Replication of the nonsegmented, plus-stranded RNA genome of Cucumber necrosis tombusvirus (CNV) requires two essential overlapping viral-coded replication proteins, the p33 replication co-factor and the p92 RNA-dependent RNA polymerase. In my thesis I describe (i) the effect of phosphorylation of p33, (ii) the RNA chaperone-like activity of p33, and (iii) the role of HSP70s a host proteins in the viral replication. To test the effect of phosphorylation on p33 function, I used in vitro phosphorylated p33. I found that phosphorylation inhibited the ability of p33 to bind to the viral RNA. Phosphorylation-mimicking mutations rendered p33 nonfunctional in plant protoplasts and in yeast. Based on these results, I propose that the primary function of phosphorylation of p33 is to regulate its RNA binding capacity, which could affect the assembly of new viral replicase complexes, recruitment of the viral RNA template into replication and/or release of viral RNA from replication. Thus, phosphorylation of p33 might help in switching the role of the viral RNA from replication to other processes, such as viral RNA encapsidation and cell-to-cell movement. Small plus-stranded RNA viruses do not code for RNA helicases that would facilitate the proper folding of viral RNAs during replication. Instead, small RNA viruses might use RNA chaperones for replication as shown here for the p33 replication protein. In vitro experiments demonstrated that the purified recombinant p33 facilitated RNA synthesis on plusstranded and double-stranded (ds)RNA templates up to 5-fold. In addition, p33 rendered dsRNA templates sensitive to single-strand specific S1 nuclease, suggesting that p33 can destabilize highly structured RNA. Altogether, the RNA chaperone activity of p33 might perform similar biological functions to the helicases. SSa a yeast HSP70 found in the viral replication complex and shown to facilitate viral replication (Serva and Nagy, 2006)To dissect the mode of action of SSA in the viral replication I used temperature sensitive and deletion mutants. Both showed miss localization of p33 compared to the wild type. Purified SSA rendered non functional bacterial expressed p92 functional in an in vitro replication assay. SSa might play a role in the transportation and assembly of viral replication proteins.

Hepatitis C virus (HCV) infects an estimated 170 million people worldwide and is a major cause of chronic hepatitis and hepatocellular carcinoma. As there are limited treatment options, the elucidation of novel host-viral interactions during HCV pathogenesis will be critical for the development of new therapeutics. My thesis work has identified cell death-inducing DFF45-like effector B (CIDEB) as a host factor that is disregulated during HCV infection, and has delineated the relevance of CIDEB’s dual roles in apoptosis and lipid metabolism in the context of the HCV lifecycle. Moreover, additional host factors necessary for the HCV lifecycle were investigated using unnatural amino acid (UAA) technology. With this technique, the photo-cross-linking UAA p-azido-phenlyalanine (AZF) and 3’-azibutyl-N-carbamoyl-lysine (Abk) were incorporated into viral proteins by expanding the genetic code of the host organism. This conferred diverse physicochemical and biological properties to these proteins that were exploited to investigate protein structure and function in vitro and in vivo. In summary, gaining insight into the numerous host-viral interactions that take place during HCV infection will both advance our understanding of HCV pathogenesis and uncover potential therapeutic targets.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(農学) / 甲第18333号 / 農博第2058号 / 新制||農||1023(附属図書館) / 学位論文||H26||N4840(農学部図書室) / 31191 / 京都大学大学院農学研究科応用生物科学専攻 / (主査)教授 奥野 哲郎, 教授 佐久間 正幸, 准教授 吉田 天士 / 学位規則第4条第1項該当

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第21996号 / 医博第4510号 / 新制||医||1037(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 小柳 義夫, 教授 朝長 啓造, 教授 杉田 昌彦 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

Orientador: Marcelo Brocchi / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Biologia / Made available in DSpace on 2018-08-27T16:38:13Z (GMT). No. of bitstreams: 1 Neves_MeirieledaSilvadas_M.pdf: 2620199 bytes, checksum: 70b2df72cfdb93196c91a87c813e12e9 (MD5) Previous issue date: 2015 / Resumo: O gênero Salmonella pertence à família Enterobacteriaceae que agrupa bacilos Gram-negativos, anaeróbios facultativos, fermentadores e geralmente flagelados. S. enterica é um dos patógenos de origem alimentar mais prevalente, sendo que infecções causadas por essa bactéria podem estar relacionadas a praticamente todos os tipos de alimentos. O trabalho foi proposto com o intuito de realizar a caracterização fenotípica e molecular de linhagens atenuadas de Salmonella enterica Typhimurium para genes codificadores de proteínas associadas ao nucleóide (NAPs Nucleoid associated Proteins). As características fenótipicas dos mutantes nulos de Salmonella enterica para os