Rozprawy doktorskie na temat „Eukaryotic gene regulation”

Die Aufklärung der Mechanismen zur Kontrolle der Genexpression ist eines der wichtigsten Probleme der modernen Molekularbiologie. Detaillierte experimentelle Untersuchungen sind enorm aufwändig aufgrund der komplexen und kombinatorischen Wechselbeziehungen der beteiligten Moleküle. Infolgedessen sind bioinformatische Methoden unverzichtbar. Diese Dissertation stellt drei Methoden vor, die die Vorhersage der regulatorischen Elementen der Gentranskription verbessern. Der erste Ansatz findet Bindungsstellen, die von den Transkriptionsfaktoren erkannt werden. Dieser sucht statistisch überrepräsentierte kurze Motive in einer Menge von Promotersequenzen und wird erfolgreich auf das Genom der Bäckerhefe angewandt. Die Analyse der Genregulation in höheren Eukaryoten benötigt jedoch fortgeschrittenere Techniken. In verschiedenen Datenbanken liegen Hunderte von Profilen vor, die von den Transkriptionsfaktoren erkannt werden. Die Ähnlichkeit zwischen ihnen resultiert in mehrfachen Vorhersagen einer einzigen Bindestelle, was im nachhinein korrigiert werden muss. Es wird eine Methode vorgestellt, die eine Möglichkeit zur Reduktion der Anzahl von Profilen bietet, indem sie die Ähnlichkeiten zwischen ihnen identifiziert. Die komplexe Natur der Wechselbeziehung zwischen den Transkriptionsfaktoren macht jedoch die Vorhersage von Bindestellen schwierig. Auch mit einer Verringerung der zu suchenden Profile sind die Resultate der Vorhersagen noch immer stark fehlerbehafted. Die Zuhilfenahme der unabhängigen Informationsressourcen reduziert die Häufigkeit der Falschprognosen. Die dritte beschriebene Methode schlägt einen neuen Ansatz vor, die die Gen-Anotation mit der Regulierung von multiplen Transkriptionsfaktoren und den von ihnen erkannten Bindestellen assoziiert. Der Nutzen dieser Methode wird anhand von verschiedenen wohlbekannten Sätzen von Transkriptionsfaktoren demonstriert.
Understanding the mechanisms which control gene expression is one of the fundamental problems of molecular biology. Detailed experimental studies of regulation are laborious due to the complex and combinatorial nature of interactions among involved molecules. Therefore, computational techniques are used to suggest candidate mechanisms for further investigation. This thesis presents three methods improving the predictions of regulation of gene transcription. The first approach finds binding sites recognized by a transcription factor based on statistical over-representation of short motifs in a set of promoter sequences. A succesful application of this method to several gene families of yeast is shown. More advanced techniques are needed for the analysis of gene regulation in higher eukaryotes. Hundreds of profiles recognized by transcription factors are provided by libraries. Dependencies between them result in multiple predictions of the same binding sites which need later to be filtered out. The second method presented here offers a way to reduce the number of profiles by identifying similarities between them. Still, the complex nature of interaction between transcription factors makes reliable predictions of binding sites difficult. Exploiting independent sources of information reduces the false predictions rate. The third method proposes a novel approach associating gene annotations with regulation of multiple transcription factors and binding sites recognized by them. The utility of the method is demonstrated on several well-known sets of transcription factors. RNA interference provides a way of efficient down-regulation of gene expression. Difficulties in predicting efficient siRNA sequences motivated the development of a library containing siRNA sequences and related experimental details described in the literature. This library, presented in the last chapter, is publicly available at http://www.human-sirna-database.net

This thesis presents data aimed at deepening our understanding of the mechanisms underlying eukaryotic gene regulation. A comprehensive understanding of these mechanisms should ultimately both allow insight into disease processes that arise from defects in gene regulatory circuits and might enable gene expression to be manipulated for application in health, agriculture and industry. The mechanisms that regulate gene expression include chromatin remodelling, post-translational modification and DNA methylation. These mechanisms are able to work together to ensure that the structure of chromatin is accessible to the machinery used for DNA based processes, such as transcription, and also to shield chromatin containing genes that remain unexpressed. These mechanisms are investigated using blood, or erythropoeisis, as the model system, with GATA1 and the NuRD complex being the elements of interest. The erythroid transcriptional factor GATA1 is subject to acetylation in two lysine rich regions, with acetylation in the second of these (at two specific lysine residues) being associated with increased chromatin occupancy as a result of a direct interaction with the bromodomain protein Brd3. As part of an effort to examine the structural and functional consequences of this interaction, it is necessary to produce site-specifically acetylated GATA1. The NuRD complex is a large multi-subunit complex involved in modulating chromatin structure to regulate gene expression. It has two activities, chromatin remodelling and histone deacetylation. Although many papers have been published on the complex, very little is available on the architecture. In order to gain a greater insight into the workings of the NuRD complex, negative stain transmission electron microscopy is used to determine the structure of a near-complete complex.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2012.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student submitted PDF version of thesis.
Includes bibliographical references.
Regulation of genes is fundamental to all living processes and can be exerted at many sequential steps. We studied several eukaryotic gene regulatory mechanisms with an emphasis on understanding the interplay between regulatory processes on a genome-wide scale. Gene splicing involves the joining of exonic RNA stretches from within a precursor messenger RNA (mRNA). Splicing typically occurs co-transcriptionally as the pre-mRNA is being produced from the DNA. We explored the relationship between the chromatin state of the gene-encoding DNA and the splicing machinery. We found a marked enrichment for nucleosomes at exonic DNA in human T cells, as compared to surrounding introns, an effect mostly explained by the biased nucleotide content of exons. The use of nucleosome positioning information improved splicing simulation models, suggesting nucleosome positioning may help determine cellular splicing patterns. Additionally, we found several histone marks enriched or depleted at exons compared to the background nucleosome levels, indicative of a histone code for splicing. These results connect the chromatin regulation and mRNA splicing processes in a genome-wide fashion. Another pre-mRNA processing step is cleavage and polyadenylation, which determines the 30 end of the mature mRNA. We found that 3P-Seq was able to quantify the levels of 30 end isoforms, in addition to the method's previous use for annotating mRNA 30 ends. Using 3P-Seq and a transcriptional shutoff experiment in mouse fibroblasts, we investigated the e?effect of nuclear alternative 30 end formation on mRNA stability, typically regulated in the cytoplasm. In genes with multiple, tandem 30 untranslated regions (30 UTRs) produced by alternative cleavage and polyadenylation, we found the shorter UTRs were significantly more stable in general than the longer isoforms. This di?difference was in part explained by the loss of cis-regulatory motifs, such as microRNA targets and PUF-binding sites, between the proximal and distal isoforms. Finally, we characterized the small interfering RNAs (siRNAs) produced from heterochromatic, silenced genomic regions in fission yeast. We observed a considerable bias for siRNAs with a 5' U, and used this bias to infer patterns of siRNA biogenesis. Furthermore, comparisons with between wild-type and the Cid14 non-canonical poly(A) polymerase mutant demonstrated that the exosome, the nuclear surveillance and processing complex, is required for RNA homeostasis. In the absence of a fully functional exosome complex, siRNAs are produced to normal exosome targets, including ribosomal and transfer RNAs, indicating these processes may compete for substrates and underscoring the interconnectedness of gene regulatory systems.
by Noah Spies.
Ph.D.

