Дисертації з теми "Xeric"
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Lawless, Patrick Joseph. "XERIC LIMESTONE PRAIRIES OF EASTERN UNITED STATES." View online, 2005. http://lib.uky.edu/ETD/ukybiol2005d00321/Lawless.pdf.
Повний текст джерелаDayton, Gage Hart. "Community assembly of xeric-adapted anurans at multiple spatial scales." Texas A&M University, 2005. http://hdl.handle.net/1969.1/3296.
Повний текст джерелаKnott, Edward Joseph. "The effect of elephants (Loxodonta africana, Blumenbach, 1797) on Xeric Succulent Thicket." Thesis, Rhodes University, 2007. http://hdl.handle.net/10962/d1005359.
Повний текст джерелаFabricius, Christo 1956. "The impact of land use on biodiversity in xeric succulent thicket, South Africa." Doctoral thesis, University of Cape Town, 1997. http://hdl.handle.net/11427/9512.
Повний текст джерелаHuman-induced land management practices are key factors which influence the dynamics of landscapes, land elements and biotic communities in Xeric Succulent Thicket, a notoriously fragile and low resilience ecosystem in the Eastern Cape, South Africa. In its natural state this vegetation type is characterized by a continuous mosaic of clumps of diverse woody shrubs in a matrix of grass, hardy dwarf shrubs and bare ground. When indigenous herbivores are replaced by domestic livestock at unsustainable stocking rates, the ratio of clump to interclump areas decreases. This has impacts on biodiversity at the landscape level, and affects the composition and species richness of plant, arthropod and reptile communities and fungal symbionts. The study has two main objectives: 1) to detennine the local-level effects of different types of land use on biodiversity in Xeric Succulent Thicket; and 2) to better understand the factors which affect biodiversity in different taxonomic groups and at different spatial scales, so that the preservation function of protected areas could be enhanced. The key questions which are addressed in the thesis are what does 'biodiversity' mean, and what are its different dimensions in Xeric Succulent Thicket; what roles do protected areas and other lands play in preserving biodiversity, how are different taxonomic groups affected by different types of land use; and how can biodiversity be monitored and measured? . The regional 'conservation landscape' is seen as the protected area plus the land immediately adjoining it, and is viewed as an integrated mosaic of anthropogenic landscapes (land management units), land elements (patches within landscapes). producer communities (assemblages of interacting plant species), consumer communities (assemblages of interacting herbivores and predators) and soil microorganisms. From a process point of view, the research relates the intensity of disturbance, mainly as a result of herbivory, to landscape complexity, the structure of land elements, and the species richness of producers and consumers within land elements. The patterns which emerge are interpreted in the context of ecosystem functioning, from the point of view of a practicing conservation biologist.
Wenk, Evelyn Susannah. "Effects of vegetation structure on fire behavior and wiregrass seedling establishment in xeric sandhills." Connect to this title online, 2009. http://etd.lib.clemson.edu/documents/1252423837/.
Повний текст джерелаHoppert, Michael, Rudolph Reimer, Anne Kemmling, Annekatrin Schröder, Bettina Günzl, and Thilo Heinken. "Structure and reactivity of a biological soil crust from a xeric sandy soil in Central Europe." Universität Potsdam, 2004. http://opus.kobv.de/ubp/volltexte/2005/587/.
Повний текст джерелаLegras, Elaine C. "Microhabitat is critical for sugar pine seedlings emergence and survival in a xeric Jeffrey pine dominated forest /." abstract and full text PDF (UNR users only), 2008. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1455660.
Повний текст джерела"May, 2008." Includes bibliographical references (leaves 39-44). Library also has microfilm. Ann Arbor, Mich. : ProQuest Information and Learning Company, [2009]. 1 microfilm reel ; 35 mm. Online version available on the World Wide Web.
Seymour, Colleen. "The influence of size and density of the Camelthorn (Acacia erioloba Meyer) on its keystone role in the Xeric Kalahari." Doctoral thesis, University of Cape Town, 2006. http://hdl.handle.net/11427/12408.
