Bacterial persistence is considered as one of the primary reason for antibiotic tolerance besides genetically acquired antibiotic resistance. Persisters are the subpopulation of a clonal bacterial population, which can survive environmental extremes and become invulnerable to stresses due to limited metabolic activities and physiological functions. Cognate toxin and antitoxin (TA) pairs, which are transcribed simultaneously from the same or different operons within the bacterial chromosomes or plasmids, play an important role for bacterial survival during stressful growth environments. Pseudomonas aeruginosa PAO1 is one of the most versatile microorganisms in the environment. Despite its ubiquitous presence, no studies have shown the differential expression pattern of its toxin-antitoxins, and persistence related genes. The purpose of the following study is to analyze differential expression of P. aeruginosa PAO1 type II toxin-antitoxins and persistence related genes under different growth conditions and to show how their stoichiometric ratio changes during different growth conditions. Differential expression analysis indicated that the toxins and antitoxin pairs behave differently under different growth conditions. In addition, the genes related to persistence presented relatively consistent differential expression pattern under different growth environment. / Master of Science / Bacterial persistence is one of the main reason for antibiotic tolerance and recurrent infections. Toxin-antitoxin molecules play an important role during bacterial persistence. Change in the expression of toxin, antitoxins, and persistence related genes and the ratio of the toxin to antitoxin mRNA molecules are important for bacterial survival in stressful environments. Pseudomonas aeruginosa PAO1 is one of most ubiquitous bacteria and responsible for recurrent infection in patients with weaker and compromised immunity. This mRNA sequence (RNA-Seq) analysis study of P. aeruginosa PAO1 showed different expression levels of toxin, antitoxin, and persistence related genes in various stressful growth conditions. This expression also showed the different ratios of the toxin to antitoxin mRNA molecules under different stress conditions. These implicate the different hypothetical roles of these toxin and antitoxin molecules in different growth conditions.

Genes are able to replicate horizontally and vertically- a given gene may be more successful on horizontally mobile elements than others. This includes genes that exhibit a post-segregational killing (PSK) phenotype. PSK is generated by expression of a toxin and antitoxin from a mobile element, such that if a bacterium loses the element the toxin becomes active in the cell and the cell dies. All PSKs described to date involve a toxin and an antitoxin function, though within a given group of toxin and antitoxin gene pairs only some are likely to exhibit this phenotype. Here, I investigate what differentiates genes that induce PSK from biochemically similar genes that do not. One group of genes of which some are known to induce PSK is toxinantitoxin (TA) systems, composed of a stable toxin and an unstable antitoxin. I analyzed computational data on the distribution of type I TA systems (RNA antitoxin), which appear to be less mobile than type II TA systems (protein toxin). Data on validated TAs suggests a correlation between distribution, mobility and the PSK phenotype. Differences in phylogeny could be due to differences in tendency to exhibit PSK in different environments. This connection between distribution and PSK was explored by experimentally testing a computationally described operon, plasmid_Toxin-ptaRNA1, that exhibited structural and distributional similarities to a mobile type I TA system. Despite this, expression of the predicted toxin ORFs did not reduce growth (as measured by saturation density) in E. coli, and the operon did not induce PSK. The conditions of PSK were further tested with the toxin (barnase) and antitoxin (barstar), which are not known to have the phenotype. A number of heterologous expression systems were developed with these genes in E. coli to test their ability to exhibit PSK in a manner akin to both type II TA systems, with a cytoplasmic toxin, and bacteriocins,which have a secreted toxin. I used equations of logarithmic decay to model the necessary expression of the proteins in the cell and their rate of decay after plasmid loss to enable PSK. My results suggest there is likely to be an evolutionary trend toward TA systems with high expression levels of very unstable antitoxins. Secreted barnase was also tested experimentally for its ability to induce PSK similar to bacteriocins, which exhibit a PSK-like phenotype in monoculture by driving maintenance of the immunity encoding plasmid. Barnase did not induce PSK, possibly due to its inability to cause antibiosis in our test system. Structural similarities and biochemical similarities are not sufficient to determine whether a given system will act as a PSK because numerous contextual factors have an effect on whether the genes are addictive. A given set of genes may have the phenotype in one species but not another, under one set of environmental conditions but not another, or on one replicon but not another. This is consistent with the competition hypothesis, which states that genes will be selected for on mobile elements due to their ability to increase horizontal reproductive success, depending on the environmental conditions.