genes ihfA ou ihfB, codificadores das subunidades A e B de IHF, foram avaliadas através de crescimento in vitro, motilidade, sobrevivência frente ao estresse nutricional (sobrevivência em fase estacionária), sob condições ácidas, na presença de sais biliares e quanto à capacidade de invasão e sobrevivência em macrófagos (linhagem J774A.1). Testes de confirmação da atenuação e avaliação da capacidade de induzir proteção em caso de infecção por S. enterica foram realizados utilizando o modelo murino. Os mutantes não apresentaram diferença no crescimento in vitro e na capacidade de sobreviver na presença de sais biliares em comparação com a linhagem selvagem. As linhagens mutantes para os genes ihfA ou ihf ihf ihfB) apresentaram uma menor capacidade de sobrevivência sob condições ácidas quando comparadas com a linhagem selvagem. A motilidade dos mutantes simples também foi reduzida. Os mutantes simples e duplo apresentaram maior capacidade de sobreviver sob estresse nutricional quando comparados com a linhagem selvagem. O mutante para o gene ihfA e o duplo mutante apresentaram um aumento na capacidade de invadir macrófagos. ihf ihfB mostraram uma capacidade aumentada em sobreviver no interior de macrófagos quando comparadas com a linhagem selvagem. Os mutantes nulos viii de Salmonella enterica para os genes ihfA ou ihfB apresentam atenuação, em diferentes graus, quanto à virulência e apresentaram capacidade de induzir proteção no modelo murino de infecção por S. enterica. Esses resultados demonstram que essa proteína apresenta função relacionada com a virulência bacteriana, sendo um importante alvo de estudo na busca de linhagens atenuadas / Abstract: The genus Salmonella belongs to the Enterobacteriaceae family that comprises Gram-negative bacillus, facultative anaerobe, fermenting and generally flagellate. S. enterica is one of the most prevalent food-borne pathogen, and infections caused by this bacterium can be associated to almost all types of food. The work was proposed with the purpose of performing phenotypic and molecular characterization of attenuated strains of Salmonella enterica Typhimurium for genes encoding proteins associated with the nucleoid (NAPs - Nucleoid associated Proteins). The phenotypic characteristics of the null mutants of Salmonella enterica for genes ihfA or ihfB, encoding the A and B subunits of IHF, were evaluated by in vitro growth, motility, survival under nutritional stress (survival in the stationary phase), under acidic conditions, in the presence of bile salts and for the ability of invasion and survival in macrophages (J774A.1 strain). Attenuation tests and evaluation of the capacity to induce protection in case of infection by S. enterica were performed using the murine model. The mutants showed no difference in the in vitro growth and the ability to survive in the presence of bile salts in comparison with the wild type strain. The single mutant for ihfA or ihf ihf ihfB) showed decreased survival under acidic conditions when compared to the wild type strain. Motility of single mutants was also reduced. Single and double mutants showed higher ability to survive under nutritional stress when compared with the wild type strain. The mutant gene for ihfA and the double mutant showed an increased ability to invade ihf ihfB mutants showed an increased ability to survive within macrophages when compared with the wild type strain. Null mutants of Salmonella enterica for ihfA or ihfB genes exhibited attenuation, to varying degrees, for virulence and showed ability to induce protection in a murine model of infection by S. enterica. x These results demonstrate that this protein has function associated to bacterial virulence and is an important subject of study in search for attenuated strains / Mestrado / Genetica de Microorganismos / Mestra em Genética e Biologia Molecular

Rinderpest virus (RPV) belongs to the order Mononegavirale which comprises non segmented negative sense RNA viruses including human pathogens such as Measles, Ebola and Marburg virus. RPV is the causative agent of Rinderpest disease in large ruminants, both domesticated and wild. The viral genome contains a non segmented negative sense RNA encapsidated by nucleocapsid protein (N-RNA). Viral transcription/replication is carried out by the virus encoded RNA dependent RNA polymerase represented by the large protein L and phosphoprotein P as (L-P) complex. Viral transcription begins at the 3’ end of the genome 3’le-N-P-M-F-H-N-tr-5’ with the synthesis of 55nt leader RNA followed by the synthesis of other viral mRNAs. A remarkable feature common to all members of Paramyxoviridae family is the gradient of transcription from 3’ end to the 5’ end of the genome due to attenuation of polymerase transcription at each gene junction. The present study aims at functional characterization of Rinderpest virus transcription and the associated activities required for viral mRNA capping. In addition, an attempt has been made to understand the novel role of a host factor, Ebp1, playing a key role in virus multiplication in infected cells. The specific aims of the study are presented in detail below. 