Повний текст джерелаWithin the savannas of the southern Kalahari, land use practices can change habitat structure profoundly, so that rangelands either became densely wooden through bush thickening or are cleared completely using arboricides or manual tree felling and bush clearance, or are at some stage in between. Demands for the wood of camelthorn trees (Acacia reioloba), (the largest tree species growing on semi-arid and arid Kalahari sands) for firewood has also impacted habitat structure. Large trees are important in the Kalahari ecosystem because they provide shade, nesting and foraging sites for birds and mammals, as well as microhabitats that facilitate the existence of a suite of subcanopy plants. Indeed, large A. erioloba tress have been mooted as a context-depend keystones species, important to other biota and ecological prcesess. The primary aim of this study was to investigate the importance of A. erioloba to biodiversity, focusing on plants and birds.
Seine, Rüdiger. "Vegetation von Inselbergen in Zimbabwe : Struktur, Diversität und ökogeographische Differenzierung einer tropischen Lebensgemeinschaft /." Wiehl : M. Galunder-Verlag, 1996. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=008418634&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
Повний текст джерелаBeiler, Kevin Jon. "The complex socio-spatial architecture of Rhizopogon spp. mycorrhizal networks in xeric and mesic old-growth interior Douglas-fir forest plots." Thesis, University of British Columbia, 2011. http://hdl.handle.net/2429/40000.
Повний текст джерелаLa, Cock Graeme Dennis. "The conservation status of subtropical transitional thicket, and regeneration through seeding of shrubs in the Xeric succulent thicket of the Eastern Cape." Thesis, Rhodes University, 1992. http://hdl.handle.net/10962/d1003777.
Повний текст джерелаZorzi, Vinicius Gaburro de. "Endemismo e conservação de refúgios xéricos pleistocênicos da Serra do Itapetinga." Universidade de São Paulo, 2016. http://www.teses.usp.br/teses/disponiveis/41/41134/tde-13122016-120248/.
Повний текст джерелаIn the mountain landscapes of Brazilian Southeast, outcrops report to the observer the geological details of a vegetation whose its local existency and distribution is related to characteristics such as soil depth and effects of the topography. Serra do Itapetinga is an elevation located in the north of the city of São Paulo, where over a hundred of corps belonging to the Neoproterozoic spring out and contain persistent Pleistocene vegetation groups as part of the “granite cave formations”. This vegetation finds itself sheltered in sites in whose soil and climate conditions provide great biological diversity, besides endemisms and poikilohydry species. These soil and microclimate nuances allow the formation of microhabitats equipped with interesting peculiarities and specifc groups of plants. Among them, the soil islands compose vegetal formations cut off by a rock matrix that might be used as a model of ecological islands when studying communities. The forms that lives prevailing in the rupestrian fields of Brazilian southeast are determined by hemichryptophyte and chamaephyte plants. The proportion of phanerophyte varies according to the existence of microhabitats that propitiate accumulation of sediments, besides abruptly rise in the rock perimeter. The importance of therophyte is reduced in comparison with all the other forms, normally referring to species with ruderal behavior. Besides its smaller presence in the outcrops of Brazilian southeast, geophyte plants, after the hemichryptophyte, represent the most frequent form of life in the soil islands - the most characteristic microhabitats of the rupestral flora. The recent disappearance of this vegetation form is strictly related to the increase of the search of these locations for observation of nature, adventure sports and harvest of native plants, where the fire use is common for clearing lands. This study aimed to determinate the vegetation structures and composition on the outcrops of Itapetinga, followed by the analysis of the disturbance sources on ecosystem to identify the evidences of impact on the ancestry vegetation communities. Ten different outcrops were studied within its relation with the local human interference in other to identify evidences of impact. The more representative groups were Asteraceae, Poaceae, Orchidaceae and Myrtaceae. The study showed the expected proportions for the Raunkier classes which supports characterization of an essentially rural physiognomy. The analysis showed the prevalence of negative impact and significant touristic, sportive and infrastructure activities. The outcrops that concentrate rare and threatened richness are the ones frequented by the public in what way conservation strategies intervene changes in the rock use and its surroundings
Pomp, Jonathan A. "Disturbances, prescribed fire, and invasion by exotic plants in a xeric mixed-oak and oak-pine dominated area of the Ridge and Valley in eastern West Virginia." Morgantown, W. Va. : [West Virginia University Libraries], 2008. https://eidr.wvu.edu/etd/documentdata.eTD?documentid=5709.