Transcriptional elongation is a crucial step in eukaryotic gene regulation whose mis-regulation leads to cellular pathologies. This makes it quite imperative to aim for a better understanding of the processes regulating transcriptional elongation. An important process promoting the association of RNA Polymerase II (RNAPII) with the coding region of the active gene and hence transcriptional elongation is the monoubiquitination of histone H2B at lysine 123. A complex of an E2 conjugase, Rad6p, and an E3 ligase, Bre1p, is essential for this process. Consistent with the role of histone H2B monoubiquitination in promoting the association of RNAPII with the active gene, this process was found to be impaired in the absence of Rad6p or point mutation of lysine 123 to arginine (H2B-K123R). Intriguingly, the association of RNAPII with the coding region of the active gene was not impaired in the absence of Bre1p, even though Bre1p is essential for histone H2B monoubiquitination. However, deletion of Bre1p’s RING domain that is essential for histone H2B monoubiquitination led to an impaired RNAPII association with the active gene. This observation indicates a role of the non-RING domain of Bre1p in repressing the association of RNAPII with the active gene, resulting in no net decrease in RNAPII occupancy in the absence of Bre1p. Taken together, my results implicated both the stimulatory and repressive roles of the histone H2B ubiquitin ligase Bre1p in regulation of RNAPII association with the coding regions of active genes and hence transcriptional elongation. Interestingly, my work also revealed that for efficient transcriptional elongation by histone H2B monoubiquitination, its optimum level needs to be maintained by a proper balance between Rad6p-Bre1p-mediated ubiquitination and de-ubiquitination (DUB) by the DUB module of SAGA. It was found that Sus1p, a subunit of the DUB module, promotes transcriptional elongation, DNA repair and replication via regulation of histone H2B DUB. In addition to Rad6p- Bre1p and the DUB module, global level of histone H2B monoubiquitination is also critically regulated by Cdk9, a kinase essential for phosphorylation of the serine 2 residue in the C-terminal domain (CTD) of RNAPII, which promotes transcriptional elongation. Apart from serine phosphorylation, proline residues at RNAPII-CTD undergo isomerization by proline isomerases, which also regulate transcription. One of the proline isomerases, Rrd1p, has been previously implicated in transcription in response to rapamycin treatment. Based on this fact and Rrd1p’s known interaction with RNAPII-CTD, we predicted that Rrd1p might regulate transcription independently of rapamycin treatment. In agreement with this hypothesis, our work revealed Rrd1p’s role in facilitating transcription of both rapamycin responsive and non-responsive genes in the absence of rapamycin treatment. Consistently, the absence of Rrd1p led to an impaired nucleosomal disassembly at the active gene, which correlates with the role of Rrd1p in promoting transcription. This is because maintenance of proper nucleosomal dynamics is essential for efficient transcription. It is known that transcriptional elongation is facilitated by the regulation of nucleosomal dynamics via the histone chaperone, FACT. Efficient chromatin reassembly in the wake of elongating RNAPII contributing to the fidelity of transcription is promoted by FACT. Being evolutionarily conserved among eukaryotes, FACT is also known to regulate DNA replication and repair, apart from transcription. Intriguingly, FACT has been found to be upregulated in cancers while its downregulation leads to tumor cell death. However, the mechanism which fine-tunes FACT for normal cellular functions remained unknown. My studies revealed a novel mechanism of regulation of FACT by the ubiquitin-proteasome system in yeast. San1p, an E3 ligase involved in nuclear protein quality control, was found to associate with the active gene and regulate transcriptional elongation through its E3 ligase activity- mediated turnover of Spt16p component of FACT. This regulation was found to maintain optimum level of Spt16p/FACT to engage with the active gene for proper transcriptional elongation, DNA repair and replication. In spite of playing such crucial roles in gene regulation, it was not known how FACT is targeted to the active gene. We discovered that a direct physical interaction between FACT and Cet1p, the mRNA capping enzyme, targets FACT to the active gene independently of Cet1p’s mRNA capping activity. Such targeting of FACT to the active gene leads to the release of promoter proximally paused-RNAPII into transcriptional elongation. However, the progress of RNAPII along the active gene during transcriptional elongation is frequently impeded by various kinds of damages along the underlying template DNA. Even though some of these lesions are co-transcriptionally repaired, it was not known whether the repair of extremely toxic DNA double-strand breaks (DSBs) was coupled to transcription. My results showed that DSBs at the transcriptionally active state of a gene are repaired faster than at the inactive state but such repair was not mediated by a co-transcriptional recruitment of DSB repair factors. This observation is in contrast to other DNA repair pathways such as nucleotide excision repair (NER) where repair factors are co-transcriptionally recruited to the lesion containing DNA. In this regard, we found that an NER factor, Rad14p, co-transcriptionally associates with the active gene in the absence of DNA damage to promote transcription, which unraveled a new role of Rad14p in transcription in addition its established role in NER. In summary, my results provide significant novel insights into the regulation of transcriptional elongation and associated processes leading to better understanding of eukaryotic gene expression.

The flood of RNA-related research in recent decades has revealed RNA to be a structurally and functionally diverse class of molecule, one that generates an intricate network of regulation that has been pivotal to the evolution of complex lifeforms. In order to elucidate how RNA achieves biological function through the formation of ribonucleoprotein (RNP) complexes, characterisation of RNA recognition by RNA-binding proteins (RBPs) is an essential step. The rules governing the interaction of RNA and RBPs have proved difficult to define, and in many instances, it is not understood how specificity is achieved. Knowledge of these rules is crucial to our understanding of RNA-related functions and their role in disease, and requires further in-depth characterisation of a wide variety of RNP complexes. The research in this Thesis details the RNA-binding behaviour of two reported RBPs. Firstly, the RNA-binding behaviour of the Drosophila transcription factor bicoid is investigated. For many years it has been believed that the bicoid homeodomain binds the 3′-UTR of the caudal mRNA transcript, yet no binding site or specificity determinants have been reported. The work here attempts to characterise this interaction. Further, other domains in the protein are examined with a view to understanding how biological specificity might be achieved. Secondly, characterisation of the RNA-binding behaviour of the heterodimeric pair of transcription elongation factors, Spt4 and Spt5, is reported. This heterodimer is known to be an important player in transcription and yet remarkably little is known about its function. In the present work, the AA-repeat RNA-binding properties of these proteins are investigated, and complex binding behaviour is reported. Overall, it is shown that the elucidation of RNA-binding activity by proteins is often not straightforward, requiring the application of multiple and increasingly sophisticated techniques if we are to grasp the underlying biology.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2010.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Transcription of mRNA appears to occur in random, intermittent bursts in a large variety of organisms. The statistics of mRNA expression can be described by two parameters: the frequency at which bursts occur (burst frequency) and the average number of mRNA produced within each burst (burst size). The mean steady-state abundance of mRNA is the product of the burst size and burst frequency. Although the experimental evidence for bursty gene transcription is abundant, little is known about its origins and consequences. We utilize single-molecule mRNA imaging and simple stochastic kinetic models to probe and understand both the mechanistic details and functional responses of transcriptional bursting in budding yeast. At the molecular level, we show that gene-specific activators can control both burst size and burst frequency by differentially utilizing kinetically distinct promoter elements. We also recognize the importance of activator residence time and nucleosome positioning on bursting. This investigation exemplifies how we can exploit spontaneous fluctuations in gene expression to uncover the molecular mechanisms and kinetic pathways of transcriptional regulation. At the network level, we demonstrate the important phenotypic consequences of transcriptional bursting by showing how noise itself can generate a bimodal, all-or-none gene expression profile that switches spontaneously between the low and high expression states in a transcriptional positive-feedback loop. Such bimodality is a hallmark in decision-making circuitry within metabolic, developmental, and synthetic gene regulatory networks. Importantly, we prove that the bimodal responses observed in our system are not due to deterministic bistability, which is an often-stated necessary condition for allor- none responses in positive-feedback loops. By clarifying a common misconception, this investigation provides unique biological insights into the molecular components, pathways and mechanisms controlling a measured phenotype.
by Tsz-Leung To.
Ph.D.

Eukaryotic gene expression is a highly synchronized cellular process whose nuclear phase is comprised of transcription, and mRNA processing and export. Transcription can be further comprised of transcription initiation, and elongation. Regulation of transcription initiation, transcription elongation, and mRNA processing and export are crucial for normal cellular function, since misregulation of these processes are associated with various diseases including cancer. Many factors or proteins are associated with these cellular processes which are modulated by different regulatory processes to maintain normal cellular function. Ubiquitin-proteasome system (UPS) is one of the recently studied regulatory processes. Over the years, ubiquitin and 26S proteasome have emerged as important regulatory factors in coordination of transcription and coupled mRNA export. However, the mechanisms as to how the ubiquitin and 26S proteasome regulate transcription and coupled mRNA export have not been clearly elucidated. Therefore, my dissertation has focused on understanding the role of UPS in these important cellular processes: transcription initiation, transcription elongation and mRNA export. The results have shown the non-proteolytic role of 19S RP of 26S proteasome in regulation of transcriptional initiation of SAGA and TFIID-dependent PHO84 gene. It was found that 19S RP facilitates both SAGA- and NuA4-TFIID-dependent transcriptional initiations of PHO84 via increased recruitment of the coactivators SAGA and NuA4 HAT, which promote TFIID-independent and -dependent PIC formation in the presence and absence of an essential nutrient, Pi, in the growth media for transcriptional initiation, respectively. Next, our studies have uncovered the role of UPS in regulation of transcriptional elongation. It was found that E3 ubiquitin ligase, San1, mediated UPS regulation of transcription elongation factor, FACT is required for stimulating nucleosomal reassembly at the coding sequence of active genes for proper transcription elongation. We also found the interaction of FACT with another important transcription elongation factor, Paf1C via NTD (N-ter domain) of Cet1p (mRNA capping enzyme) to regulate transcription elongation.Subsequently, our results revealed a novel regulation of Paf1 component of Paf1C by UPS to regulate its abundance for proper cellular function. Transcription of genes could be blocked by DNA damage which can be repaired by transcription-coupled DNA repair (TCR) pathways. SUMOylation, another PTM (Post-translational modifications) like ubiquitination, is implicated in regulation of many DNA repair pathways including TCR, but it is not clearly understood how SUMOylation and associated enzymes are involved in regulation of such pathways. Here, we revealed the distinct role of SUMO ligases Siz1 and Siz2 in response to several DNA damaging agents such as UV, MMS (methyl methanesulfonate), HU (Hydroxyurea) and H2O2 (Hydrogen peroxide). Finally, we have extended our research works to understand the regulatory mechanisms of mRNA export by UPS. We found the interaction of TREX (Transcription/Export) component Sub2 with Mdm30 (F-box protein) for ubiquitination and proteasomal degradation of Sub2 in a transcription-dependent manner to regulate mRNA export. We also found the role CBC (Cap binding complex) in regulation of nuclear mRNA export. Collectively, the results of this study postulate a better understanding of regulation of transcription initiation, transcription elongation, and mRNA export by UPS.

