might be regulated in response to environmental signals. Bioinformatic analyses, based on the complete genome sequence of the type strain ATCC 23270, identified five cbb gene clusters four of which we show experimentally to be operons. These operons are predicted to encode: (i) the components of the carboxysome and one copy of form I RubisCO (cbb1 operon), (ii) a second copy of form I RubisCO (cbb2 operon), (iii) enzymes of central carbon metabolism (cbb3 operon), (iv) a phosphoribulokinase and enzymes of sulfur metabolism (cbb4 operon) and RubisCO form II (cbb5 gene cluster). In addition, the gene for a LysR-type transcriptional regulator CbbR was identified immediately upstream and in divergent orientation to the cbb1 operon and another associated with the cbb5 gene cluster. A. ferrooxidans was grown under different concentrations of CO2 (2.5 to 20% [v/v]), and levels of mRNA and protein were evaluated by qPCR and Western blotting, respectively. CbbR binding to predicted promoter regions of operons cbb1-4 was assayed by EMSA This information permitted the formulation of models explaining how these operons might be regulated by environmental CO2 concentrations. These models were evaluated in vivo in a heterologous host, using cloned A. ferrooxidans cbbR to complement a mutant of the facultative chemoautotroph Ralstonia eutropha H16 lacking a functional cbbR. Cloned copies of A. ferrooxidans promoter regions were also introduced into R. eutropha to evaluate their ability to drive reporter gene expression. This work lays the framework for further studies that should result in a more comprehensive picture of how CO2 fixation is regulated in A. ferrooxidans.
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Chetal, Kashish, and Sarath Chandra Janga. "OperomeDB: A Database of Condition-Specific Transcription Units in Prokaryotic Genomes." BioMed Research International 2015 (2015): 1–10. http://dx.doi.org/10.1155/2015/318217.
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Background. In prokaryotic organisms, a substantial fraction of adjacent genes are organized into operons—codirectionally organized genes in prokaryotic genomes with the presence of a common promoter and terminator. Although several available operon databases provide information with varying levels of reliability, very few resources provide experimentally supported results. Therefore, we believe that the biological community could benefit from having a new operon prediction database with operons predicted using next-generation RNA-seq datasets.Description. We present operomeDB, a database which provides an ensemble of all the predicted operons for bacterial genomes using available RNA-sequencing datasets across a wide range of experimental conditions. Although several studies have recently confirmed that prokaryotic operon structure is dynamic with significant alterations across environmental and experimental conditions, there are no comprehensive databases for studying such variations across prokaryotic transcriptomes. Currently our database contains nine bacterial organisms and 168 transcriptomes for which we predicted operons. User interface is simple and easy to use, in terms of visualization, downloading, and querying of data. In addition, because of its ability to load custom datasets, users can also compare their datasets with publicly available transcriptomic data of an organism.Conclusion. OperomeDB as a database should not only aid experimental groups working on transcriptome analysis of specific organisms but also enable studies related to computational and comparative operomics.
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Guardabassi, Luca, Bruno Perichon, Jean van Heijenoort, Didier Blanot, and Patrice Courvalin. "Glycopeptide Resistance vanA Operons in Paenibacillus Strains Isolated from Soil." Antimicrobial Agents and Chemotherapy 49, no. 10 (October 2005): 4227–33. http://dx.doi.org/10.1128/aac.49.10.4227-4233.2005.
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ABSTRACT The sequence and gene organization of the van operons in vancomycin (MIC of >256 μg/ml)- and teicoplanin (MIC of ≥32 μg/ml)-resistant Paenibacillus thiaminolyticus PT-2B1 and Paenibacillus apiarius PA-B2B isolated from soil were determined. Both operons had regulatory (vanR and vanS), resistance (vanH, vanA, and vanX), and accessory (vanY, vanZ, and vanW) genes homologous to the corresponding genes in enterococcal vanA and vanB operons. The vanA PT operon in P. thiaminolyticus PT-2B1 had the same gene organization as that of vanA operons whereas vanA PA in P. apiarius PA-B2B resembled vanB operons due to the presence of vanW upstream from the vanHAX cluster but was closer to vanA operons in sequence. Reference P. apiarius strains NRRL B-4299 and NRRL B-4188 were found to harbor operons indistinguishable from vanA PA by PCR mapping, restriction fragment length polymorphism, and partial sequencing, suggesting that this operon was species specific. As in enterococci, resistance was inducible by glycopeptides and associated with the synthesis of pentadepsipeptide peptidoglycan precursors ending in d-Ala-d-Lac, as demonstrated by d,d-dipeptidase activities, high-pressure liquid chromatography, and mass spectrometry. The precursors differed from those in enterococci by the presence of diaminopimelic acid instead of lysine in the peptide chain. Altogether, the results are compatible with the notion that van operons in soil Paenibacillus strains and in enterococci have evolved from a common ancestor.
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Ohshima, Hideyuki, Satoshi Matsuoka, Kei Asai, and Yoshito Sadaie. "Molecular Organization of Intrinsic Restriction and Modification Genes BsuM of Bacillus subtilis Marburg." Journal of Bacteriology 184, no. 2 (January 15, 2002): 381–89. http://dx.doi.org/10.1128/jb.184.2.381-389.2002.