Повний текст джерелаTitle from document title page. Document formatted into pages; contains ix, 173 p. : ill. (some col.), col. maps. Includes abstract. Includes bibliographical references.
Lieske, Kathrin [Verfasser]. "Vegetative reproduction and clonal diversity in pleurocarpous mosses (Bryopsida) of xeric habitats : a combined molecular and morpho-anatomical study in the three mosses Abietinella abietina (Hedw.) Fleisch. (Thuidiaceae), Homalothecium lutescens (Hedw.) Robins. (Brachytheciaceae) and Homalothecium sericeum (Hedw.) Schimp. (Brachytheciaceae) / Kathrin Lieske." Berlin : Freie Universität Berlin, 2010. http://d-nb.info/1024744086/34.
Повний текст джерелаNeilson, Liam Michael. "Xer site-specific recombination at dif by Haemophilus influenzae XerC." Thesis, University of Oxford, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.299484.
Повний текст джерелаFerreira, Henrique. "Dissecting the roles of XerC and XerD in Xer site-specific recombination." Thesis, University of Oxford, 2002. http://ora.ox.ac.uk/objects/uuid:1e2ed560-ab23-43b7-b095-c18ebb0a8bb9.
Повний текст джерелаRobertson, Malcolm. "Investigation of the effects of accessory factors on HJ resolution by XerC and XerD." Thesis, University of Oxford, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.414223.
Повний текст джерелаScola, Vincent J. "Desert soil microbial communities across a xeric stress gradient." Diss., 2016. http://hdl.handle.net/2263/57286.
Повний текст джерелаDissertation (MSc)--University of Pretoria, 2016.
tm2016
Microbiology and Plant Pathology
MSc
Unrestricted
Qureshi, Maqsood Hassan. "Nitrogen available to winter wheat as influenced by previous crop in a moist xeric environment." Thesis, 1999. http://hdl.handle.net/1957/33521.
Повний текст джерелаGraduation date: 1999
Almeida, Carolina Cristiano de. "Structural and functional diversity of the diazotrophic community in xeric ecosystems: response to nitrogen availability." Master's thesis, 2019. http://hdl.handle.net/10451/41864.
Повний текст джерелаThe effects of increased nitrogen (N) input in the ecosystems have brought concern for some time, however, the long-term consequences of this input to Mediterranean ecosystems are poorly studied. The Mediterranean ecosystem is considered a hotspot of biodiversity; the Mediterranean basin was considered one of the places in the world with most biodiversity. Taking in consideration that these ecosystems are regarded as environments under N limitation, changes in N availability will alter the relationships between the organisms. The research presented here was carried out on the Mediterranean basin and it is part of an ongoing study over the last 10 years, in which fields were fertilized with different quantities and forms of N. To understand the consequences of this fertilization, diazotrophic bacteria were isolated from rhizospheric soil of Ulex densus subjected to different N addition and characterized in terms of the structural and functional diversity of the communities, using this community as an ecological marker for comparing the different treatments. The methodology applied to understand the ecological consequences was the isolation of the rhizospheric bacteria in N free media and characterization of this collection of isolates, in terms of diversity (phenotypic and genotypic), morphophysiology (distribution of the isolates by different morphotypes), structure (identification of the genera associated with the treatments), functionality (ability to perform ammonification, nitrification and/or denitrification) and metabolic plasticity (ability to use different carbon sources). The N fixation of these isolates was confirmed by their growth in solid and liquid N free media and by isotopic characterization of the ratio of 15N over 14N; a search for the nifH gene was also performed. For this community there were no differences in diversity nor at the morphophysiological level between the different treatments. However, a higher number of isolates was detected in the treatments with higher NH4+ inputs (40A, and 80AN). The ability to perform N fixation was confirmed through multiple passages in solid and liquid N-free media and nifH gene was detected through PCR and dot-blot hybridization, though this was not possible for every representative isolate. Differences with genera associated with the form and quantity of N added to the soil were observed, with some genera only appearing in certain treatments. Every isolate presented more than one function in the N cycle and a decrease in nitrification together with an increase of denitrification was observed along with the increase