Thesis (Ph.D.)--University of Missouri-Columbia, 2005.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (January 25, 2007) Vita. Includes bibliographical references.

A characteristic feature of gene expression in eukaryotes is the addition of a 5' terminal 7-methylguanine "cap" to nascent pre-messenger RNA (mRNA) in the nucleus. It is the 5'capping process, which proves vital to creating a mature mRNA. The synthesis of an mRNA followed by its capping is a complex undertaking which requires a set of protein factors. The capped mRNA is then exclusively bound by a cap-binding complex (CBC). CBC shields mRNA from exonucleases as well as regulates downstream post-transcriptional events, translational initiation and nonsense mediated mRNA decay (NMD). Any misregulation during capping or in the binding of CBC can lead a number of diseases/disorders. Thus, the process and regulation of capping and CBC binding to mRNA are important fields to study the control of gene expression. Over the years, capping apparatus and CBC have been implicated in post-transcriptional regulation. However, it is not yet known whether CBC plays any role in controlling transcriptional initiation or elongation. Thus, the major research focus in my thesis had been to analyze the role of CBC and capping enzymes in regulation of transcriptional initiation and elongation. The results have revealed the role of CBC in stimulating the formation of pre-initiation complex (PIC) at the promoter in vivo via Mot1p (modifier of transcription). Subsequently, we have demonstrated the roles of CBC in transcription elongation, splicing and nuclear export of mRNA. Interestingly, we find that the capping enzyme, Cet1p, decreases promoter proximal accumulation of RNA polymerase II. These results support that Cet1p promotes the release of paused-RNA polymerase II to get engaged into elongating form for productive transcription. Such function of Cet1p appears to be mediated via the Facilitates chromatin transcription (FACT) complex. We find that FACT is targeted to the active gene by the N-terminal domain of Cet1p independently of its capping activity. In the absence of Cet1p, recruitment of FACT to the active gene is impaired, leading to paused-RNA polymerase II. Collectively, the results of my thesis work provide significant insight on the regulation of gene expression by CBC and capping enzyme, Cet1p.

The initial work focused on the interaction between the general coactivator CBP (SID domain) and members of the p160 family of coactivators (AD1 domain), which is a key step in the activation of transcription by nuclear receptors. The solution structure of the CBP SID / SRC1 AD1 complex described in this thesis shows that the two helical domains are intimately associated, with the first helix in SRC1 AD1 and the first three helices in CBP SID forming a four helix bundle, which is capped by the fourth helix of the AD 1 domain. Comparisons with the structure of the related CBP SID / ACTR AD 1 complex showed that while the CBP SID domain adopts a similar fold in complex with different p160 proteins, the topologies of the AD1 domains are strikingly different, a feature that is likely to contribute to functional specificity of these complexes. The second part of the work described here focused on the interaction between the MA-3 domains of the tumour suppressor Pdcd4 and the translation factor eIF4A, which has been shown to inhibit cap-dependent translation. The C-terminal MA-3 domain (Pdcd4 MA-3C) was shown to consist of three atypical HEAT repeats capped by a final helix. This domain was found to interact with the N-terminal domain of eIF4A through a conserved surface region. The comparison of NMR spectra obtained from Pdcd4 MA-3 C and the tandem MA-3 region strongly suggests that the tandem MA-3 region is composed of two equivalent domains connected by a semi-flexible linker. The high resolution structural information obtained provides important insights into the interactions and functional specificity of the protein complexes studied.

Thesis (Ph. D.)--University of California, Riverside, 2010.
Includes abstract. Available via ProQuest Digital Dissertations. Title from first page of PDF file (viewed April 24, 2010). Includes bibliographical references. Also issued in print.

The 5$ sp prime$ untranslated region (UTR) of c-myc and human immunodeficiency virus type 1 (HIV-1) mRNAs were used as models in a variety of in vitro and in vivo systems in order to study the role of this region in the control of eukaryotic gene expression. Using an ultraviolet light-induced crosslinking assay, a 55 kilodalton protein was identified in extracts of HeLa, mouse erythroleukemia, and other cell lines, which interacts specifically with a purine-rich RNA sequence in the 5$ sp prime$ UTR of c-myc. The function of this protein in control of c-myc expression is not known, but may be implicated in the process of transcriptional elongation. The 5$ sp prime$ UTR of HIV-1 mRNAs was shown to inhibit strongly the translation of a heterologous mRNA; this inhibition was dependent on the secondary structure predicted to form in this region, and on the accessibility of the cap structure to initiation factors. The structural requirements in the HIV-1 5$ sp prime$ UTR for trans-activation by the viral tat gene product were examined by mutagenesis studies; the base-pairing in the stem-loop structure, the sequence of the loop, and the presence of a three nucleotide bulge were found to be critical features necessary for complete trans-activation. These findings indicate that the 5$ sp prime$ UTR can have important effects on the expression of eukaryotic genes.

Endoplasmic reticulum (ER) stress activates an integrated stress response which causes inhibition of overall protein synthesis via phosphorylation of the eukaryotic initiation factor 2alpha (eIF2alpha). However, ER stress also results in selective translation of mRNAs, one of which is a transcription factor ATF4. ATF4 activates transcription of downstream stress-induced genes such as growth arrest and DNA-damage inducible gene 34 (GADD34) under ER stress. The function of GADD34 is to dephosphorylate eIF2alpha by interacting with protein phosphatase 1, thus leading to recovery of overall protein synthesis and translation of stress-induced transcripts through a negative feedback mechanism. In this thesis, we showed that GADD34 is not only transcriptionally induced, but also translationally regulated for maximal expression under ER stress. Translational regulation of GADD34 was mediated by its 5’ untranslated region (5’ UTR), which was found to contain two upstream open reading frames (uORFs) in human and mouse. It was revealed that the downstream uORF2 is required for basal repression and translational upregulation under ER stress, while the upstream uORF1 is dispensable in this regulation. In addition, the uORF2 is readily recognized and translated, but the uORF1 is bypassed by the scanning ribosomes. Further mutational analysis on the GADD34 5’ UTR demonstrated that the uORF2 and the intercistronic region between the uORF2 and the main ORF are sufficient to direct translation when eIF2alpha is phosphorylated. In this process, the amino acid/nucleotide identity of the uORF2 was not required, but its conserved size was important. The sequence conservation within the intercistronic region also was identified, but changing the length and pyrimidine:purine ratio in this region did not significantly affect translational regulation. Finally, we set up in vitro translation systems where cap-dependent translation is compromised by inhibiting ternary complex and eIF4F formation in order to test GADD34 translational regulation. The results from the current thesis suggest that GADD34 translation is mediated through its 5’ UTR via a unique mechanism, which may serve as a model to understand translational regulation of other uORFs-containing mRNAs under cellular stress.