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ABSTRACT Transcriptional analysis and disruption of five open reading frames (ORFs), ydiO, ydiP, ydiR, ydiS, and ydjA, in the prophage 3 region of the chromosome of Bacillus subtilis Marburg revealed that they are component genes of the intrinsic BsuM restriction and modification system of this organism. The classical mutant strain RM125, which lacks the restriction and modification system of B. subtilis Marburg, lacks the prophage 3 region carrying these five ORFs. These ORFs constitute two operons, the ydiO-ydiP operon and the ydiR-ydiS-ydjA operon, both of which are expressed during the logarithmic phase of growth. The predicted gene products YdiO and YdiP are the orthologues of cytosine DNA methyltransferases. The predicted YdiS product is an orthologue of restriction nucleases, while the predicted YdiR and YdjA products have no apparent paralogues and orthologues whose functions are known. Disruption of the ydiR-ydiS-ydjA operon resulted in enhanced transformation by plasmid DNA carrying multiple BsuM target sequences. Disruption of ydiO or ydiP function requires disruption of at least one of the following genes on the chromosome: ydiR, ydiS, and ydjA. The degrees of methylation of the BsuM target sequences on chromosomal DNAs were estimated indirectly by determining the susceptibility to digestion with XhoI (an isoschizomer of BsuM) of DNAs extracted from the disruptant strains. Six XhoI (BsuM) sites were examined. XhoI digested at the XhoI sites in the DNAs from disruptants with disruptions in both operons, while XhoI did not digest at the XhoI sites in the DNAs from the wild-type strain or from the disruptants with disruptions in the ydiR-ydiS-ydjA operon. Therefore, the ydiO-ydiP operon and the ydiR-ydiS-ydjA operon are considered operons that are responsible for BsuM modification and BsuM restriction, respectively.
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White, Andrea K., and William W. Metcalf. "Two C—P Lyase Operons in Pseudomonas stutzeri and Their Roles in the Oxidation of Phosphonates, Phosphite, and Hypophosphite." Journal of Bacteriology 186, no. 14 (July 15, 2004): 4730–39. http://dx.doi.org/10.1128/jb.186.14.4730-4739.2004.
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ABSTRACT DNA sequencing and analysis of two distinct C—P lyase operons in Pseudomonas stutzeri WM88 were completed. The htxABCDEFGHIJKLMN operon encodes a hypophosphite-2-oxoglutarate dioxygenase (HtxA), whereas the predicted amino acid sequences of HtxB to HtxN are each homologous to the components of the Escherichia coli phn operon, which encodes C—P lyase, although homologs of E. coli phnF and phnO are absent. The genes in the htx operon are cotranscribed based on gene organization, and the presence of the intergenic sequences is verified by reverse transcription-PCR with total RNA. Deletion of the htx locus does not affect the ability of P. stutzeri to grow on phosphonates, indicating the presence of an additional C—P lyase pathway in this organism. To identify the genes comprising this pathway, a Δhtx strain was mutagenized and one mutant lacking the ability to grow on methylphosphonate as the sole P source was isolated. A ca.-10.6-kbp region surrounding the transposon insertion site of this mutant was sequenced, revealing 13 open reading frames, designated phnCDEFGHIJKLMNP, which were homologous to the E. coli phn genes. Deletion of both the htx and phn operons of P. stutzeri abolishes all growth on methylphosphonate and aminoethylphosphonate. Both operons individually support growth on methylphosphonate; however, the phn operon supports growth on aminoethylphosphonate and phosphite, as well. The substrate ranges of both C—P lyases are limited, as growth on other phosphonate compounds, including glyphosate and phenylphosphonate, was not observed.
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Williams, Carol, Lei Xu, and Thomas Blumenthal. "SL1 trans Splicing and 3′-End Formation in a Novel Class of Caenorhabditis elegansOperon." Molecular and Cellular Biology 19, no. 1 (January 1, 1999): 376–83. http://dx.doi.org/10.1128/mcb.19.1.376.
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ABSTRACT Many Caenorhabditis elegans genes exist in operons in which polycistronic precursors are processed by cleavage at the 3′ ends of upstream genes and trans splicing 100 to 400 nucleotides away, at the 5′ ends of downstream genes, to generate monocistronic messages. Of the two spliced leaders, SL1 istrans spliced to the 5′ ends of upstream genes, whereas SL2 is reserved for downstream genes in operons. However, there are isolated examples of what appears to be a different sort of operon, in which trans splicing is exclusively to SL1 and there is no intercistronic region; the polyadenylation signal is only a few base pairs upstream of the trans-splice site. We have analyzed the processing of an operon of this type by inserting the central part of mes-6/cks-1 into an SL2-type operon. In this novel context, cks-1 is trans spliced only to SL1, and mes-6 3′-end formation occurs normally, demonstrating that this unique mode of processing is indeed intrinsic to this kind of operon, which we herein designate “SL1-type.” An exceptionally long polypyrimidine tract found in the 3′ untranslated regions of the three known SL1-type operons is shown to be required for the accumulation of both upstream and downstream mRNAs. Mutations of thetrans-splice and poly(A) signals indicate that the two processes are independent and in competition, presumably due to their close proximity, raising the possibility that production of upstream and downstream mRNAs is mutually exclusive.
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He, Hongjun, Holly A. Snyder, and Steven Forst. "Unique organization and regulation of the mrx fimbrial operon in Xenorhabdus nematophila." Microbiology 150, no. 5 (May 1, 2004): 1439–46. http://dx.doi.org/10.1099/mic.0.26853-0.