Chromosomal toxin-antitoxin (TA) modules have been proposed to function as regulators of cell growth in response to environmental perturbations. The objective of this study was to characterize the MazEF TA system of the human pathogen Streptococcus mutans. Our data showed that the mazEF genes form a bicistronic operon. MazF toxin had a toxic effect on cells and this effect can be neutralized by coexpression of its cognate antitoxin MazE. Furthermore, we demonstrated that MazE and MazF proteins interact with each other in vivo, confirming the nature of this TA as a type II addiction system. We also demonstrated that MazF is a toxic nuclease arresting cell growth through the mechanism of RNA cleavage and that MazE inhibits the RNase activity of MazF by forming a protein complex. Our results suggest that the MazEF TA might represent a cell growth modulator facilitating the persistence of S. mutans in the oral cavity.

Chromosomal toxin-antitoxin (TA) modules have been proposed to function as regulators of cell growth in response to environmental perturbations. The objective of this study was to characterize the MazEF TA system of the human pathogen Streptococcus mutans. Our data showed that the mazEF genes form a bicistronic operon. MazF toxin had a toxic effect on cells and this effect can be neutralized by coexpression of its cognate antitoxin MazE. Furthermore, we demonstrated that MazE and MazF proteins interact with each other in vivo, confirming the nature of this TA as a type II addiction system. We also demonstrated that MazF is a toxic nuclease arresting cell growth through the mechanism of RNA cleavage and that MazE inhibits the RNase activity of MazF by forming a protein complex. Our results suggest that the MazEF TA might represent a cell growth modulator facilitating the persistence of S. mutans in the oral cavity.

All bacteria initiate translation using formylated methionine, yet directly after translation, the formyl-group is removed. This sequence of addition and removal appears futile, yet every sequenced bacterial genome encodes the enzymes for formylation and deformylation, suggesting this process is essential. Puzzlingly, the process is absent from both Archaea and Eukaryotes, and moreover, bacterial mutants lacking both the formylase and deformylase activities are viable, albeit with a diminished growth rate. We created an Escherichia coli strain devoid of formylase and deformylase activity. This strain was then allowed to evolve over 1500 generations whereupon it reached wild-type growth rate, demonstrating that formylation can be completely dispensed with. This raises an additional question: if the formylation cycle is unnecessary, how did it emerge and why has it persisted? Our results show that the formylation-deformylation cycle could have evolved as a toxin-antitoxin pair (TA) with post-segregational killing (PSK) activity. TAs ‘addict’ cells to the plasmids that carry them by inducing PSK. We measured the stability of formylase-deformylase encoding plasmids and their ability to elicit PSK in our evolved E. coli strain. We report several lines of evidence consistent with the formylation-cycle having evolved from a plasmid-borne PSK element: 1) in the absence of deformylation, formyl-methionine on proteins is cytotoxic in bacteria 2) deformylation relieves the cytotoxicity of formyl-methionine, 3) the loss of a plasmid containing formylase and deformylase genes from evolved cells results in cessation of growth – a standard PSK phenotype. In addition, we introduced the E. coli formylase and deformylase genes into yeast and demonstrate that Met-tRNA formylation is not lethal, even in the absence of deformylation. This suggests PSK would be ineffectual in yeast, accounting for the absence of formylation from eukaryotic cytoplasmic translation. We also report the presence of formylase and deformylase genes in the two representative members of the archaeal Methanocopusculum genus. Moreover, we demonstrate that these genes have been acquired by a recent horizontal gene transfer from bacteria. Our results indicate that formylmethionine use in bacteria evolved, not through a direct functional benefit to cells, but through competition between infectious genetic elements.