Setdb1 est une «histone» lysine méthyltransférase (KMT) appartenant à la famille SUV39, l’une des principales machineries épigénétiques de répression des gènes. Setdb1 établit notamment la mono-, la di- et la tri- méthylation sur la lysine 9 de l'histone H3 (H3K9).Setdb1 est essentiel pour la survie, la pluripotence et l'auto-renouvellement des cellules souches embryonnaires de souris (mESCs); son knock-out est mortel au stade de la péri-implantation à 3,5 dpc chez la souris. Setdb1 est également nécessaire pour la différenciation de nombreux types de cellules progénitrices : spermatogenèse, neurogenèse, différenciation des chondrocytes et différenciation des muscles squelettiques. De plus, Setdb1 a été associé à plusieurs maladies: il est amplifié dans le mélanome et le cancer du poumon et il est dérégulé dans les cancers du foie, de la prostate, colorectal et du sein, dans la maladie de Huntington et la schizophrénie. Remarquablement, au-delà des histones, Setdb1 méthyle nombreux substrats non-histones, y compris UBF, p53, Akt, Tat et Ing2.Bien que Setdb1 ait toujours été associé à son rôle nucléaire, il s'avère que Setdb1 est la seule KMT de la famille SUV39 à avoir également une localisation cytoplasmique, dans plusieurs types de cellules, y compris les mESCs, les fibroblastes embryonnaires de souris (MEFs) et les cellules HeLa. Cependant, la fonction de Setdb1 dans le cytoplasme reste totalement inconnue. Pour étudier le rôle cytoplasmique de Setdb1, nous avons utilisé des cellules souches embryonnaires de souris (mESCs), dans lesquelles Setdb1 est essentiel. Nos résultats montrent que Setdb1 cytoplasmique est crucial pour la survie des mESCs: en effet, le nombre de cellules apoptotiques augmente après la perte de Setdb1 cytoplasmique. Nous avons constaté que le Setdb1 cytoplasmique affecte la synthèse de protéines dans les mESCs. Nous montrons en outre que le Setdb1 cytoplasmique interagit avec la protéine Trim71 spécifique de mESC (également appelée Lin41) et avec le facteur de traduction d'initiation eIF3c dans les mESC. Enfin, nous avons démontré que Setdb1 et Trim71 co-régulent ensemble la stabilité et la traduction des mARNs. Nos données actuelles mettent au jour la fonction cytoplasmique essentielle d'une lysine méthyltransférase appelée Setdb1, au début considérée comme étant spécifique uniquement des histones et apportent de nouvelles informations sur la régulation post-transcriptionnelle de l'expression génique médiée par un régulateur épigénétique fondamental
Setdb1 is a “histone” lysine methyltransferase (KMT) belonging to the SUV39 family that methylates lysine 9 of histone H3 (H3K9), one of the major epigenetic machineries mainly involved in gene repression. Notably, Setdb1 establishes mono-, di- and tri-methylation of H3K9. Setdb1, or Eset in mice, is essential for the survival, the pluripotency and the self-renewal of mouse embryonic stem cells (mESCs); Eset knockout is lethal at the peri-implantation stage at 3.5 dpc in mice. Setdb1 is also required for the differentiation of many progenitor cell types: spermatogenesis, neurogenesis, chondrocyte differentiation and skeletal muscle differentiation. Moreover, Setdb1 has been associated with several diseases: it is amplified in melanoma and lung cancer and it is dysregulated in liver, prostate, colorectal and breast cancers, Huntington disease and schizophrenia.Remarkably, beyond histones, Setdb1 methylates many non-histone substrates, such as UBF, p53, AKT, Tat and ING2 proteins. Although Setdb1 has been always associated with its nuclear role, it turns out that Setdb1 is the only H3K9 KMT to have also a cytoplasmic localization, in several cell types, including mESCs, mouse embryonic fibroblasts (MEFs) and HeLa cells. However, the function of Setdb1 in the cytoplasm remains totally unknown. To investigate Setdb1 cytoplasmic role, we have used mouse embryonic stem cells (mESCs), in which Setdb1 is essential. Our results show that cytoplasmic Setdb1 is crucial for the survival of mESCs: indeed, the number of apoptotic cells increases after the loss of cytoplasmic Setdb1. We found that cytoplasmic Setdb1 affects newly protein synthesis in mESCs. We further show that cytoplasmic Setdb1 interacts with mESCs-specific protein Trim71 (also called Lin41) and with the initiation translation factor eIF3c in mESCs. Finally, we reported that Setdb1 and Trim71 together co-regulate mRNA stability and translation. Our current data unravel the essential cytoplasmic function of Setdb1, for long time considered exclusively an “histone” lysine methyltransferase, and provide new insights into the post-transcriptional regulation of gene expression mediated by a fundamental epigenetic regulator

O sistema celulolítico do fungo filamentoso Trichoderma reesei é induzido transcricionalmente em pelo menos 1000 vezes pelo crescimento do fungo na presença de celulose e fortemente reprimido por glicose. Usando a abordagem de deleção no promotor, determinou-se que a região localizada entre -241 e -72 bp, em relação ao TATA box, denominada UARcb1, é responsável pela transcrição estimulada por celulose da enzima celobiohidrolase I (cbhl). Neste trabalho mostramos que essa região controla a transcrição de um gene repórter, sofrendo repressão por glicose, em Saccharomyces cerevisiae, um microrganismo que não possui os genes necessários para a utilização de celulose. A transcrição mediada por UARcbl, que é controlada por glicose, requer o produto do gene SNFl, uma proteína quinase, e dois repressores: SSN6 e TUP1, cujos papéis no controle de genes reprimidos por glicose, na levedura, são bem estabelecidos. Nossos resultados indicam um mecanismo conservado de controle por glicose em microrganismos eucarióticos.
The cellulotic system of the filamentous fungus Trichoderma reesei is transcriptionally induced 1000 -fold in presence of cellulose and is strongly repressed by glucose. Using the promoter deletion approach, the upstream activating region (UARcbl) responsible for cellulose-stimulated transcription of the major member of the cellulase system, cellobiohydrolase I, was localized between -241 and -72 relative to the TATA box. In this work we show that this region controls transcription and mediates glucose repression of a reporter gene in Saccharomyces cerevisiae, a unicellular microorganism that lacks the genes required for the utilization of cellulose. Glucose-controlled transcription mediated by the UARcbl requires the product of SNF1 gene, a protein kinase, and two repressors SSN6 and TUP1, which are well estalished in controlling glucose-represible yeast genes. Our results indicate a conserved mechanism of glucose control in eukariotic microorganisms.

Glicose e oxigênio são moléculas essenciais para a maioria dos organismos vivos. Além de sua importância nos processos de produção de energia - glicose como fonte de carbono e energia e oxigênio como aceptor dos elétrons doados por NADH e FADH2 - estes dois compostos funcionam como efetuadores, modulando vários processos metabólicos e fisiológicos nas células. Visto que a mitocôndria é um dos alvos afetados pelas disponibilidades destas duas moléculas, nós isolamos e seqüenciamos o genoma mitocondrial de Trichoderma reesei, um fungo multicelular empregado neste trabalho como sistema modelo. Foi estudado o efeito da variação de concentração de glicose e oxigênio sobre a expressão de transcritos do genoma mitocondrial, bem como sua implicação no metabolismo de glicose. São apresentadas análises da expressão gênica de aproximadamente 2000 transcritos de T. reesei submetido a concentrações limitantes de oxigênio dissolvido, realizadas com o emprego da técnica de microarrays de cDNA. Pelo menos 330 transcritos foram diferencialmente expressos em função da disponibilidade de oxigênio. Aqueles envolvidos nos processos de síntese protéica e divisão celular foram regulados negativamente, enquanto transcritos relacionados com funções de defesa celular e síntese de RNA foram positivamente regulados. Uma fração substancial de outros genes afetados pela baixa disponibilidade de oxigênio não possui, atualmente, funções celulares conhecidas. Esta observação deve contribuir para a posterior anotação funcional do genoma de T. reesei. Também foram identificados reguladores transcricionais diferencialmente expressos em baixas tensões de oxigênio. O perfil de expressão destes reguladores aponta-os como potenciais candidatos ao envolvimento com a expressão de genes afetados pela disponibilidade de oxigênio.
Glucose and oxygen are essential molecules in most of living organisms. In addition to their importance in production of energy - glucose as a carbon and energy source and oxygen as an acceptor of electrons donated by NADH and FADH2 - both molecules function as effectors modulating various metabolic and physiological processes in the cell. Because one of the targets affected by both molecules is the mitochondrion, we isolated and sequenced the mitochondrial genome of Trichoderma reesei, a multicellular fungus that is used in this study as a model system. The effect of varying the concentration of glucose and oxygen on the expression of the transcripts of the mitochondrial genome, and its implication on the metabolism of glucose, was studied. Gene-wide expression analyses of nearly 2000 transcripts of T. reesei under limited concentration of dissolved oxygen, using cDNA microarry technique, are presented. At least 330 transcripts were differentially expressed with respect to oxygen availability. Those involved in protein synthesis and cell division processes were downregulated, while transcripts involved in cell defense and RNA synthesis were upregulated. A substantive fraction of other anaerobically affected genes have currently unknown cellular roles, and these results should therefore contribute to further functional annotation of the genome. ln addition, we have identified transcriptional regulators that are differentially expressed at a low oxygen tensions. The expression profile of these regulators points them out as potential candidates involved in the expression of genes affected by oxygen availability.