of bioavailable N. Isolates belonging to genera detected exclusively in treatment 80AN presented higher N content and higher denitrification rates. The variation of the metabolic plasticity between the treatments increased with N fertilization, since the control treatment had the more consistent response for metabolic plasticity. The increase on the isolates number followed the same pattern as the proliferation of U. densus, showing that the potential for N fixation was not inhibited by NH4+ addition. The differences in genera associated with the treatments evidenced a change in the structure of the community. The results of multifunctionality showed a shift in the diazotrophic community, since nitrification and denitrification presented specificity towards different forms of fertilization. The higher variation of the metabolic plasticity associated with N fertilization can be related to changes in carbon source availability in the soil. From the work presented here and future work can arise a platform to direct alterations on the agriculture and ecological guidelines. These alterations are essential to protect this kind of ecosystem, which is extremely important due to the unique biodiversity associated with it.
Há algum tempo que os efeitos do aumento de nitrogénio (N) introduzido nos ecossistemas leva a preocupação, no entanto, as consequências a longo prazo desta introdução nos ecossistemas mediterrânicos estão pouco estudadas. O ecossistema mediterrânico é considerado um hotspot de biodiversidade; a bacia mediterrânica é um dos locais com mais biodiversidade no mundo. Tendo em consideração que estes ecossistemas são considerados ambientes com limitações ao nível do N, as mudanças na disponibilidade de N vão alterar as relações entre os organismos. O estudo aqui apresentado foi levado a cabo na bacia mediterrânica e faz parte de um estudo que já começou há 10 anos e continua a ser efetuado, em que os talhões foram fertilizados com diferentes formas e quantidades de N. De forma a entender as consequências desta fertilização, foi isolada a comunidade de bactérias diazotróficas a partir de solo rizosférico de Ulex densus sujeito a diferentes adições de N e foi caracterizada a diversidade estrutural e funcional da comunidade. A comunidade diazotrófica foi utilizada como um marcador ecológico para a comparação entre os diferentes tratamentos. Os microrganismos que conseguem realizar a fixação de N, ou seja, a conversão de N2 em NH3, são chamados de diazotróficos. Esta conversão ocorre através da ação da enzima nitrogenase, e existem três tipos desta enzima que variam no cofator associado: Mo, Va ou Fe. A estrutura da comunidade diazotrófica não apresentou diferenças entre os tratamentos em termos de diversidade, no entanto o índice de diversidade genotípico apresentou um poder de discriminação maior que o índice de diversidade fenotípico entre os isolados e, portanto, é essencial complementar os resultados fenotípicos com o perfil genotípico obtido por PCR fingerprinting. Foi detetado um número mais elevado de isolados diazotróficos heterotróficos nos tratamentos com maior adição de NH4+ (40A e 80AN), estes são os mesmos tratamentos em que existe proliferação de Ulex densus, mostrando assim que o potencial para a fixação de N não foi inibido pela adição desta forma de N. Foram observadas diferenças entre os géneros identificados dependendo da forma e quantidade de N adicionado ao solo, mostrando mudanças na estrutura da comunidade. Para esta identificação foram escolhidos 40 representantes, sendo 10 de cada tratamento, e foram considerados uma boa representação da coleção de isolados por não apresentarem valores de semelhança superiores a 55%. A maioria dos géneros que foram identificados estão associados com a fixação de N ou pelo menos apresentam um espécie pertencente ao género que apresenta a capacidade de realizar fixação de N. A capacidade de realizar fixação de N foi confirmada através de múltiplas passagens por meio sem N quer na forma líquida quer na forma sólida e o gene nifH foi detetado através da técnica de PCR e por hibridação dot-blot. Apesar de os isolados representantes terem sido identificados como sendo pertencentes a géneros associados com a capacidade de realizar a fixação de N e os pares de primers escolhidos serem capazes de amplificar o gene nifH dos clusters onde estes géneros estão incluídos, não foi possível amplificar e/ou detetar o gene nifH da maioria dos isolados. Isto pode significar que o gene nifH destes isolados é diferente das sequências utilizadas para desenhar os primers. Até há pouco tempo, era comum associar determinados