1. Development of in vitro transcription system for RPV mRNA synthesis and role of phosphorylation of P protein in transcription. The transition of viral polymerase from transcription to replication in infected cells has been a long standing puzzle in all paramyxoviruses. Earlier work carried out using RPV minigenome with a CAT reporter gene and studies with phosphorylation null mutant P, has revealed the importance of P phosphorylation for viral transcription in vivo. However, the contribution of other cellular factors in the viral transcription/replication switch could not be ruled out in these assays. In order to understand the specific role of P protein in transcription/replication, it was necessary to develop a cell free transcription system for viral mRNA synthesis. Hence, viral genomic RNA (N-RNA) was purified from RPV infected cells using CsCl density gradient centrifugation. The viral RNA polymerase consisting of L-P complex was separately expressed in insect cells and partially purified by glycerol gradient centrifugation. Glycerol gradient fraction containing the L-P complex was found to be active in viral transcription. Notably, the gradient of transcription of viral mRNA was observed in vitro with the partially purified recombinant L-P complex similar to in vivo. However, the recombinant polymerase complex failed to synthesis the 55nt leader RNA, in agreement with the recent finding in VSV that the transcriptase complex was unable to synthesize leader RNA and viral transcription is initiated at the N gene start site unlike the conventional 3’ entry mode. The newly developed in vitro reconstituted transcription system was used to analyze the effect of P phosphorylation on viral transcription. The results presented in chapter 2, indicate that phosphorylated P supports transcription whereas unphosphorylated P transdominantly inhibits the transcription in vitro suggesting the possible role of the status of P protein phosphorylation in determining transcription/replication switch. 2. Enzymatic activities associated with RPV L protein- role in viral mRNA capping. Post transcriptional modification of mRNA such as capping and methylation determines the translatability of viral mRNA by cellular ribosome. In negative sense RNA viruses, synthesis of viral mRNA is carried out by the viral encoded RNA polymerase in the host cell cytoplasm. Since the host capping and methylation machinery is localized to the nucleus, viruses should either encode their own mRNA modification enzymes or adopt alternative methods as has been reported for orthomyxoviruses (cap snatching) and picornaviruses (presence of IRES element). In order to test, if RPV RNA polymerase possesses any of the capping and methylation activities, both virus as well as the RNP complex containing the viral N-RNA and RNA polymerase (L-P) were purified from infected cells. Using the purified virus and RNP complex, the first two activities required for mRNA capping vis-à-vis, RNA triphosphatase and guanylyltransferase were tested and the results are described in chapter 3 and 4. Purified virus as well as the RNP complex showed both RNA triphosphatase (RTPase) and Nucleotide triphosphatase activities. Neither purified N-RNA or recombinant P proteins show these activities suggesting that it is indeed mediated by viral L protein. By the metal dependency of the reaction and by the motif conservation with other reported RTPases, RPV L protein was assigned to the metal dependent RTPase tunnel family. Capping activity was also seen with the L protein present in RNP complex by its ability to form a covalent complex with GMP moiety of GTP. The specificity of the reaction with GTP, inhibition of Enzyme-GMP complex formation by the inorganic pyrophosphate and the susceptibility of Enzyme-GMP complex under acidic conditions clearly indicated that RPV L represents the viral guanylyl transferase. Further confirmation was obtained by the indirect capping assay in which Enzyme-GMP complex was formed when recombinant L protein was incubated with the cap labeled RNA due to the reversible nature of capping reaction. Owing to the large size of L protein (240 KDa), it is conceivable that the L protein functions in a modular fashion for different activities pertaining to RNA synthesis and modification. Sequence comparison of L proteins from different morbilliviruses revealed the presence of three conserved domains namely domain I (aa 1-606), domain II (aa 650-1694) and domain III (aa 1717-2183). Since domain II has already been assigned as the viral RNA dependent RNA polymerase, domain I and domain III were chosen for further characterization. Both domains were cloned, expressed and purified to homogeneity using recombinant baculovirus expression system. However, the recombinant domain III alone showed the NTPase activity where as neither domain I or III showed