Bacillus subtilis is the model organism for low GC Gram-positive bacteria and is of great biotechnological interest. Protein phosphorylation is an important regulatory mechanism in bacteria and it has not been extensively studied yet. Recent site-specific phosphoproteomic studies identified a large number of novel serine/threonine phosphorylation sites in B. subtilis, including a) two transition phase global gene regulators DegS and AbrB and b) RecA, that plays a major role in double-strand break repair and DNA recombination. .B. subtilis disposes of several putative Ser/Thr kinases like PrkA, YbdM, YabT and a characterizd kinase PrkC, but very few physiological substrates for these have been defined so far. In vitro phosphorylation assays were used to identify which of these kinases were able to phosphorylate DegS, RecA and AbrB. DegS phosphorylation on serine 76 by the kinase YbdM influenced its activity towards DegU both in vitro and in vivo, and expression of DegS S76D( on replacing serine to aspartate) in B. subtilis perturbed cellular processes regulated by the DegS/DegU two component system. This suggests a link between DegS phosphorylation at serine 76 and the level of DegU phosphorylation, establishing this post-translational modification as an additional trigger for this two-component system. At the onset of sporulation, B. subtilis expresses an unusual serine/threonine kinase YabT, which exhibits a septal localization and is activated by non-sequence-specific DNA binding. Activated YabT phosphorylates RecA at the residue serine 2, which in turn promotes the formation of RecA foci at the onset of spore development. On the other hand, non-phosphorylatable RecA or inactivated YabT lead to reduced spore formation in the presence of DNA lesions . This suggests a functional similarity between B. subtilis developmental stage dependent RecA phosphorylation and its eukaryal homologous Rad51 phosphorylation, which leads to its recruitment to the lesion sites. We therefore proposed that RecA phosphorylation serves as an additional signal mechanism that promotes focus formation during spore development. AbrB is phosphorylated by YabT, YbdM and PrkC in vitro and AbrB phosphorylation leads to reduced affinity for its target DNA and abolished binding cooperativity in vitro and in vivo. Expression of the phosphomimetic AbrB-S86D or of the non-phosphorylatable AbrB-S86A mutant protein in B. subtilis disturbed some stationary phase phenomena such as exoprotease production, competence and the onset of sporulation, probably by deregulation of AbrB-target genes and operons. We therefore, proposed that AbrB phosphorylation as an additional regulatory mechanism needed to switch off this ambiactive gene regulator during the transition phase.

Quantifying the function of mammalian enhancers at the genome or population scale has been longstanding challenge in the field of gene regulation. Studies of individual enhancers have provided anecdotal evidence on which many foundational assumptions in the field are based. Genome-scale studies have revealed that the number of sites bound by a given transcription factor far outnumber the genes that the factor regulates. In this dissertation we describe a new method, chromatin immune-enriched reporter assays (ChIP-reporters), and use that approach to comprehensively test the enhancer activity of genomic loci bound by the glucocorticoid receptor (GR). Integrative genomics analyses of our ChIP-reporter data revealed an unexpected mechanism of glucocorticoid (GC)-induced gene regulation. In that mechanism, only the minority of GR bound sites acts as GC-inducible enhancers. Many non-GC-inducible GR binding sites interact with GC-induced sites via chromatin looping. These interactions can increase the activity of GC-induced enhancers. Finally, we describe a method that enables the detection and characterization of the functional effects of non-coding genetic variation on enhancer activity at the population scale. Taken together, these studies yield both mechanistic and genetic evidence that provides context that informs the understanding of the effects of multiple enhancer variants on gene expression.

Dissertation

Synthetic biological systems promise to combine the spectacular diversity of biological functionality with engineering principles to design new life to address many pressing needs. As these engineered systems advance in sophistication, there is ever-greater need for customizable, situation-specific expression of desired genes. However, existing gene control platforms are generally not modular, or do not display performance requirements required for robust phenotypic responses to input signals. This work expands the capabilities of eukaryotic gene control in two important directions.

For development of greater modularity, we extend the use of synthetic self-cleaving ribozyme switches to detect changes in input protein levels and convey that information into programmed gene expression in eukaryotic cells. We demonstrate both up- and down-regulation of levels of an output transgene by more than 4-fold in response to rising input protein levels, with maximal output gene expression approaching the highest levels observed in yeast. In vitro experiments demonstrate protein-dependent ribozyme activity modulation. We further demonstrate the platform in mammalian cells. Our switch devices do not depend on special input protein activity, and can be tailored to respond to any input protein to which a suitable RNA aptamer can be developed. This platform can potentially be employed to regulate the expression of any transgene or any endogenous gene by 3’ UTR replacement, allowing for more complex cell state-specific reprogramming.

We also address an important concern with ribozyme switches, and riboswitch performance in general, their dynamic range. While riboswitches have generally allowed for versatile and modular regulation, so far their dynamic ranges of output gene modulation have been modest, generally at most 10-fold. We address this shortcoming by developing a modular genetic amplifier for near-digital control of eukaryotic gene expression. We combine ribozyme switch-mediated regulation of a synthetic TF with TF-mediated regulation of an output gene. The amplifier platform allows for as much as 20-fold regulation of output gene expression in response to input signal, with maximal expression approaching the highest levels observed in yeast, yet being tunable to intermediate and lower expression levels. EC50 values are more than 4 times lower than in previously best-performing non-amplifier ribozyme switches. The system design retains the modular-input architecture of the ribozyme switch platform, and the near-digital dynamic ranges of TF-based gene control.

Together, these developments suggest great potential for the wide applicability of these platforms for better-performing eukaryotic gene regulation, and more sophisticated, customizable reprogramming of cellular activity.

Copper (Cu) is a co-factor that is essential for oxidative phosphorylation, protection from oxidative stress, angiogenesis, signaling, iron acquisition, peptide hormone maturation, and a number of other cellular processes. However, excess copper can lead to membrane damage, protein oxidation, and DNA cleavage. To balance the need for copper with the necessity to prevent accumulation to toxic levels, cells have evolved sophisticated mechanisms to regulate copper acquisition, distribution, and storage. The basic components of these regulatory systems are remarkably conserved in most eukaryotes, and this has allowed the use of a variety of model organisms to further our understanding of how Cu is taken into the cell and utilized.

While the components involved in Cu uptake, distribution, and storage are similar in many eukaryotes, evolution has led to differences in how these processes are regulated. For instance, fungi regulate the components involved in Cu uptake and detoxification primarily at the level of transcription while mammals employ a host of post-translational homeostatic mechanisms. In Saccharomyces cerevisiae, transcriptional responses to copper deficiency are mediated by the copper-responsive transcription factor Mac1. Although Mac1 activates the transcription of genes involved in high affinity copper uptake during periods of deficiency, little is known about the mechanisms by which Mac1 senses or responds to reduced copper availability. In the first part of this work, we show that the copper-dependent enzyme Sod1 (Cu,Zn superoxide dismutase) and its intracellular copper chaperone Ccs1 function in the activation of Mac1 in response to an external copper deficiency. Genetic ablation of either CCS1 or SOD1 results in a severe defect in the ability of yeast cells to activate the transcription of Mac1 target genes. The catalytic activity of Sod1 is essential for Mac1 activation and promotes a regulated increase in binding of Mac1 to copper response elements in the promoter regions of genomic Mac1 target genes. Although there is precedent for additional roles of Sod1 beyond protection of the cell from oxygen radicals, the involvement of this protein in copper-responsive transcriptional regulation has not previously been observed.

Higher eukaryotes including mice and humans regulate Cu uptake predominately by means of post-translational control of the localization and stability of the Cu transport proteins. One of these proteins, Ctr1, is the primary means of Cu uptake into the cell, and members of the highly conserved Ctr family of Cu ion channels have been shown to mediate high affinity Cu(I) uptake into cells. In yeast and cultured human cells, Ctr1 functions as a homo-trimer with each monomer harboring an amino-terminal extracellular domain, three membrane spanning domains, a cytoplasmic loop, and a cytoplasmic tail. In addition to the highly conserved Ctr1 Cu ion importer, the baker's yeast S. cerevisiae expresses a related protein called Ctr2. Experimental evidence demonstrates that unlike yeast and mammalian Ctr1, yeast Ctr2 is localized to the vacuolar membrane where it mobilizes Cu stores to the cytoplasm under conditions of Cu limitation.