grupos de microrganismos a uma biotransformação especifica do ciclo do N. Contudo ultimamente tem sido exposto que a maioria dos isolados tem potencial para realizar diversas biotransformações de N, nomeadamente combinações de fixação de N, amonificação, nitrificação e desnitrificação. Isto representa um fator extremamente importante, uma vez que a multifuncionalidade é essencial para a adaptação dos microrganismos ao meio ambiente. Os isolados representantes de cada um dos tratamentos foram testados para diferentes processos do ciclo do N: amonificação, nitrificação e desnitrificação. Todos os isolados apresentaram mais do que uma função no ciclo do N. Pode ser observada uma mudança na multifuncionalidade da comunidade diazotrófica devido ao aumento da biodisponibilidade de N. De facto, a capacidade de realizar nitrificação foi encontrada mais frequentemente entre os isolados representantes da comunidade de tratamentos que recebem mais baixas quantidade de NH4+ (tratamento controlo e 40AN), o que está de concordo com o que foi reportado em estudos anteriores, em que o aumento da disponibilidade de NH4+ leva a uma inibição na nitrificação. Foi encontrada capacidade desnitrificante em todos os tratamentos, no entanto os isolados do tratamento 80AN (recebe mais nitrato) apresentaram taxas de desnitrificação mais elevadas. Taxas de desnitrificação mais elevadas podem ser relevantes como mecanismo de desintoxicação de nitrato, uma vez que elevadas concentrações de nitrato podem inibir a fixação de N. A alteração da multifuncionalidade microbiana associada com biotransformações de N está associada a mudanças na composição da comunidade diazotrófica, sendo corroborado pelas diferenças nos géneros identificados nos diferentes tratamentos. Com base nos resultados apresentados, é sugerido que o isolado identificado como pertencente ao género Pedobacter apresente uma sensibilidade para concentrações elevadas de N, e por isso pode ser possível que a ausência deste género possa estar associada a um aumento da disponibilidade de N. Por esta razão, este género pode ser um candidato a indicador biológico das alterações no ecossistema. Não foi possível associar a multifuncionalidade dos isolados com a plasticidade metabólica. No entanto, quando foram analisados dois géneros comuns a diferentes tratamentos - Pseudomonas e Stenotrophomonas - foi possível observar diferenças na resposta à adição de N. Os isolados identificados como Pseudomonas apresentaram uma resposta uniforme ao longo dos diferentes tratamentos, correspondendo a uma elevada plasticidade metabólica. Contudo, no caso dos isolados identificados como Stenotrophomonas o mesmo já não foi observado, pois a plasticidade metabólica aumenta com o aumento da disponibilidade de N. Isto pode dever-se à adaptação destes microrganismos ao aumento da disponibilidade de N ou os isolados podem pertencer a espécies distintas com diferente plasticidade metabólica. Apesar de ser possível identificar algumas diferenças entre os tratamentos em termos das propriedades funcionais da comunidade, estas diferenças não são tão evidentes como seria de esperar, de acordo com a literatura, e não corroboram completamente a teoria de que a comunidade diazotrófica devia ser afetada pela adição de N. Estas discrepâncias podem dever-se ao ecossistema que está a ser estudado, o ecossistema mediterrânico, que apresenta características distintas dos outros ecossistemas estudados em trabalhos semelhantes, e por isso os efeitos da adição de N podem não ser os mesmos. Por último, a comunidade escolhida neste trabalho como indicador das respostas do ecossistema, a comunidade diazotrófica, pode não ter sido a escolha certa, uma vez que não se verificam grandes alterações estruturais com o aumento da biodisponibilidade de N. A comunidade estudada foi isolada a partir do solo rizosférico de Ulex densus que é uma espécie que prolifera nos tratamentos com maior disponibilidade de N. Desta forma a planta pode estar a contrabalançar os efeitos da adição de N levando a que não haja diferenças na comunidade diazotrófica entre os tratamentos. Pode ainda ser o facto de a comunidade rizosférica conseguir aguentar as pressões ecológicas e manter este grupo de microrganismos, uma vez que estes apresentam uma função importante que pode ser benéfica para a comunidade rizosférica quando em condições de N limitante. O trabalho apresentado neste estudo, e trabalho futuro que possa advir dele, pode criar uma plataforma para alterações nas políticas agrícolas e ambientais, que são necessárias para a preservação deste tipo de ecossistemas que apresentam uma biodiversidade única.