RTPase activity. This is expected since a part of the conserved RTPase motif was located in domain II in the multiple sequence alignment with other viral and yeast RTPases. In addition, the recombinant domain III also showed the characteristic enzyme-GMP complex formation but failed to be active in the indirect capping assay. Therefore, both domain II and domain III are likely to be involved in the co-transcriptional capping of viral mRNA. In support of this view, recent report in VSV suggests the presence of additional motif in domain II which is essential for viral mRNA capping. Preliminary evidence has been presented in the appendix section for the presence of N7 guanine methyl transferase activity with L protein although further experiments are needed to confirm this activity. 3. Role of host factor Ebp1 in negative sense RNA virus replication - a possible antagonist In recent years, many cellular factors such as actin, tubulin and profilin have been shown to be involved in viral transcription. Ebp1-ErbB3 binding protein was initially isolated as a cellular protein which binds to Influenza viral polymerase subunit PB1. Ebp1 selectively inhibits the influenza virus transcription in vitro whereas the cap binding and endonuclease activity of PB1 subunit of viral polymerase is unaffected. Till now there are no reports of the role of Ebp1 in non segmented negative sense RNA virus infection. The fifth chapter describes the role of Ebp1 in RPV infection and vice versa. RPV infection leads to down regulation of Ebp1 mRNA levels which in turn leads to decreased protein synthesis. Subsequently, it was found that Ebp1 interacts presumably with viral N protein, being a part of the viral RNP complex in both infected cells as well as in purified virion. Further, over expression of Ebp1 inhibits viral transcription and as a consequence the virus multiplication in vivo suggesting a mutual antagonism between virus and the host cell through Ebp1 protein.

Bacteria lack nucleus and any other membrane-bound organelles. Hence all the cellular components, including proteins, DNA, RNA and other components are located within the cytoplasm. The region of the cell which encompasses the bacterial genomic DNA is termed ‘Nucleoid’. The nucleoid is composed largely of DNA and small amounts of proteins and RNA. The genomic DNA is organized in ways that are compatible with all the major DNA-related processes like replication, transcription and chromosome segregation. Proteins that play important role(s) in the structuring of DNA and having the potential to influence gene expression have been explored in all kingdoms of life. The organization of bacterial chromosome is influenced by several important factors. These factors include molecular crowding, negative supercoiling of DNA and NAPs (nucleoid-associated proteins) and transcription. Nucleoid-associated proteins are abundant and relatively low-molecular mass proteins which can bind DNA and function as architectural constituents in the nucleoid. Additionally, NAPs are involved in all the major cellular processes like replication, repair and gene transcription. At least a dozen distinct NAPs are known to be present in E. coli. HU, IHF (integration host factor), H-NS (histone-like nucleoid-structuring), Fis (Factor for inversion stimulation), Dps (DNA protection from starvation) are some of the abundant NAPs in E. coli. Most of these proteins bind DNA and show either DNA bending, bridging or wrapping which are directly relevant to their physiological role(s). As most of these proteins are involved in the regulation of transcription of many genes, they act as factors unifying gene regulation with nucleoid architecture and environment. Pathogenic bacteria have the ability to grow and colonize different environments and thus need to adapt to constantly changing conditions within the host. H-NS and IHF, being able to link environmental cues to the regulation of gene expression, play an important role in the bacterial pathogenesis. H-NS is one of the well studied NAPs in enterobacteria, and is known as a global gene silencer. It is also an important DNA structuring protein, involved in chromosome packaging. H-NS protein is a small (~15 kDa) protein, which is present at approximately 20000 copies/ cell. The most striking feature of H-NS is that although it binds DNA in a relatively sequence-independent fashion but is known to preferentially recognize and bind intrinsically curved DNA. It also constrains DNA supercoils in vitro, thereby affects DNA topology. H-NS also influences replication, recombination and genomic stability. In addition, it functions as a global regulator by regulating the expression of various genes which are linked to environmental adaptation. Various studies have shown the association of H-NS to AT-rich regions of the genome. About 5% of E. coli genes are regulated by H-NS, bulk of which are (~80%) negatively regulated. H-NS is involved in the silencing of horizontally-acquired genes, many of