Les systèmes Toxine Antitoxine (TA) sont présents chez la plupart des génomes bactériens. Nous rapportons dans cette étude la présence de tels systèmes chez la bactérie pathogène Helicobacter pylori. Ce nouveau système TA de type I, de la même manière que les autres systèmes de type I décrits, est composé d’une toxine peptidique membranaire (AapA) dont la traduction est inhibée par un ARNnc (IsoA) suivant une interaction côté 5’ non traduit de l’ARNm. La structuration particuliere de l’ARNm a permit l’identification d’orthologue de ce système dans les chromosomes des genres Helicobacter et Campylobacter mais aussi sur leur patrimoine génétique mobiles, tels les plasmides. Ceci impliquant leur potentielle acquisition dans le génome par transfert génétique horizontal. La deuxième partie de l’étude se focalise sur la résolution par RMN du liquide de la structure atomique de la toxine AapA1 dans un environnement pseudo-membranaire. Une approche par mutation révèle les determinants structuraux de la toxicité, pointant la présence d’une empreinte de charge dans la partie helicoidale transmenbranaire de la toxine présente aussi chez d’autres toxines de Type I. La dernière partie de l’étude se centre sur l’expression et la purification de la toxine afin d’en étudier la structure en environnement membranaire complexe par RMN du solide. Les résultats prometteurs ouvrent la voie à une caractérisation de la toxine par l’expérience de PISEMA. / Toxin-antitoxin (TA) systems are present in almost all bacterial genomes. Here we report that the genome of the major human gastric pathogen, Helicobacter pylori, is hosting several copies of a new family of type I TA systems. Similarly to other type I TA systems, the toxin (AapA) is a small membrane protein whose expression is controlled by a small antisense RNA (IsoA, antitoxin) that binds to the 5’ untranslated region (UTR) of the mRNA. In addition we used the strong conservation of the mRNA folding to identify homologs of this TA system not only in other Helicobacter and Campylobacter chromosomes but also on plasmids, indicating that this new TA system might have been spread over different genomes via horizontal gene transfer.The second part of the study take account of the AapA toxin itself. We acutely determine the structure of AapA1 by liquid state NMR in membrane mimicking environment. We then probe first structural insight on atomic structural determinants of its toxicity following a mutation studies.These results reveal a particular charge pattern on the transmembrane α helix domain of AapA toxin similary to other Type I toxin. A third part of the studies is based on expression and purification of the toxin in order to determine the structure in complex membrane environment by solid state NMR. The promosisng result open the way to characterize the toxin by PISEMA experiment.

Les systèmes toxine-antitoxine (STA) sont généralement constitués de deux petites protéines cytoplasmiques : une toxine stable et une antitoxine instable capable de neutraliser la toxine et de réprimer l’expression de l’opéron toxine-antitoxine. Une étude menée au laboratoire avait mis en évidence que la perte du gène hicB3 (ypo3369) de Y. pestis, codant une antitoxine solitaire putative, entraine un retard de la croissance bactérienne in vitro et une atténuation de la virulence dans un modèle murin de peste bubonique (Pradel et al., 2014). Par analyse in silico, nous avons détecté, en amont de hicB3, un petit gène non annoté candidat pour coder la toxine HicA3. La surproduction de HicA3 provoque la bactériostase chez Escherichia coli et Y. pestis et la production subséquente de HicB3 restaure la croissance. HicA3 et HicB3 constituent donc un STA fonctionnel. Cependant, la perte du STA HicA3B3 n’affecte pas la virulence de Y. pestis dans un modèle murin de peste bubonique. Nous avons ensuite purifié et caractérisé les protéines HicA3 et HicB3. La toxine HicA3 est une ribonucléase monomérique de 66 aa qui comporte un résidu histidine catalytique essentiel pour son activité. L’antitoxine HicB3 a une double fonction : elle interagit avec HicA3 pour la neutraliser et elle réprime le promoteur de l’opéron hicA3B3. Des expériences de retard sur gel et de fusions transcriptionnelles avec un gène rapporteur ont révélé que l’antitoxine HicB3 et le complexe HicA3-HicB3 se fixent sur deux opérateurs chevauchant les boîtes -10 et -35 du promoteur PhicA3. Nous avons également résolu la structure cristalline de l’antitoxine HicB3 et celle du complexe HicA3-HicB3. HicB3 est un tétramère qui comporte deux domaines de fixation à l’ADN du type ruban-hélice-hélice et deux domaines de neutralisation de la toxine. / Toxin-antitoxin systems (TAS) are generally constituted by two small cytoplasmic proteins: a stable toxin and an unstable antitoxin which neutralizes the toxin and represses the expression of the toxin-antitoxin operon. In previous research, our lab found that Yersinia pestis lacking the hicB3 (ypo3369) gene, encoding a putative orphan antitoxin, has a growth defect in vitro and is attenuated for virulence in a murine model of bubonic plague (Pradel et al., 2014). In silico analysis revealed a small gene upstream of hicB3, encoding a putative toxin that we called HicA3. HicA3 overproduction generates bacteriostasis of Escherichia coli and Y. pestis, and the subsequent production of HicB3 restores cell growth. HicA3 and HicB3 thus constitute a functional TAS. However, the lack of the HicA3B3 TAS does not affect Y. pestis virulence in a murine model of bubonic plague. We then purified and characterized the HicA3 and HicB3 proteins. The HicA3 toxin is a monomeric 66-aa ribonuclease with a catalytic histidine residue required for its activity. The HicB3 antitoxin has two functions: it binds and neutralizes HicA3 and it represses the hicA3B3 operon promoter. Gel-shift assays and transcriptional reporter fusion experiments showed that both HicB3 and the HicA3-HicB3 complex bind to two operators overlapping the -10 and -35 boxes of the PhicA3 promoter. We also solved the crystal structures of the HicB3 antitoxin and the HicA3-HicB3 complex. HicB3 is a tetramer with two DNA binding domains of the ribbon-helix-helix type and two toxin neutralization domains.