Pipkin, Ashley. "The Influence of Fire and Other Disturbance on Ericaceous Shrubs in Xeric Pine-Oak Forests of the Appalachian Mountains." Thesis, 2011. http://hdl.handle.net/1969.1/ETD-TAMU-2011-05-9450.
Повний текст джерелаTshikae, Balatlhane Power. "Local and regional factors influencing dung beetle assemblage structure across an environmental gradient in Botswana." Thesis, 2011. http://hdl.handle.net/2263/28856.
Повний текст джерелаThesis (PhD)--University of Pretoria, 2011.
Zoology and Entomology
unrestricted
Villion, Manuela. "Caractérisation des recombinases XerC et XerD de Proteus mirabilis." Thèse, 2005. http://hdl.handle.net/1866/15137.
Повний текст джерелаJia, Fuli. "Cloning and characterization of xerC gene of Streptococcus suis." Thèse, 2005. http://hdl.handle.net/1866/15151.
Повний текст джерелаLin, Chih-kai, and 林智凱. "Phonological Changes of Entering Tone in Sino-Xenic Languages: An Optimality-Theoretic Approach." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/69941710971969837694.
Повний текст джерела國立臺灣大學
語言學研究所
96
This thesis investigates the phonological change of entering tone in Sino-Xenic languages based on Optimality Theory. The research issues include (1) How is Optimality Theory applied to historical linguistics on the basis of Sino-Xenic languages? (2) How is Opacity in historical linguistics represented in Optimality Theory by means of Sino-Xenic languages? (3) How does internal syllable structure affect phonological change in Sino-Xenic languages? In this study, the data of entering tone are collected according to Fāngyán Diàochá Zìbiăo (A wordlist for dialectal research). Specifically, the layers, which are pertinent to each other, from three Sino-Xenic languages, Literal Reading (Wéndú) in Southern Min and Kan-on in Sino-Japanese and Modern Sino-Korean, are compared. In addition, instead of the traditional method, Shè, this study concentrates on vowel changes by dividing vowels into [a]-vowel group, non-[a]-vowel group and Hékŏu. First, to investigate the compatibility of historical phonology and modern phonological theories, this study adopts Faithfulness, Markedness, Dispersion-Optimality Theory (FMD-OT) (Sanders 2003), paying specific attention to vowel changes. The results show that Optimality Theory can account for the historical change of entering tone in Sino-Xenic languages and FMD-OT succinctly explains vowel changes in terms of dispersion constraints. Furthermore, this study amends Sanders’ FMD-OT by proposing a new set of constraints for central vowel. Second, this study also discusses opacity in historical phonology. To solve this problem, this study modifies and extends Candidate Chains (McCarthy 2007). It is proposed historical sound change is continual, each stage is evaluated parallelly, and new constraints are activated and reranked with previous constraints. This proposal is supported by Gĕngshè (梗攝) in Southern Min, Division III of Shēngshè (深攝) and Xiánshè (咸攝) in Sino-Japanese and Gĕngshè (梗攝) in Sino-Korean. This study finally explores the influence of syllable structure on sound changes in Sino-Xenic languages. Two factors are suggested, Phonotactic Constraints and Licensing Constraints. When the moras belong to the same branch in syllable node, there is no interaction of the segments, such as Southern Min. On the contrary, when the moras locate in different branches, the segments interact, such as Sino-Japanese and Sino-Korean. What distinguishes Sino-Japanese from Sino-Korean is whether mora is prominent or not. On one hand, if the mora is prominent, the mora is maintained, for example, Sino-Japanese. On the other hand, if the mora is not prominent, the mora is likely to be deleted, i.e. Sino-Korean, when the interaction takes place.