which are involved in pathogenesis, in a process known as xenogeneic silencing. H-NS is known to regulate the expression of various virulence factors like cytotoxins, fimbriae and siderophores in several pathogenic bacteria. Several studies have revealed that hns mutants show increased frequency of illegitimate recombination and reduction in intra-chromosomal recombination, indicating the involvement of H-NS in DNA repair/recombination. H-NS is known to act in several transposition systems, which it does so due to its ability to interact with other proteins involved and due to its DNA structure-specific binding activity. The prototypical IHF (Integration Host Factor) was originally discovered in E. coli as an essential co-factor for the site-specific recombination of phage λ. E. coli IHF belongs to DNABII structural family, along with HU and other proteins and consists of two subunits, IHFα and IHFβ. Thesubunits are ~10 kDa each and are essential for full IHF activity. Apart from its role in bacteriophage integration/excision, IHF also has roles in various processes such as DNA replication, transcription and also in several site-specific recombination systems. In most of these processes, IHF acts as an architectural component by facilitating the formation of nucleoprotein complexes by bending DNA at specific sites. IHF acts as a transcriptional regulator, influencing the global gene expression in E. coli and S. Typhimurium. Gene regulation by IHF requires its DNA architectural role, facilitating interactions between RNA polymerase and regulatory protein. The high intracellular concentration of IHF indicates that it might associate with DNA in a non-specific manner and contribute to chromatin organization. The binding of E. coli IHF causes the DNA to adopt U-turn and brings the non-adjacent sequences into close juxtaposition. IHF is also involved in gene regulation in several pathogenic organisms and is shown to regulate expression of many virulence factors. Despite extensive literature on NAPs, very little is known about NAPs and nucleoid architecture in M. tuberculosis. In the light of significant physiological roles played by NAPs in adaptation to environmental changes and in growth and virulence of bacteria, elucidation of their roles in M. tuberculosis is of paramount importance for a better understanding of its pathogen city. M. tuberculosis Rv3852 (hns) gene is predicted to encode a 134 amino acid protein with a molecular mass of 13.8 kDa. The amino acid sequence alignment revealed that M. tuberculosis H-NS and E. coli H-NS showed very low degree of sequence identity (6%). To explore the biochemical properties of M. tuberculosis H-NS, the sequence corresponding to Rv3852 was amplified via PCR, cloned and plasmid expressing M. tuberculosis hns was constructed. M. tuberculosis H-NS was over expressed and purified to homogeneity. E. coli H-NS was also over expressed and purified. Comparison of experimentally determined secondary structure showed considerable differences between M. tuberculosis and E. coli H-NS proteins. Chemical cross linking suggested that M. tuberculosis H-NS protein exists in both monomeric and dimeric forms in solution, consistent with the diametric nature of E. coli H-NS protein. Our studies have revealed that M. tuberculosis H-NS binds in a more structure-specific manner to DNA replication and repair intermediates, but displays lower affinity for double stranded DNA with relatively higher GC content. It bound to the Holliday junction (HJ), the central recombination intermediate, with high affinity. Furthermore, similar to M. tuberculosis H-NS, E. coli H-NS was able to bind to replication and recombination intermediates, but at a lower affinity than M. tuberculosis H-NS. To gain insights into homologous recombination in the context of nucleoid, we investigated the ability of M. tuberculosis RecA to catalyze DNA strand exchange between single-strand DNA and linear duplex DNA in the presence of increasing amounts of H-NS. We found that M. tuberculosis H-NS inhibited strand exchange mediated by its cognate RecA in a concentration dependent manner. Similar effect was seen in the case of E. coli H-NS, where it was able to suppress DNA strand exchange promoted by E. coli RecA, but at relatively higher concentrations, suggesting that H-NS proteins act as ‘roadblocks’ to strand exchange promoted by their cognate RecA proteins. H-NS and members of H-NS-family of NAPs are known to form rigid nucleoprotein filament structures on binding to DNA, which results in gene-silencing and is also implicated in chromosomal organization. Studies have also shown that H-NS mutants defective in gene silencing also lack the ability to form rigid nucleoprotein filament structure and that nucleoprotein filament structure is responsive to environmental factors. Our studies employing ligase-mediated DNA circularization assays reveal that both E. coli and M. tuberculosis H-NS proteins abrogate the circularization of linear DNA substrate by