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Xenorhabdus nematophila, a Gram-negative bacterium belonging to the Proteus clade of the family Enterobacteriaceae, forms a mutualistic association with the soil nematode Steinernema carpocapsae. The nematode invades insects and releases Xenorhabdus into the haemolymph, where it participates in insect killing. To begin to understand the role of fimbriae in the unique life cycle of Xenorhabdus, the organization and expression of the mrx fimbrial operon was analysed. The mrx operon contained only five structural genes (mrxACDGH), making it one of the smallest chaperone-usher fimbrial operons studied to date. Unlike the mrp operon of Proteus mirabilis, a site-specific recombinase was not linked to the mrx operon. The intergenic region between the major fimbrial gene (mrxA) and the usher gene (mrxC) lacked a mrpB-like gene, but contained three tandem inverted repeat sequences located downstream of mrxA. A 940 nt mrxA-containing mRNA was the major transcript produced in cells growing on agar, while an mrx polycistronic mRNA was produced at low levels. A canonical σ 70 promoter, identified upstream of mrxA, was not subject to promoter inversion. Fimbriae were not produced in an lrp-mutant strain, suggesting that the leucine-responsive regulatory protein, Lrp, plays a role in the regulation of the mrx operon. These findings show that the genetic organization and regulation of the mrx operon is in several respects distinct from other chaperone-usher fimbrial operons.
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Goldman, J. D., D. G. White, and S. B. Levy. "Multiple antibiotic resistance (mar) locus protects Escherichia coli from rapid cell killing by fluoroquinolones." Antimicrobial Agents and Chemotherapy 40, no. 5 (May 1996): 1266–69. http://dx.doi.org/10.1128/aac.40.5.1266.
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The multiple antibiotic resistance (mar) locus in Escherichia coli consists of two divergently expressed operons (marC and marRAB), both of which contribute to the Mar phenotype. Overexpression of the marRAB operon protected E. coli against rapid cell killing by fluoroquinolones. Inactivation of the operon in mar mutants restored a wild-type bactericidal susceptibility. Both operons of the locus were required for protection from the quinolone-mediated bactericidal activity in mar locus deletion mutants. The effect was lost at high concentrations of fluoroquinolones, unlike the case for the previously described genes hipA and hipQ. The inducible mar locus appears to specify a novel antibactericidal mechanism which may play a role in the emergence of fluoroquinolone-resistant clinical E. coli isolates.
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Dong, Shengli, Sylvia A. McPherson, Li Tan, Olga N. Chesnokova, Charles L. Turnbough, and David G. Pritchard. "Anthrose Biosynthetic Operon of Bacillus anthracis." Journal of Bacteriology 190, no. 7 (February 1, 2008): 2350–59. http://dx.doi.org/10.1128/jb.01899-07.
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ABSTRACT The exosporium of Bacillus anthracis spores consists of a basal layer and an external hair-like nap. The nap is composed primarily of the glycoprotein BclA, which contains a collagen-like region with multiple copies of a pentasaccharide side chain. This oligosaccharide possesses an unusual terminal sugar called anthrose, followed by three rhamnose residues and a protein-bound N-acetylgalactosamine. Based on the structure of anthrose, we proposed an enzymatic pathway for its biosynthesis. Examination of the B. anthracis genome revealed six contiguous genes that could encode the predicted anthrose biosynthetic enzymes. These genes are transcribed in the same direction and appear to form two operons. We introduced mutations into the B. anthracis chromosome that either delete the promoter of the putative upstream, four-gene operon or delete selected genes in both putative operons. Spores produced by strains carrying mutations in the upstream operon completely lacked or contained much less anthrose, indicating that this operon is required for anthrose biosynthesis. In contrast, inactivation of the downstream, two-gene operon did not alter anthrose content. Additional experiments confirmed the organization of the anthrose operon and indicated that it is transcribed from a σE-specific promoter. Finally, we demonstrated that anthrose biosynthesis is not restricted to B. anthracis as previously suggested.
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Liu, Tao, Hao Luo, and Feng Gao. "Position preference of essential genes in prokaryotic operons." PLOS ONE 16, no. 4 (April 22, 2021): e0250380. http://dx.doi.org/10.1371/journal.pone.0250380.
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Essential genes, which form the basis of life activities, are crucial for the survival of organisms. Essential genes tend to be located in operons, but how they are distributed in operons is still unclear for most prokaryotes. In order to clarify the general rule of position preference of essential genes in operons, an index of the average position of genes in an operon was proposed, and the distributions of essential and non-essential genes in operons in 51 bacterial genomes and two archaeal genomes were analyzed based on this new index. Consequently, essential genes were found to preferentially occupy the front positions of the operons, which tend to be expressed at higher levels.
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Nanamiya, Hideaki, Makiko Sato, Kenta Masuda, Mikiko Sato, Tetsuya Wada, Shota Suzuki, Yousuke Natori, Masato Katano, Genki Akanuma, and Fujio Kawamura. "Bacillus subtilis mutants harbouring a single copy of the rRNA operon exhibit severe defects in growth and sporulation." Microbiology 156, no. 10 (October 1, 2010): 2944–52. http://dx.doi.org/10.1099/mic.0.035295-0.
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The number of copies of rRNA genes in bacterial genomes differs greatly among bacterial species. It is difficult to determine the functional significance of the heterogeneity of each rRNA operon fully due to the existence of multiple rRNA operons and because the sequence heterogeneity among the rRNA genes is extremely low. To overcome this problem, we sequentially deleted the ten rrn operons of Bacillus subtilis and constructed seven mutant strains that each harboured a single rrn operon (either rrnA, B, D, E, I, J or O) in their genome. The growth rates and sporulation frequencies of these mutants were reduced drastically compared with those of the wild-type strain, and this was probably due to decreased levels of ribosomes in the mutants. Interestingly, the ability to sporulate varied significantly among the mutant strains. These mutants have proved to be invaluable in our initial attempts to reveal the functional significance of the heterogeneity of each rRNA operon.