Lin, Chih-kai. "Phonological Changes of Entering Tone in Sino-Xenic Languages: An Optimality-Theoretic Approach." 2008. http://www.cetd.com.tw/ec/thesisdetail.aspx?etdun=U0001-1407200818541300.
Повний текст джерелаLiu, Hua. "The role of Caulobacter crescentus XerC and XerD recombinases in site-specific recombination." Thèse, 2010. http://hdl.handle.net/1866/6856.
Повний текст джерелаIn most bacteria, the chromosomal dimer resolution process is mediated by two tyrosine recombinases, XerC and XerD, which bind cooperatively and perform the recombination reaction at the dif site near the terminus of replication. This reaction also requires the C-terminal domain of the cell division protein FtsK. Caulobacter crescentus is an aquatic Gram-negative bacterium found in various environments. This bacterium has an asymmetric cell cycle which can be used to synchronize cell growth in order to study the temporal expression of a gene and the interconnection between the cell cycle and development. The binding activity and the formation of phosphotyrosyl complex of the C. crescentus recombinases, ccXerC and ccXerD, were tested on the C. crescentus dif (ccdif) site. Both ccXerC and ccXerD bound preferentially to the left half-site of ccdif and showed reduced cooperative binding, unlike what was found with the E. coli dif site. Covalent complex formation activity was tested by using fluorescently labelled linear “nicked suicide substrates” and labelled proteins. Heat and SDS-resistant protein-DNA complexes were formed when both wild-type ccXerC and ccXerD reacted with ccdif but not in the presence of active-site tyrosine mutant proteins. Phosphotyrosine complexes formed on the top-nicked suicide substrate were found to be more efficient than on the bottom-nicked suicide substrates and surprisingly ccXerC remained bound to both top and bottom-nicked ccdif suicide substrates.
Boer, Stephanie Charlotte. "Understanding chromosome dimer resolution systems of pathogenic bacteria at a molecular level." Thesis, 2019. http://hdl.handle.net/1959.13/1407912.
Повний текст джерелаAlthough bacteria appear to be simple organisms when compared to eukaryotes, the systems by which they undergo chromosome replication and cell division are exceedingly complex. Due to the circular nature of many bacterial chromosomes, an odd number of crossing-over events that can result from homologous recombination lead to the formation of covalently linked chromosomes, known as “chromosome dimers”. These dimers need to be resolved before cell division can occur. Bacteria undergo this resolution of chromosome dimers via site-specific recombination. In Escherichia coli, Pseudomonas aeruginosa, Vibrio cholerae and other bacteria, the action of FtsK, XerC and XerD produces two pairs of strand-exchange reactions which resolve chromosome dimers, thus allowing cell division to proceed. Some bacteria (e.g. Lactococcus lactis) and most archaea encode only a single Xer recombinase that does both strand exchanges. FtsK interacts with specific sequences that orient the protein toward “dif” sites, positioning the chromosome so XerC/ XerD can undertake the recombination reaction. FtsK also has a role during cell division in which it pumps DNA, at an extraordinary rate, to hasten cytokinesis and chromosome segregation. The N-terminus of FtsK is primarily responsible for localisation of the protein during cell division and assembly of cell-division machinery, whilst the C-terminus forms the motor domain, responsible for translocase activity and interactions with XerD. The work conducted herein, sought to confirm structural and functional information about FtsK from E. coli and P. aeruginosa. Functionally, it was important to identify the conserved nature of the protein by performing in vitro recombination experiments across both species. These recombination assays were used in downstream FtsK inhibition experiments where a suite of inhibitory compounds was designed to target its ATPase activity in vitro and in vivo. As part of structural studies, mutagenesis of the amino acids in the E. coli FtsKγ – XerD interaction was completed, elucidating important interactions between these two protein subunits. The final stage of experimentation was dedicated to crystallography studies, aimed at identifying a high-resolution structure of FtsK in its active state. The results of the mutagenesis studies were published in October 2016 in a paper titled “Activation of Xer-recombination at dif: structural basis of the FtsKγ-XerD interaction.” This paper featured structural information of FtsKγ and XerD and how mutations in this interaction can affect its recombination effect in vitro and in vivo.