rigidifying the DNA backbone. These results suggest that M. tuberculosis H-NS could form nucleoprotein filament-like structures upon binding to DNA and these structures might be involved in transcriptional repression, chromosomal organization and protection of genomic DNA. In summary, these findings provide insights into the role of M. tuberculosis H-NS in homologous and/or homeologous recombination as well as transcriptional regulation and nucleoid organization. The second part of the thesis concerns the characterization of M. tuberculosis integration host factor (mIHF). The annotation of whole-genome sequence of M. tuberculosis H37Rv showed the presence of Mtihf gene (Rv1388) which codes for a putative 20-kDa integration host factor (mIHF). Amino acid sequence alignment revealed very low degree of sequence identity between mIHF and E. coli IHFαβ subunits. Unlike E. coli IHF, mIHF is essential for the viability of M. tuberculosis. The three-dimensional molecular modeling of mIHF based upon co crystal structure of Streptomycin coelicolor IHF (sIHF) duplex DNA, showed the presence of conserved Arg170, Arg171, Arg173, which were predicted to be involved in DNA binding and a conserved Pro150, in the tight turn. The coding sequence corresponding to the M. tuberculosis H37Rv ihf gene (Rv1388) was amplified, cloned and plasmid over expressing M. tuberculosis ihf (pMtihf) was constructed. Using pMtihf as a template and using specific primers, mutant ihf encoding plasmids were constructed in which, the arginine at position 170, 171, or 173 was replaced with alanine or aspartate and proline at position 150 was substituted with alanine. To explore the role of mIHF in cell viability, we investigated the ability of M. tuberculosis ihf to complement E. coli ΔihfA or ΔihfB strains against genotoxic stress. Despite low sequence identity between Mtihf and E. coli ihfA and ihfB, wild type Mtihf was able to rescue the UV and MMS sensitive phenotypes of E. coli ΔihfAand ΔihfBstrains, whereas Mtihf alleles bearing mutations in the DNA-binding residues failed to confer resistance against DNA-damaging agents. To further characterize the functions of mIHF, wild type and mutant versions of mIHF proteins were over expressed and purified to near homogeneity. Circular dichroism spectroscopy of wild type mIHF and mIHF mutant proteins revealed that they have similar secondary structures. By employing size-exclusion chromatography and blue-native PAGE, we determined that mIHF exists as a dimmer in solution. To understand the mechanistic basis of mIHF functions, we carried out electrophoretic mobility shift assays. In these assays, we found that wild-type mIHF showed high affinity and stable binding to DNA containing attB and attP sites and also to curved DNA, but not those mIHF mutants bearing mutations in DNA-binding residues. Because wild type mIHF was able to rescue the UV and MMS sensitive phenotypes of E. coli ΔihfA and ΔihfB strains, we ascertained the effect of overexpression of mIHF proteins on the bacterial nucleoid. Our results revealed that wild type mIHF was also able to cause significant nucleoid compaction upon its overexpression, but mutant mIHF proteins were unable to cause compaction of E. coli nucleoid structure. M. smegmatis IHF is known to stimulate L5 phage integrase mediated site-specific recombination, we investigated the ability of mIHF to promote site-specific recombination. In vitro recombination assays showed that M. tuberculosis IHF effectively stimulated the L5 integrase mediated site-specific recombination. Since DNA-bending activity of E. coli IHF is necessary for its functions in various processes like initiation of replication, site-specific recombination, transcriptional regulation and chromosomal organization, we asked whether mIHF possesses DNA bending activity. We employed ligase mediated DNA circularization assays, which revealed that like E. coli IHF, mIHF was able to bend DNA resulting in the covalent closure of DNA to yield circular DNA molecules. Interestingly mIHF also resulted in the formation of slower migrating linear DNA multimers, albeit to a lesser extent, which suggest that both E. coli IHF and mIHF show DNA-bending, but the mechanism is distinct. Further studies using atomic force microscopy showed that depending upon the placement of preferred binding site (curved-DNA sequence) mIHF promotes DNA compaction into nucleoid-like or higher order filamentous structures. Together, these findings provide insights into functions of mIHF in the organization of bacterial nucleoid and formation of higher-order nucleoprotein structures. Importantly, our studies revealed that the DNA-binding residues, the DNA bending mechanism and mechanism of action of mIHF during site-specific recombination were different from E. coli IHF protein. Together with extensive biochemical and in vitro data of bacterial growth, the findings presented in this thesis provide novel insights into the biological roles of H-NS and mIHF in M. tuberculosis.