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Yeon, Jihye, Stephen M. Miller, and Wipawee Dejtisakdi. "New Synthetic Operon Vectors for Expressing Multiple Proteins in the Chlamydomonas reinhardtii Chloroplast." Genes 14, no. 2 (January 31, 2023): 368. http://dx.doi.org/10.3390/genes14020368.
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Microalgae are a promising platform for generating valuable commercial products, including proteins that may not express well in more traditional cell culture systems. In the model green alga Chlamydomonas reinhardtii, transgenic proteins can be expressed from either the nuclear or chloroplast genome. Expression in the chloroplast has several advantages, but technology is not yet well developed for expressing multiple transgenic proteins simultaneously. Here, we developed new synthetic operon vectors to express multiple proteins from a single chloroplast transcription unit. We modified an existing chloroplast expression vector to contain intercistronic elements derived from cyanobacterial and tobacco operons and tested the ability of the resulting operon vectors to express two or three different proteins at a time. All operons containing two of the coding sequences (for C. reinhardtii FBP1 and atpB) expressed the products of those genes, but operons containing the other two coding sequences (C. reinhardtii FBA1 and the synthetic camelid antibody gene VHH) did not. These results expand the repertoire of intercistronic spacers that can function in the C. reinhardtii chloroplast, but they also suggest that some coding sequences do not function well in the context of synthetic operons in this alga.
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Wang, Henian, Ching-Ping Tseng, and Robert P. Gunsalus. "The napF and narG Nitrate Reductase Operons in Escherichia coli Are Differentially Expressed in Response to Submicromolar Concentrations of Nitrate but Not Nitrite." Journal of Bacteriology 181, no. 17 (September 1, 1999): 5303–8. http://dx.doi.org/10.1128/jb.181.17.5303-5308.1999.
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ABSTRACT Escherichia coli synthesizes two biochemically distinct nitrate reductase enzymes, a membrane-bound enzyme encoded by thenarGHJI operon and a periplasmic cytochromec-linked nitrate reductase encoded by thenapFDAGHBC operon. To address why the cell makes these two enzymes, continuous cell culture techniques were used to examinenapF and narG gene expression in response to different concentrations of nitrate and/or nitrite. Expression of thenapF-lacZ and narG-lacZ reporter fusions in strains grown at different steady-state levels of nitrate revealed that the two nitrate reductase operons are differentially expressed in a complementary pattern. The napF operon apparently encodes a “low-substrate-induced” reductase that is maximally expressed only at low levels of nitrate. Expression is suppressed under high-nitrate conditions. In contrast, the narGHJI operon is only weakly expressed at low nitrate levels but is maximally expressed when nitrate is elevated. The narGHJI operon is therefore a “high-substrate-induced” operon that somehow provides a second and distinct role in nitrate metabolism by the cell. Interestingly, nitrite, the end product of each enzyme, had only a minor effect on the expression of either operon. Finally, nitrate, but not nitrite, was essential for repression of napF gene expression. These studies reveal that nitrate rather than nitrite is the primary signal that controls the expression of these two nitrate reductase operons in a differential and complementary fashion. In light of these findings, prior models for the roles of nitrate and nitrite in control ofnarG and napF expression must be reconsidered.
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Alcántara, Cristina, Luz Adriana Sarmiento-Rubiano, Vicente Monedero, Josef Deutscher, Gaspar Pérez-Martínez, and María J. Yebra. "Regulation of Lactobacillus casei Sorbitol Utilization Genes Requires DNA-Binding Transcriptional Activator GutR and the Conserved Protein GutM." Applied and Environmental Microbiology 74, no. 18 (August 1, 2008): 5731–40. http://dx.doi.org/10.1128/aem.00230-08.
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ABSTRACT Sequence analysis of the five genes (gutRMCBA) downstream from the previously described sorbitol-6-phosphate dehydrogenase-encoding Lactobacillus casei gutF gene revealed that they constitute a sorbitol (glucitol) utilization operon. The gutRM genes encode putative regulators, while the gutCBA genes encode the EIIC, EIIBC, and EIIA proteins of a phosphoenolpyruvate-dependent sorbitol phosphotransferase system (PTSGut). The gut operon is transcribed as a polycistronic gutFRMCBA messenger, the expression of which is induced by sorbitol and repressed by glucose. gutR encodes a transcriptional regulator with two PTS-regulated domains, a galactitol-specific EIIB-like domain (EIIBGat domain) and a mannitol/fructose-specific EIIA-like domain (EIIAMtl domain). Its inactivation abolished gut operon transcription and sorbitol uptake, indicating that it acts as a transcriptional activator. In contrast, cells carrying a gutB mutation expressed the gut operon constitutively, but they failed to transport sorbitol, indicating that EIIBCGut negatively regulates GutR. A footprint analysis showed that GutR binds to a 35-bp sequence upstream from the gut promoter. A sequence comparison with the presumed promoter region of gut operons from various firmicutes revealed a GutR consensus motif that includes an inverted repeat. The regulation mechanism of the L. casei gut operon is therefore likely to be operative in other firmicutes. Finally, gutM codes for a conserved protein of unknown function present in all sequenced gut operons. A gutM mutant, the first constructed in a firmicute, showed drastically reduced gut operon expression and sorbitol uptake, indicating a regulatory role also for GutM.
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Johansen, Lars Engholm, Per Nygaard, Catharina Lassen, Yvonne Agersø, and Hans H. Saxild. "Definition of a Second Bacillus subtilis pur Regulon Comprising the pur and xpt-pbuX Operons plus pbuG, nupG (yxjA), and pbuE (ydhL)." Journal of Bacteriology 185, no. 17 (September 1, 2003): 5200–5209. http://dx.doi.org/10.1128/jb.185.17.5200-5209.2003.