In mice and humans a gene encoding a protein with significant similarity to the Ctr family has been identified, denoted Ctr2. Publications from others suggest that mammalian Ctr2 may either be a low affinity Cu importer at the plasma membrane or, similar to yeast Ctr2, may mobilize Cu from intracellular organelles such as the lysosome to the cytosol. In agreement with a previous report we found that a fraction of mouse Ctr2 is localized to the plasma membrane and that its membrane topology is the same as Ctr1. Interestingly, over-expression of Ctr2 by stable transfection results in decreased intracellular bioavailable Cu. To begin to understand the physiological role of Ctr2, mice bearing a systemic deletion of the Ctr2 gene were generated. The Ctr2-/- mice are viable but hyper-accumulate Cu in all tissues analyzed. Moreover, protein levels of the Ctr1 Cu importer are dramatically altered in tissues from the Ctr2 knock out mice, and over-expression of Ctr2 in cultured mammalian cells enhances processing of the Ctr1 protein into a less active form. Taken together these results suggest that mammalian Ctr2 functions in the cell as a negative regulator of Cu import via Ctr1.

Dissertation

Transcription regulation in eukaryotes is a complex process governed by the concerted action of different factors. The work in this thesis is focused on transcriptional regulation in Saccharomyces cerevisiae. I analyzed the regulation of the phospholipid biosynthetic gene INO1 , which has been a model gene for transcription studies for over three decades. Some major questions that I have addressed are: what kinds of cis regulatory sequences and trans factors are important for regulation of INO1? What is the sequence of events in this regulation? How is the recruitment of these trans factors consequential for INO1 transcription? I present my results here for the role of the basic helix loop helix transcription factor (bHLH) family in coordinated regulation of INO1 transcription. I report that the centromeric binding factor 1 (Cbf1p) together with two other members of the bHLH protein family, Ino2p and Ino4p, are required for efficient derepression of INO1 transcription. Together these bHLH transcription factors recruit the ISW2 chromatin-remodeling complex onto the INO1 promoter to drive productive transcription from the INO1 locus. My efforts in studying the regulation of INO1 led me to study the regulation of SNA3, a gene found in tandem upstream (→→) to the INO1 gene and regulated by the same environmental conditions as INO1. Studies on the mechanism of coregulation of adjacent genes in budding yeast have been largely speculative. I provide evidence that the same bHLH proteins which regulate INO1 also regulate SNA3, albeit differentially. Significantly, my results also show that the regulation of both SNA3 and INO1 is dictated from the intergenic region between the two genes. This is a novel mechanism of transcription regulation in yeast as regulation from downstream of ORF is unknown in yeast. Thus, my results with both SNA3 and INO1 provide novel details on how the process of transcription is regulated in response to an environmental cue.

MicroRNAs (miRNA) are endogenous, small non coding RNAs, which play a prominent role in eukaryotic gene regulation. Perturbations leading to an altered abundance of miRNAs can lead to pathological conditions like neurological disorders, auto-immune diseases, and even cancer. Thus regulating the regulators is of utmost importance. While miRNA biogenesis is well delineated pathway, currently sparse information is available about active miRNA turnover. A mature miRNA is thought to be formed within the pre-miRISC (RNA induced silencing complex), where one of the strands, miRNA/ guide strand from the miRNA: miRNA*/ guide: passenger strand duplex, is preferentially selected and loaded onto the Argonaute (AGO). This miRNA acts as an ‘address label’, and guides the miRISC to cognate mRNA targets, and repress their expression. Researchers while purifying human siRISC have found that the small RNA remains very strongly bound by AGO, even under highly stringent conditions applied during the purification. The notion got strengthened when the crystal structure of Thermus thermophilus AGO bound to a 5’ phosphorylated guide DNA revealed that AGO has a bi-lobed structure comprising of four domains (N, PAZ, MID, and PIWI), where except the nucleotides in the seed region (2nd-8th nucleotide from the 5’ end), the entire guide sequence remains buried inside a channel. The co-crystal structure also showed that the 5’ nucleotide of the guide sequence remains anchored at the basic pocket of the MID domain, and 3’ end is bound by the PAZ domain. Later, co-crystal structures of human Ago2 with endogenous RNA, and that of yeast AGO with guide RNA, revealed that multiple general and specific contacts exist between the residues of AGO and the guide sequence. This endorsed the earlier notion that only seed sequence of the guide strand remains free for interaction with cognate targets through antisense mechanism. Together all these findings contributed to the notion that miRISC (as well as siRISC) is a super-stable structure, and the guide sequence remains inaccessible to the nucleases in the cell. Hence, miRNAs might undergo passive degradation with host AGO protein. Interestingly, this notion got upturned, when researchers working with Caenorhabditis elegans (C. elegans) demonstrated through explicit biochemical assays that miRNAs do get released from AGO, before they undergo degradation by bona fide miRNase (XRN-2), without affecting the integrity of the AGO protein. The researchers also showed that XRN-2 depleted worm lysate exhibits compromised miRNA release activity. This could possibly be because of a direct role of XRN-2 in miRNA release. Conversely, XRN-2 might function downstream of a dedicated miRNA release factor, and in XRN-2 depleted state, as the release and turnover kinetics are inter-linked, the overall miRNA release activity got diminished. However, the identity of this endogenous, and dedicated ‘miRNA release factor’ has remained elusive, so far. Here, we report, that a small 26 kDa pur-alpha family protein (PLP-1) is responsible for release of a subset of miRNAs from miRNA AGO of C. elegans, without affecting the integrity of AGO. The protein is not only capable of freeing the miRNA from the grasp of AGO, but can also bind, and deliver it to a defined component of miRNA turnover machinery (miRNasome-1). We have also found that this protein can oligomerize, and that binding of specific miRNAs can induce unique pattern of oligomerization of the protein (by enhancing specific oligomeric forms over others). This work unfurls the potential of a single protein to perform multiple functions through its different oligomeric forms, and connect miRISC/AGO to a miRNA turnover complex (miRNAsome-1) in a two-step miRNA turnover pathway. The Chapter 1 forms the Introduction to the thesis, and presents extensive literature survey on topics pertaining to the current work. The chapter begins with a brief history of non-coding RNAs, miRNAs in particular. Next, an account of the different small non-coding RNAs is presented. This is followed by a description of the well-delineated miRNA biogenesis pathway, with special emphasis on RISC (RNA induced silencing complex) and AGO proteins, the core of RISC. Next, a short summary on RNAi is documented. Further, an account of miRNA functions in general, miRNA stability, and active miRNA turnover is presented chronologically. Thereafter, a brief description of the model organism C. elegans, used for the current study is presented. The Chapter 2 constitutes the first data chapter titled ‘Identification of a miRNA releasing activity in Caenorhabditis elegans.’ This chapter embodies the primary working hypothesis, and establishes the platform for the identification of the miRNA release factor. Here an account of the step-wise fractionation of C. elegans total lysate, yielding a fraction enriched in miRNA release activity is presented. After subsequent biochemical assays with the enriched fraction, mass spectrometric analysis is carried out, which reveals Pur alpha like protein-1 (PLP-1), as a putative miRNA release factor. Using recombinant PLP-1 in in vitro assays, the protein is validated as the miRNA release factor, which can dislodge the miRNA from the grasp of AGO/ miRISC, in C. elegans. Further, a probable role of the paralogous protein PLP-2 from C. elegans in miRNA binding and release, is studied and discussed. The Chapter 3 titled ‘in vivo effects upon plp-1 knockdown in C. elegans’ follows the PLP-1 identification chapter. In this section, the consequent in vivo effects upon plp-1 knockdown is depicted. Effects on worms, both phenotypic and developmental are accounted for briefly. Next, effect on endogenous miRNAs (precursor and mature forms), and their cognate targets is assessed. This is followed by illustrating effects on other major players of miRNA metabolism namely miRNA AGO, and miRNases: XRN-1 and XRN-2, in PLP-1 depleted condition. The Chapter 4 titled ‘Functional characterization of PLP-1’, encompasses the brunt of the work, where through in vitro assays, three major functional aspects of the protein PLP-1: miRNA binding, miRNA release, and delivery of miRNA to bona fide miRNA turnover machinery, are elucidated. While demonstrating the miRNA binding property of the protein, an intriguing observation led to the revelation that the protein possess an exceptional oligomerization property. Together with biochemical assays and mutational studies, the importance of the different regions and residues of the protein, for individual functions is depicted. This chapter ends with summarising the independence or inter-dependence of the different functionalities of the protein. The thesis is concluded with a combined Discussion and Future Perspective section. This section gives a holistic picture of the work with respect to the existing knowledge on miRNA metabolism, and how the current study enhances our understanding of this process leading to the next level of perception. The following chapter contains the Materials and Methods, used in the present study. In this chapter, a comprehensive description of all the methodologies employed in the experiments is provided. References for already established methodologies are indicated and incorporated. As an addendum Appendix is included where detailed information on the different clones used in the study is provided. Also, figures corresponding to some of the data not shown in the previous version of the thesis is incorporated in this section. Finally, the closure is done with the Bibliography section.
UGC