L'abondance d'éléments génétiques mobiles dans le génome humain a un impact critique sur son évolution et son fonctionnement. Même si la plupart des éléments transposables sont inactifs en raison de l'accumulation de mutations, le rétrotransposon LINE-1 (Long Interspersed Element-1 ; ou L1) continue de se mobiliser et d'influer sur notre génome. Il a ainsi contribué à l'évolution de l'homme moderne, mais aussi à l'apparition de maladies génétiques. Les séquences du rétrotransposon L1 correspondent à 17% de la masse totale de l’ADN humain. Une copie active de L1 est capable de se mobiliser de manière autonome par un mécanisme de type «copier-coller» qui met en jeu un intermédiaire ARN et une étape de transcription inverse. Cependant, peu de choses sont connues sur les voies cellulaires impliquées dans la mobilité de L1. Notre laboratoire a découvert, par des cribles double-hybride, une interaction entre la protéine ORF2p de L1 et le récepteur α associé aux œstrogènes (ERRα), un membre de la famille des récepteurs nucléaires. Ici, nous avons confirmé et étendu cette observation à plusieurs autres membres de la superfamille des récepteurs de stéroïdes en utilisant un test de double-hybride fluorescent (F2H) en culture cellulaire. Pour mieux comprendre le rôle potentiel d’ERRα dans le cycle de rétrotransposition de L1, nous avons effectué des expériences de suppression et de surexpression qui suggèrent qu’ERRα est un régulateur positif de la rétrotransposition. Collectivement, ces données relient les voies de signalisation des stéroïdes avec la régulation post-traductionnelle de la rétrotransposition de L1, ce qui suggère un modèle par lequel ERRα et probablement autres récepteurs nucléaires peuvent recruter le RNP L1 vers des emplacements chromosomiques spécifiques. / The abundance of genetic mobile elements in our DNA has a critical impact on the evolution and function of the human genome. Even if most transposable elements are inactive due to the accumulation of mutational events, the Long INterspersed Element-1 (LINE-1 or L1) retrotransposon continues to diversify and impact our genome, being involved in the evolution of modern humans and in the appearance of genetic diseases or in tumorigenesis. L1 forms 17% of human DNA. It is autonomously active being replicated through an RNA-mediated ‘copy-and-paste’ mechanism. The L1 element encodes two proteins, ORF1p and ORF2p, which associate with the L1 mRNA to form L1 ribonucleoprotein particles, the core of the retrotransposition machinery. However, little is known about the cellular pathways involved in L1 replication. Our laboratory has discovered by yeast 2-hybrid screens an interaction between L1 ORF2p and the estrogen-related receptor α (ERRα), a member of the nuclear receptor family. Here, we confirmed and extended this observation to several other members of the steroid receptor superfamily using a fluorescent two-hybrid assay (F2H) in human cultured cells. To get further insight into the potential role of ERR in L1 replication cycle, we performed ERR siRNA-mediated knock-down and overexpression experiments, which suggest that ERR is a positive regulator of retrotransposition. Moreover, the artificial tethering and concentration of ERR to a large and repetitive genomic array inhibits retrotransposition. Collectively, these data link steroid signaling pathways with the post-translational regulation of L1 retrotransposition, suggesting a model by which ERRα, and probably several other nuclear receptors, can recruit the L1 RNP to specific chromosomal locations, acting as tethering factors.