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ABSTRACT In Bacillus subtilis expression of genes or operons encoding enzymes and other proteins involved in purine synthesis is affected by purine bases and nucleosides in the growth medium. The genes belonging to the PurR regulon (purR, purA, glyA, guaC, pbuO, pbuG, and the pur, yqhZ-folD, and xpt-pbuX operons) are controlled by the PurR repressor, which inhibits transcription initiation. Other genes are regulated by a less-well-described transcription termination mechanism that responds to the presence of hypoxanthine and guanine. The pur operon and the xpt-pbuX operon, which were studied here, are regulated by both mechanisms. We isolated two mutants resistant to 2-fluoroadenine in which the pur operon and the xpt-pbuX operon are expressed at increased levels in a PurR-independent manner. The mutations were caused by deletions that disrupted a potential transcription terminator structure located immediately upstream of the ydhL gene. The 5′ part of the ydhL leader region contained a 63-nucleotide (nt) sequence very similar to the 5′ ends of the leaders of the pur and xpt-pbuX operons. Transcripts of these regions may form a common tandem stem-loop secondary structure. Two additional genes with potential leader regions containing the 63-nt sequence are pbuG, encoding a hypoxanthine-guanine transporter, and yxjA, which was shown to encode a purine nucleoside transporter and is renamed nupG. Transcriptional lacZ fusions and mutations in the 63-nt sequence encoding the possible secondary structures provided evidence that expression of the pur and xpt-pbuX operons and expression of the ydhL, nupG, and pbuG genes are regulated by a common mechanism. The new pur regulon is designated the XptR regulon. Except for ydhL, the operons and genes were negatively regulated by hypoxanthine and guanine. ydhL was positively regulated. The derived amino acid sequence encoded by ydhL (now called pbuE) is similar to the amino acid sequences of metabolite efflux pumps. When overexpressed, PbuE lowers the sensitivity to purine analogs. Indirect evidence indicated that PbuE decreases the size of the internal pool of hypoxanthine. This explains why the hypoxanthine- and guanine-regulated genes are expressed at elevated levels in a mutant that overexpresses pbuE.
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Mackey, Michael C., Moisés Santillán, and Necmettin Yildirim. "Modeling operon dynamics: the tryptophan and lactose operons as paradigms." Comptes Rendus Biologies 327, no. 3 (March 2004): 211–24. http://dx.doi.org/10.1016/j.crvi.2003.11.009.
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Hefty, P. Scott, and Richard S. Stephens. "Chlamydial Type III Secretion System Is Encoded on Ten Operons Preceded by Sigma 70-Like Promoter Elements." Journal of Bacteriology 189, no. 1 (October 20, 2006): 198–206. http://dx.doi.org/10.1128/jb.01034-06.
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ABSTRACT Many gram-negative bacterial pathogens employ type III secretion systems for infectious processes. Chlamydiae are obligate intracellular bacteria that encode a conserved type III secretion system that is likely requisite for growth. Typically, genes encoding type III secretion systems are located in a single locus; however, for chlamydiae these genes are scattered throughout the genome. Little is known regarding the gene regulatory mechanisms for this essential virulence determinant. To facilitate identification of cis-acting transcriptional regulatory elements, the operon structure was determined. This analysis revealed 10 operons that contained 37 genes associated with the type III secretion system. Linkage within these operons suggests a role in type III secretion for each of these genes, including 13 genes encoding proteins with unknown function. The transcriptional start site for each operon was determined. In conjunction with promoter activity assays, this analysis revealed that the type III secretion system operons encode σ70-like promoter elements. Transcriptional initiation by a sigma factor responsible for constitutive gene expression indicates that undefined activators or repressors regulate developmental stage-specific expression of chlamydial type III secretion system genes.
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Goh, Ee-Been, Peggy J. Bledsoe, Li-Ling Chen, Prasad Gyaneshwar, Valley Stewart, and Michele M. Igo. "Hierarchical Control of Anaerobic Gene Expression in Escherichia coli K-12: the Nitrate-Responsive NarX-NarL Regulatory System Represses Synthesis of the Fumarate-Responsive DcuS-DcuR Regulatory System." Journal of Bacteriology 187, no. 14 (July 2005): 4890–99. http://dx.doi.org/10.1128/jb.187.14.4890-4899.2005.
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ABSTRACT Hierarchical control ensures that facultative bacteria preferentially use the available respiratory electron acceptor with the most positive standard redox potential. Thus, nitrate is used before other electron acceptors such as fumarate for anaerobic respiration. Nitrate regulation is mediated by the NarX-NarL two-component system, which activates the transcription of operons encoding nitrate respiration enzymes and represses the transcription of operons for other anaerobic respiratory enzymes, including enzymes involved in fumarate respiration. These are fumarate reductase (encoded by the frdABCD operon), fumarase B, which generates fumarate from malate, and the DcuB permease for fumarate, malate, and aspartate. The transcription of the corresponding structural genes is activated by the DcuS-DcuR two-component system in response to fumarate or its dicarboxylate precursors. We report results from preliminary transcription microarray experiments that revealed two previously unknown members of the NarL regulon: the aspA gene encoding aspartate-ammonia lyase, which generates fumarate; and the dcuSR operon encoding the dicarboxylate-responsive regulatory system. We measured beta-galactosidase expression from monocopy aspA-lacZ, frdA-lacZ, and dcuS-lacZ operon fusions in response to added nitrate and fumarate and with respect to the dcuR and narL genotypes. Nitrate, acting through the NarX-NarL regulatory system, repressed the transcription of all three operons. Only frdA-lacZ expression, however, was responsive to added fumarate or a dcuR + genotype. Phospho-NarL protein protected operator sites in the aspA and dcuS promoter regions from DNase I cleavage in vitro. The overall results are consistent with the hypothesis that nitrate represses frdA operon transcription not only directly, by repressing frdA promoter activity, but also indirectly, by repressing dcuS promoter activity.