Indiana University-Purdue University Indianapolis (IUPUI)
Gene expression is a highly coordinated process that relies upon appropriate regulation of translation for protein homeostasis. Regulation of protein synthesis largely occurs at the initiation step in which the translational start site is selected by ribosomes and associated initiating factors. In addition to the coding sequences (CDS) for protein products, short upstream open reading frames (uORFs) located in the 5’-leader of mRNAs are selected for translation initiation. While uORFs are largely considered to be inhibitory to translation at the downstream CDS, uORFs can also promote initiation of CDS translation in response to environmental stresses. Multiple transcripts associated with stress adaptation are preferentially translated through uORF-mediated mechanisms during activation of the Integrated Stress Response (ISR). In the ISR, phosphorylation of α subunit of the translation initiation factor eIF2α (eIF2α~P) during environmental stresses results in a global reduction in protein synthesis that functions to conserve energy and nutrient resources and facilitate reprogramming of gene expression. Many key regulators of the ISR network are subject to preferential translation in the response to eIF2α-P. These preferentially translated genes include the pro-apoptotic transcriptional activator Chop that modifies gene expression programs, feedback regulator Gadd34 that targets the catalytic subunit of protein phosphatase 1 to dephosphorylate eIF2α~P, and glutamyl-prolyl tRNA synthetase Eprs that increases the charged tRNA pool and primes the cell for resumption of protein synthesis after stress remediation. Ribosome bypass of at least one inhibitory uORF is a common theme between Chop, Gadd34, and Eprs, which allows for their regulated expression in response to cellular stress. However, different features encoded within the uORFs of the Chop, Gadd34, and Eprs mRNAs provide for regulation of their inhibitory functions, illustrating the complexities of uORF-mediated regulation of gene-specific translation. Importantly, preferentially translated ISR targets can also be transcriptionally regulated in response to cellular stress and misregulation of transcriptional or translational expression of Gadd34 can elicit maladaptive cell responses that contribute to disease. These mechanisms of translation control are conserved throughout species, emphasizing the importance of translation control in appropriate gene expression and the maintenance of protein homeostasis and health in diverse cellular conditions.

The chb operon constitutes the genes essential for utilization of chitooligosaccharides in Escherichia coli and related species. The six genes of the operon code for a transcriptional regulator (ChbR) of the operon, a permease (ChbBCA), a monodeacetylase (ChbG), and a phospho-beta-glucosidase (ChbF). In the absence of the substrate, the operon is maintained in a transcriptionally repressed state, while presence of the substrate leads to transcriptional activation. Regulation of the chb operon is brought about by the concerted action of three proteins, the negative regulator NagC coded by the nag operon, the dual function regulator ChbR coded by the chb operon and the universal regulatory protein CRP. Mutations that lead to alterations in the regulation of the operon can facilitate utilization of cellobiose, in addition to chitooligosaccharides by E. coli. The studies presented in Chapter II were aimed at understanding the evolution of cellobiose utilization in Shigella sonnei, which is phylogenetically very close to E. coli. Cel+ mutants were isolated from a Cel- wild type S. sonnei strain. Interestingly, Cel+ mutants arose relatively faster on MacConkey cellobiose agar from the S. sonnei wild type strain compared to E. coli. Similar to E. coli, the Cel+ phenotype in S. sonnei mutants was linked to the chb operon. Deletion of the phospho-β-glucosidase gene, chbF also resulted in loss of the Cel+ phenotype, indicating that ChbF is responsible for hydrolysis of cellobiose in these mutants. Previous work from the lab has shown that acquisition of two classes of mutations is necessary and sufficient to give rise to Cel+ mutants in E. coli. The first class of mutations either within the nagC locus or at the NagC binding site within the chb promoter, lead to NagC derepression. The second class consisting of gain-of-function mutations in chbR enable the recognition of cellobiose as an inducer by ChbR and subsequent activation of the operon. However, in S. sonnei a single mutational event of an IS element insertion resulted in acquisition of this phenotype. Depending on the type and location of the insertion, the mutants were grouped as Type I, and Type II. In Type I mutants an 1S600 insertion between the inherent -10 and -35 elements within the chb promoter leads to ChbR-independent constitutive activation of the operon, while in Type II mutants, an IS2/600 insertion at -113/-114, leads to ChbR-dependent, cellobiose-inducible expression of the operon. The results presented also indicate that in addition to relieving NagC mediated repression, the insertion in Type II mutants also leads to increase in basal transcription from the chb promoter. Constitutive expression of the chb operon also results in utilization of the aromatic β-glucosides salicin and arbutin, in addition to cellobiose in Type I mutants, which indicates the promiscuous nature of permease and hydrolysis enzyme of the chb operon. This part of the thesis essentially demonstrates the different trajectories taken for the evolution of new metabolic function under conditions of nutrient stress by two closely related species. It emphasizes the significance of the strain background, namely the diversity of transposable elements in the acquisition of the novel function. The second part of this research investigation, detailed in Chapter III deals with experiments to characterize the eukaryotic orthologs of the last gene of the chb operon. The chbG gene of E. coli codes for a monodeacetylase of chitooligosaccharides like chitobiose and chitotriose. The protein belongs to a highly conserved, but less explored family of proteins called YdjC, whose orthologs are present in many prokaryotes and eukaryotes including mammals. The human YDJC locus located on chromosome 22 is linked to a variety of inflammatory diseases and the transcript levels are relatively high in stem cells and a few cancer cells. In silico analysis suggested that the mammalian YdjC orthologs possess sequence and structural similarity with the prokaryotic counterpart. The full length mouse YdjC ortholog, which is 85% identical to the human ortholog was cloned into a bacterial vector and expressed in a chbG deletion strain of E. coli. The mouse YdjC ortholog could neither promote growth of the strain on chitobiose nor induce transcription from the chb promoter. The purified mouse YdjC ortholog could not deacetylate chitobiose in vitro as well, suggesting that the mouse ortholog failed to complement the function of the E. coli counterpart, ChbG under the conditions tested in this study. In order to characterize the mammalian YdjC orthologs more elaborately, further experimentation was performed in mammalian cell lines. The results indicate that YdjC is expressed in mammalian cell lines of different tissue origin and the expression was seen throughout the cell. Overexpression of mouse Ydjc in a few mammalian cells also resulted in increased proliferation and migration, indicating a direct or indirect role of this protein in cell growth/proliferation. The mammalian orthologs of ChbG therefore appear to have related but distinct activities and substrates compared to the bacterial counterpart that need to be elucidated further.