Duplancic, Videla María Andrea del Carmen. "Autoecología de Araucaria araucana en el Noroeste Extra-Andino Xérico de Patagonia." Doctoral thesis, 2019. http://hdl.handle.net/11086/12783.
Повний текст джерелаEn el ambiente xérico, extra andino, del norte de Patagonia, Araucaria araucana crece en afloramientos rocosos (morros) de superficies variables, formando pequeños bosques puros, probablemente restos de una vegetación boscosa más extensa. Este tipo de bosque abarca aproximadamente un tercio del área de distribución de la especie en Argentina. Se ha postula do que en estos ambientes la regeneración de A. araucana por semilla es escasa o nula. El objetivo general de este trabajo fue estudiar las vías de reproducción y establecimiento de bosques xéricos de A. araucana. Se evaluó la abundancia de renovales de origen agámico y de semilla, la capacidad de regeneración por semillas y la supervivencia de plantines. Además, se caracterizó la condición de micrositios favorables-desfavorables para la regeneración, a través de parámetros ambientales. Se concluye, que la variación interanual en la producción de semillas, la predación y la disponibilidad de micrositios adecuados para la germinación y posterior supervivencia de plantines de A. araucana, serían limitantes para la regeneración sexual en este bosque. La vía de regeneración dominante de A. araucana en ambientes xéricos es la asexual, principalmente asociada a microambientes rocosos.
Castillo, Martinez Fabio Andres. "Molecular characterization of XerS/difSL site-specific recombination system in Streptococcus suis." Thesis, 2020. http://hdl.handle.net/1866/24509.
Повний текст джерелаThe circular state of the bacterial chromosome presents a specific problem during replication. An odd number of homologous recombination events results in concatenated dimer chromosomes that cannot be partitioned into daughter cells. To solve this problem, bacteria have developed a mechanism of dimer resolution based on site-specific recombination system. This is performed by the Xer/dif system. In this system, the Xer proteins perform a recombination reaction in the dif site at the cell septum immediately prior to cell division. In most bacteria this reaction is performed by two recombinases, XerC and XerD. However, an important zoonotic pathogen; Streptococcus suis harbors a different recombination system, composed by a single recombinase enzyme called XerS, that catalyzes the recombination reaction in an unconventional dif site; difSL. A region characterized by two imperfect inverted repeat regions that flank a central region of 11 bp.To characterize the mode of cleavage of XerS, EMSA experiments were performed by using HEX-labelled PCR fragments and “nicked suicide substrates”. Our data suggests that; 1.) XerS is able to bind the entire difSL sequence; 2.) XerS binds more efficiently the left half side on incomplete difSL mutants than the right half side; 3.) XerS cleaves both the top and bottom strands of the difSL site, with a more efficient reaction at the bottom strand; 4.) Nucleotides at the outermost region of a T rich region seem to be determinant for binding selectivity and modifications of the extra spacing between the inverted repeat arms as well as length modifications of the central region change cleavage preference. 5.) XerS did not show any specific activity on another unconventional dif site in Firmicutes, as tested on difH. 6.) XerS interacts with FtsK-y subunit. This research aims to understand how XerS recombination works in the single recombinase system of Streptococcus and how this recombination is regulated by host factors. Exploration of these recombinases will provide a better understanding of the mechanisms of DNA exchange and genome stability in bacteria. It can also increase our knowledge of the evolution and speciation of recombinogenic bacteria.