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White, Andrea K., and William W. Metcalf. "The htx and ptx Operons of Pseudomonas stutzeri WM88 Are New Members of the Pho Regulon." Journal of Bacteriology 186, no. 17 (September 1, 2004): 5876–82. http://dx.doi.org/10.1128/jb.186.17.5876-5882.2004.
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ABSTRACT The htx and ptx operons of Pseudomonas stutzeri WM88 allow for the use of the inorganic reduced phosphorus (P) compounds hypophosphite (P valence, +1) and phosphite (P valence, +3) as sole P sources. To support the proposed in vivo role for the htx and ptx operons, namely the use of phosphite and hypophosphite as alternative P sources, we used reporter gene fusions to examine their expression levels with respect to various P conditions. Expression of the htx and ptx operons was induced up to 17- and 22-fold, respectively, in cultures grown under phosphate starvation conditions relative to expression in medium with excess phosphate (Pi). However, the presence of the reduced P substrate hypophosphite, phosphite, or methylphosphonate, in addition to excess Pi, did not result in an increase in the expression of either operon. To provide further support for a role of the htx and ptx operons in Pi acquisition, we identified P. stutzeri phoBR homologs and constructed deletion mutants. Induction of the htx and ptx reporter gene fusions in response to growth on limiting Pi was abolished in ΔphoB, ΔphoR, and ΔphoBR mutants, demonstrating that htx and ptx expression is phoBR dependent. The putative LysR-type regulator encoded by ptxE has no apparent role in the expression of the htx and ptx operons, as no effect was observed on the level of induction of either operon in a ΔptxE mutant.
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Chaurasia, Akhilesh Kumar, and Shree Kumar Apte. "Overexpression of the groESL Operon Enhances the Heat and Salinity Stress Tolerance of the Nitrogen-Fixing Cyanobacterium Anabaena sp. Strain PCC7120." Applied and Environmental Microbiology 75, no. 18 (July 24, 2009): 6008–12. http://dx.doi.org/10.1128/aem.00838-09.
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ABSTRACT The bicistronic groESL operon, encoding the Hsp60 and Hsp10 chaperonins, was cloned into an integrative expression vector, pFPN, and incorporated at an innocuous site in the Anabaena sp. strain PCC7120 genome. In the recombinant Anabaena strain, the additional groESL operon was expressed from a strong cyanobacterial P psbA1 promoter without hampering the stress-responsive expression of the native groESL operon. The net expression of the two groESL operons promoted better growth, supported the vital activities of nitrogen fixation and photosynthesis at ambient conditions, and enhanced the tolerance of the recombinant Anabaena strain to heat and salinity stresses.
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Tobe, Toru, Tetsuya Hayashi, Chang-Gyun Han, Gary K. Schoolnik, Eiichi Ohtsubo, and Chihiro Sasakawa. "Complete DNA Sequence and Structural Analysis of the Enteropathogenic Escherichia coli Adherence Factor Plasmid." Infection and Immunity 67, no. 10 (October 1, 1999): 5455–62. http://dx.doi.org/10.1128/iai.67.10.5455-5462.1999.
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ABSTRACT The complete nucleotide sequence and organization of the enteropathogenic Escherichia coli (EPEC) adherence factor (EAF) plasmid of EPEC strain B171 (O111:NM) were determined. The EAF plasmid encodes two known virulence-related operons, thebfp operon, which is composed of genes necessary for biosynthesis of bundle-forming pili, and the bfpTVW(perABC) operon, composed of regulatory genes required forbfp transcription and also for transcriptional activation of the eae gene in the LEE pathogenicity island on the EPEC chromosome. The 69-kb EAF plasmid, henceforth designated pB171, contains, besides the bfp and bfpTVW(perABC) operons, potential virulence-associated genes, plasmid replication and maintenance genes, and many insertion sequence elements. Of the newly identified open reading frames (ORFs), two which comprise a single operon had the potential to encode proteins with high similarity to a C-terminal region of ToxB whose coding sequence is located on pO157, a large plasmid harbored by enterohemorrhagicE. coli. Another ORF, located between the bfpand bfpTVW operons, showed high similarity withtrcA, a bfpT-regulated chaperone-like protein gene of EPEC. Two sites were found to be putative replication regions: one similar to RepFIIA of p307 or F, and the other similar to RepFIB of R100 (NR1). In addition, we identified a third region that contains plasmid maintenance genes. Insertion elements were scattered throughout the plasmid, indicating the mosaic nature of the EAF plasmid and suggesting evolutionary events by which virulence genes may have been obtained.
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Darwin, Andrew J., Eva C. Ziegelhoffer, Patricia J. Kiley, and Valley Stewart. "Fnr, NarP, and NarL Regulation of Escherichia coli K-12 napF (Periplasmic Nitrate Reductase) Operon Transcription In Vitro." Journal of Bacteriology 180, no. 16 (August 15, 1998): 4192–98. http://dx.doi.org/10.1128/jb.180.16.4192-4198.1998.