The chb operon constitutes the genes essential for utilization of chitooligosaccharides in Escherichia coli and related species. The six genes of the operon code for a transcriptional regulator (ChbR) of the operon, a permease (ChbBCA), a monodeacetylase (ChbG), and a phospho-beta-glucosidase (ChbF). In the absence of the substrate, the operon is maintained in a transcriptionally repressed state, while presence of the substrate leads to transcriptional activation. Regulation of the chb operon is brought about by the concerted action of three proteins, the negative regulator NagC coded by the nag operon, the dual function regulator ChbR coded by the chb operon and the universal regulatory protein CRP. Mutations that lead to alterations in the regulation of the operon can facilitate utilization of cellobiose, in addition to chitooligosaccharides by E. coli. The studies presented in Chapter II were aimed at understanding the evolution of cellobiose utilization in Shigella sonnei, which is phylogenetically very close to E. coli. Cel+ mutants were isolated from a Cel- wild type S. sonnei strain. Interestingly, Cel+ mutants arose relatively faster on MacConkey cellobiose agar from the S. sonnei wild type strain compared to E. coli. Similar to E. coli, the Cel+ phenotype in S. sonnei mutants was linked to the chb operon. Deletion of the phospho-β-glucosidase gene, chbF also resulted in loss of the Cel+ phenotype, indicating that ChbF is responsible for hydrolysis of cellobiose in these mutants. Previous work from the lab has shown that acquisition of two classes of mutations is necessary and sufficient to give rise to Cel+ mutants in E. coli. The first class of mutations either within the nagC locus or at the NagC binding site within the chb promoter, lead to NagC derepression. The second class consisting of gain-of-function mutations in chbR enable the recognition of cellobiose as an inducer by ChbR and subsequent activation of the operon. However, in S. sonnei a single mutational event of an IS element insertion resulted in acquisition of this phenotype. Depending on the type and location of the insertion, the mutants were grouped as Type I, and Type II. In Type I mutants an 1S600 insertion between the inherent -10 and -35 elements within the chb promoter leads to ChbR-independent constitutive activation of the operon, while in Type II mutants, an IS2/600 insertion at -113/-114, leads to ChbR-dependent, cellobiose-inducible expression of the operon. The results presented also indicate that in addition to relieving NagC mediated repression, the insertion in Type II mutants also leads to increase in basal transcription from the chb promoter. Constitutive expression of the chb operon also results in utilization of the aromatic β-glucosides salicin and arbutin, in addition to cellobiose in Type I mutants, which indicates the promiscuous nature of permease and hydrolysis enzyme of the chb operon. This part of the thesis essentially demonstrates the different trajectories taken for the evolution of new metabolic function under conditions of nutrient stress by two closely related species. It emphasizes the significance of the strain background, namely the diversity of transposable elements in the acquisition of the novel function. The second part of this research investigation, detailed in Chapter III deals with experiments to characterize the eukaryotic orthologs of the last gene of the chb operon. The chbG gene of E. coli codes for a monodeacetylase of chitooligosaccharides like chitobiose and chitotriose. The protein belongs to a highly conserved, but less explored family of proteins called YdjC, whose orthologs are present in many prokaryotes and eukaryotes including mammals. The human YDJC locus located on chromosome 22 is linked to a variety of inflammatory diseases and the transcript levels are relatively high in stem cells and a few cancer cells. In silico analysis suggested that the mammalian YdjC orthologs possess sequence and structural similarity with the prokaryotic counterpart. The full length mouse YdjC ortholog, which is 85% identical to the human ortholog was cloned into a bacterial vector and expressed in a chbG deletion strain of E. coli. The mouse YdjC ortholog could neither promote growth of the strain on chitobiose nor induce transcription from the chb promoter. The purified mouse YdjC ortholog could not deacetylate chitobiose in vitro as well, suggesting that the mouse ortholog failed to complement the function of the E. coli counterpart, ChbG under the conditions tested in this study. In order to characterize the mammalian YdjC orthologs more elaborately, further experimentation was performed in mammalian cell lines. The results indicate that YdjC is expressed in mammalian cell lines of different tissue origin and the expression was seen throughout the cell. Overexpression of mouse Ydjc in a few mammalian cells also resulted in increased proliferation and migration, indicating a direct or indirect role of this protein in cell growth/proliferation. The mammalian orthologs of ChbG therefore appear to have related but distinct activities and substrates compared to the bacterial counterpart that need to be elucidated further.

Indiana University-Purdue University Indianapolis (IUPUI)
In response to different environmental stresses, phosphorylation of eIF2 (eIF2P) represses global translation coincident with preferential translation of ATF4. ATF4 is a transcriptional activator of the integrated stress response, a program of gene expression involved in metabolism, nutrient uptake, anti-oxidation, and the activation of additional transcription factors, such as CHOP/GADD153, that can induce apoptosis. Although eIF2P elicits translational control in response to many different stress arrangements, there are selected stresses, such as exposure to UV irradiation, that do not increase ATF4 expression despite robust eIF2P. In this study we addressed the underlying mechanism for variable expression of ATF4 in response to eIF2P during different stress conditions and the biological significance of omission of enhanced ATF4 function. We show that in addition to translational control, ATF4 expression is subject to transcriptional regulation. Stress conditions such as endoplasmic reticulum stress induce both transcription and translation of ATF4, which together enhance expression of ATF4 and its target genes in response to eIF2P. By contrast, UV irradiation represses ATF4 transcription, which diminishes ATF4 mRNA available for translation during eIF2∼P. eIF2P enhances cell survival in response to UV irradiation. However, forced expression of ATF4 and its target gene CHOP leads to increased sensitivity to UV irradiation. In this study, we also show that C/EBPβ is a transcriptional repressor of ATF4 during UV stress. C/EBPβ binds to critical elements in the ATF4 promoter resulting in its transcriptional repression. The LIP isoform of C/EBPβ, but not the LAP version is regulated following UV exposure and directly represses ATF4 transcription. Loss of the LIP isoform results in increased ATF4 mRNA levels in response to UV irradiation, and subsequent recovery of ATF4 translation, leading to enhanced expression of its target genes. Together these results illustrate how eIF2P and translational control, combined with transcription factors regulated by alternative signaling pathways, can direct programs of gene expression that are specifically tailored to each environmental stress.

Indiana University-Purdue University Indianapolis (IUPUI)
In response to different environmental stresses, phosphorylation of eukaryotic initiation factor-2 (eIF2) rapidly reduces protein synthesis, which lowers energy expenditure and facilitates reprogramming of gene expression to remediate stress damage. Central to the changes in gene expression, eIF2 phosphorylation also enhances translation of ATF4, a transcriptional activator of genes subject to the Integrated Stress Response (ISR). The ISR increases the expression of genes important for alleviating stress, or alternatively triggering apoptosis. One ISR target gene encodes the transcriptional regulator CHOP whose accumulation is critical for stress-induced apoptosis. In this dissertation research, I show that eIF2 phosphorylation induces preferential translation of CHOP by a mechanism involving a single upstream ORF (uORF) located in the 5’-leader of the CHOP mRNA. In the absence of stress and low eIF2 phosphorylation, translation of the uORF serves as a barrier that prevents translation of the downstream CHOP coding region. Enhanced eIF2 phosphorylation during stress facilitates ribosome bypass of the uORF, and instead results in the translation of CHOP. Stable cell lines were also constructed that express CHOP transcript containing the wild type uORF or deleted for the uORF and each were analyzed for expression changes in response to the different stress conditions. Increased CHOP levels due to the absence of inhibitory uORF sensitized the cells to stress-induced apoptosis when compared to the cells that express CHOP mRNA containing the wild type uORF. This new mechanism of translational control explains how expression of CHOP and the fate of cells are tightly linked to the levels of phosphorylated eIF2 and stress damage.

碩士
國防醫學院
生物化學研究所
86
Thymidine kinase, a crucial enzyme in the salvage pathway of thymidine triphosphate formation, is involved in the process of DNA replication. The level of thymidine kinase(TK) is known to be increased at the G1/S phase of the cell cycle. Transcriptional activation is important for the increase of human thymidine kinase(hTK) expression at the G1/S phase of the cell cycle . In this study, in order to examine the DNA sequence responsible for the transcriptional activation of hTK gene in young cells during growth stimulation and transcriptional repression in old ones, the interaction between nuclear protein factors and hTK promoter was examined by in vivo footprinting in young and old IMR-90 cells, respectively. Results from in vivo footprinting of hTK promoter in young and old IMR-90 cells showed that NF-Y binding sites could be one of key cis-elements responsible for transcriptional activation of hTK gene. In addition, in vivo footprints of old IMR- 90 cells on the DNA sequence between two adjacent NF-Y binding sites, CCAAT boxes, were apparently different from those of young cells. Thus, CCAAT-binding proteins might be associated ageing. On the other hand, in order to understand the molecular basis underlying the E1A-mediated transcriptional repression of the c-erbB-2 overexpression in human cancer cells, we compared the nuclear factor-binding patterns of c-erbB-2 promoter in vivo in cell lines with and without exogenous E1A expression. However, we did not observe any difference of nuclear factor-binding in vivo of E1A-non-expressing cells versus E1A-expressing cells. Therefore, E1A might repress the transcriptional overexpression of c-erbB-2 gene through protein-protein interactions with some endogenous factors. Furthermore, in order to show the correlation between the distribution of Sp1 elements and the demethylation patterns in the CpG islands of the human globin gene, we have analyzed the methylation patterns of human globin promoter and its interaction with Sp1 factor by genomic sequencing and in vivo footprinting, respectively. HeLa cells with high passage number didn't show significant protein- DNA interactions at Sp1 binding sites within the human globin gene as K562 cells did and its genomic DNA was highly methylated downstream of the cap site (+1). This indicated that Sp1 sites in human adult globin gene promoters might correlate to demethylation of erythroid-specific CpG islands.