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ABSTRACT The expression of several Escherichia coli operons is activated by the Fnr protein during anaerobic growth and is further controlled in response to nitrate and nitrite by the homologous response regulators, NarL and NarP. Among these operons, thenapF operon, encoding a periplasmic nitrate reductase, has unique features with respect to its Fnr-, NarL-, and NarP-dependent regulation. First, the Fnr-binding site is unusually located compared to the control regions of most other Fnr-activated operons, suggesting different Fnr-RNA polymerase contacts during transcriptional activation. Second, nitrate and nitrite activation is solely dependent on NarP but is antagonized by the NarL protein. In this study, we used DNase I footprint analysis to confirm our previous assignment of the unusual location of the Fnr-binding site in the napFcontrol region. In addition, the in vivo effects of Fnr-positive control mutations on napF operon expression indicate that the napF promoter is atypical with respect to Fnr-mediated activation. The transcriptional regulation of napF was successfully reproduced in vitro by using a supercoiled plasmid template and purified Fnr, NarL, and NarP proteins. These in vitro transcription experiments demonstrate that, in the presence of Fnr, the NarP protein causes efficient transcription activation whereas the NarL protein does not. This suggests that Fnr and NarP may act synergistically to activate napF operon expression. As observed in vivo, this activation by Fnr and NarP is antagonized by the addition of NarL in vitro.
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Ordóñez, Efrén, Michal Letek, Noelia Valbuena, José A. Gil, and Luis M. Mateos. "Analysis of Genes Involved in Arsenic Resistance in Corynebacterium glutamicum ATCC 13032." Applied and Environmental Microbiology 71, no. 10 (October 2005): 6206–15. http://dx.doi.org/10.1128/aem.71.10.6206-6215.2005.
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ABSTRACT Corynebacterium glutamicum is able to grow in media containing up to 12 mM arsenite and 500 mM arsenate and is one of the most arsenic-resistant microorganisms described to date. Two operons (ars1 and ars2) involved in arsenate and arsenite resistance have been identified in the complete genome sequence of Corynebacterium glutamicum. The operons ars1 and ars2 are located some distance from each other in the bacterial chromosome, but they are both composed of genes encoding a regulatory protein (arsR), an arsenite permease (arsB), and an arsenate reductase (arsC); operon ars1 contains an additional arsenate reductase gene (arsC1′) located immediately downstream from arsC1. Additional arsenite permease and arsenate reductase genes (arsB3 and arsC4) scattered on the chromosome were also identified. The involvement of ars operons in arsenic resistance in C. glutamicum was confirmed by gene disruption experiments of the three arsenite permease genes present in its genome. Wild-type and arsB3 insertional mutant C. glutamicum strains were able to grow with up to 12 mM arsenite, whereas arsB1 and arsB2 C. glutamicum insertional mutants were resistant to 4 mM and 9 mM arsenite, respectively. The double arsB1-arsB2 insertional mutant was resistant to only 0.4 mM arsenite and 10 mM arsenate. Gene amplification assays of operons ars1 and ars2 in C. glutamicum revealed that the recombinant strains containing the ars1 operon were resistant to up to 60 mM arsenite, this being one of the highest levels of bacterial resistance to arsenite so far described, whereas recombinant strains containing operon ars2 were resistant to only 20 mM arsenite. Northern blot and reverse transcription-PCR analysis confirmed the presence of transcripts for all the ars genes, the expression of arsB3 and arsC4 being constitutive, and the expression of arsR1, arsB1, arsC1, arsC1′, arsR2, arsB2, and arsC2 being inducible by arsenite.
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Martin-Verstraete, Isabelle, Josef Deutscher, and Anne Galinier. "Phosphorylation of HPr and Crh by HprK, Early Steps in the Catabolite Repression Signalling Pathway for the Bacillus subtilis Levanase Operon." Journal of Bacteriology 181, no. 9 (May 1, 1999): 2966–69. http://dx.doi.org/10.1128/jb.181.9.2966-2969.1999.
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ABSTRACT Carbon catabolite repression (CCR) of Bacillus subtiliscatabolic genes is mediated by CcpA and in part by P-Ser–HPr. For certain operons, Crh, an HPr-like protein, is also implicated in CCR. In this study we demonstrated that in ptsH1 crh1and hprK mutants, expression of the lev operon was completely relieved from CCR and that both P-Ser–HPr and P-Ser–Crh stimulated the binding of CcpA to the cresequence of the lev operon.
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Gifford, Christine M., and Susan S. Wallace. "The Genes Encoding Formamidopyrimidine and MutY DNA Glycosylases in Escherichia coli Are Transcribed as Part of Complex Operons." Journal of Bacteriology 181, no. 14 (July 15, 1999): 4223–36. http://dx.doi.org/10.1128/jb.181.14.4223-4236.1999.
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ABSTRACT Escherichia coli formamidopyrimidine (Fpg) DNA glycosylase and MutY DNA glycosylase are base excision repair proteins that work together to protect cells from the mutagenic effects of the commonly oxidized guanine product 7,8-dihydro-8-oxoguanine. The genes encoding these proteins, fpg and mutY, are both cotranscribed as part of complex operons. fpg is the terminal gene in an operon with the gene order radC,rpmB, rpmG, and fpg. This operon has transcription initiation sites upstream of radC, in theradC coding region, and immediately upstream offpg. There is a strong attenuator in therpmG-fpg intergenic region and three transcription termination sites downstream of fpg. There is an additional site, in the radC-rpmB intergenic region, that corresponds either to a transcription initiation site or to an RNase E or RNase III cleavage site. mutY is the first gene in an operon with the gene order mutY, yggX, mltC, andnupG. This operon has transcription initiation sites upstream of mutY, in the mutY coding region, and immediately upstream of nupG. There also appear to be attenuators in the yggX-mltC and mltC-nupGintergenic regions. The order of genes in these operons has been conserved or partially conserved only in other closely related gram-negative bacteria, although it is not known whether the genes are cotranscribed in these other organisms.
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Manzoor, Irfan, Sulman Shafeeq, Muhammad Afzal, and Oscar P. Kuipers. "Fucose-Mediated Transcriptional Activation of the fcs Operon by FcsR in Streptococcus pneumoniae." Journal of Molecular Microbiology and Biotechnology 25, no. 2-3 (2015): 120–28. http://dx.doi.org/10.1159/000377724.
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In this study, we explore the impact of fucose on the transcriptome of <i>S. pneumoniae</i> D39. The expression of various genes and operons, including the fucose uptake PTS and utilization operon (<i>fcs</i> operon) was altered in the presence of fucose. By means of quantitative RT-PCR and β-galactosidase analysis, we demonstrate the role of the transcriptional regulator FcsR, present upstream of the <i>fcs</i> operon, as a transcriptional activator of the <i>fcs</i> operon. We also predict a 19-bp putative FcsR regulatory site (5′-ATTTGAACATTATTCAAGT-3′) in the promoter region of the <i>fcs</i> operon. The functionality of this predicted FcsR regulatory site was further confirmed by promoter-truncation experiments, where deletion of half of the FscR regulatory site or full deletion led to the abolition of expression of the <i>fcs</i> operon.
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Takeda, Yasuo, Kazuma Takase, Ichiro Yamato, and Keietsu Abe. "Sequencing and Characterization of the xyl Operon of a Gram-Positive Bacterium, Tetragenococcus halophila." Applied and Environmental Microbiology 64, no. 7 (July 1, 1998): 2513–19. http://dx.doi.org/10.1128/aem.64.7.2513-2519.1998.
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ABSTRACT The xyl operon of a gram-positive bacterium,Tetragenococcus halophila (previously calledPediococcus halophilus), was cloned and sequenced. The DNA was about 7.7 kb long and contained genes for a ribose binding protein and part of a ribose transporter, xylR (a putative regulatory gene), and the xyl operon, along with its regulatory region and transcription termination signal, in this order. The DNA was AT rich, the GC content being 35.8%, consistent with the GC content of this gram-positive bacterium. The xyl operon consisted of three genes, xylA, encoding a xylose isomerase, xylB, encoding a xylulose kinase, andxylE, encoding a xylose transporter, with predicted molecular weights of 49,400, 56,400, and 51,600, respectively. The deduced amino acid sequences of the XylR, XylA, XylB, and XylE proteins were similar to those of the corresponding proteins in other gram-positive and -negative bacteria, the similarities being 37 to 64%. Each polypeptide of XylB and XylE was expressed functionally inEscherichia coli. XylE transported d-xylose in a sodium ion-dependent manner, suggesting that it is the first described xylose/Na+ symporter. The XylR protein contained a consensus sequence for binding catabolites of glucose, such as glucose-6-phosphate, which has been discovered in glucose and fructose kinases in bacteria. Correspondingly, the regulatory region of this operon contained a putative binding site of XylR with a palindromic structure. Furthermore, it contained a consensus sequence, CRE (catabolite-responsive element), for binding CcpA (catabolite control protein A). We speculate that the transcriptional regulation of this operon resembles the regulation of catabolite-repressible operons such as the amy, lev, xyl, andgnt operons in various gram-positive bacteria. We discuss the significance of the regulation of gene expression of this operon inT. halophila.
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Prammananan, Therdsak, Peter Sander, Burkhard Springer, and Erik C. Böttger. "RecA-Mediated Gene Conversion and Aminoglycoside Resistance in Strains Heterozygous for rRNA." Antimicrobial Agents and Chemotherapy 43, no. 3 (March 1, 1999): 447–53. http://dx.doi.org/10.1128/aac.43.3.447.
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ABSTRACT Clinical resistance to aminoglycosides in general is due to enzymatic drug modification. Mutational alterations of the small ribosomal subunit rRNA have recently been found to mediate acquired resistance in bacterial pathogens in vivo. In this study we investigated the effect of 16S rRNA heterozygosity (wild-type [wt] and mutant [mut] operons at position 1408 [1408wt/1408mut]) on aminoglycoside resistance. Using an integrative vector, we introduced a single copy of a mutated rRNA operon (1408 A→G) into Mycobacterium smegmatis, which carries two chromosomal wild-type rRNA operons; the resultant transformants exhibited an aminoglycoside-sensitive phenotype. In contrast, introduction of the mutated rRNA operon into anM. smegmatis rrnB knockout strain carrying a single functional chromosomal wild-type rRNA operon resulted in aminoglycoside-resistant transformants. Subsequent analysis by DNA sequencing and RNase protection assays unexpectedly demonstrated a homozygous mutant genotype, rRNAmut/rRNAmut, in the resistant transformants. To investigate whether RecA-mediated gene conversion was responsible for the aminoglycoside-resistant phenotype in the rRNAwt/rRNAmut strains, recAmutant strains were generated by allelic exchange techniques. Transformation of the recA rrnB M. smegmatis mutant strains with an integrative vector expressing a mutated rRNA operon (Escherichia coli position 1408 A→G) resulted in transformants with an aminoglycoside-sensitive phenotype. Subsequent analysis showed stable heterozygosity at 16S rRNA position 1408 with a single wild-type allele and a single resistant allele. These results demonstrate that rRNA-mediated mutational resistance to aminoglycosides is recessive.