Cell biology is a branch of science with a seemingly infinite abundance of interesting phenomena which are essential to our understanding of life and which may potentially drive the development of technology that improves our lives. Among the open ended questions within the field, an understanding of how gene networks are affected by limited cellular components is both broad and rich with interest. Common to all cellular systems are enzymes which perform many tasks within cells without which organisms could not remain healthy. Here are presented several explorations of enzymatic processing as well as a tool constructed for this purpose. More specifically, these works consider the effect of coupling of gene networks via competition for enzymes found within the cell. It is shown that a limitation on the number of available enzymes permits the formation of bottlenecks which drastically affect molecular dynamics within cells. These effects potentially afford cell behaviors that in part explain the impressive robustness of life to constantly fluctuating environments. / Ph. D.

A major challenge in biology is to understand and predict the effect of mutations on protein structure, stability and function. Chapter 1 provides a general introduction on protein sequence-structure relationships and use of the CcdAB toxin-antitoxin system as a model to study molecular determinants of mutant phenotypes. In Chapter 2, we describe the use of saturation mutagenesis combined with deep sequencing to determine phenotypes for 1664 single-site mutants of the E. coli cytotoxin, CcdB. We examined multiple expression levels, effects of multiple chaperones and proteases and employed extensive in vitro characterization to understand how mutations affect these phenotypes. While general substitution preferences are known, eg polar residues preferred at exposed positions and non-polar ones at buried positions, we show that depth from the surface is important and that there are distinctly different energetic penalties for each specific polar, charged and aromatic amino acid introduced at buried positions. We also show that over-expression of ATP independent chaperones can rescue mutant phenotypes. Other studies have primarily looked at effects of ATP dependent chaperone expression on phenotype, where it is not possible to say whether mutational effects on folding kinetics or thermodynamic stability are the primary determinant of altered phenotypes, since there is energy input with these chaperones. The data suggest that mutational effects on folding rather than stability determine the in vivo phenotype of CcdB mutants. This has important implications for efforts to predict phenotypic effects of mutations and in protein design. While looking at the mutational landscape of a given gene from an evolutionary perspective, it is important to establish the genotype-phenotype relationships under physiologically relevant conditions. At the molecular level, the relationship between gene sequence and fitness has implications for understanding both evolutionary processes and functional constraints on the encoded proteins. Chapter 3 describes a methodology to test the fitness of individual CcdB mutants in E.coli over several generations by monitoring the rate of plasmid loss. We also propose a methodology for high throughput analysis of a pool of CcdB mutants using deep sequencing to quantitate the relative population of each mutant in a population of E.coli cells, grown for several generations and build the fitness landscape. While the F-plasmid based CcdAB system is known to be involved in plasmid maintenance through post-segregational killing, recent identification of ccdAB homologs on the chromosome, including in pathogenic strains of E.coli and other bacteria, has led to speculations on their functional role on the chromosome. In Chapter 4, we show that both the native ccd operon of the E.coli O157 strain as well as the ccd operon from the F- plasmid when inserted on the E.coli chromosome lead to protection from cell death under multiple antibiotic stress conditions through formation of persisters. Both the ccdF and ccdO157 operons may share common mechanisms for activation under stress conditions and also display weak cross activation. The chromosomal toxin shows weaker activity as compared to the plasmidic counterpart and is therefore less efficient in causing cell death. This has important implications in generation of potential therapeutics that target these TA systems. Chapter 5 describes the use of site-saturation mutagenesis coupled with deep sequencing to infer mutational sensitivity for the intrinsically disordered antitoxin, CcdA. The data allows us to make comparisons between overall as well as residue specific mutational sensitivity patterns with that of globular proteins, like CcdB (described in Chapter 2) and study toxin- antitoxin interaction and regulation through saturation suppressor mutagenesis. Interestingly, we found several examples of synonymous point mutations in CcdA that lead to loss of its activity. In Chapter 6 we attempt to explore the molecular bases for some of these synonymous mutations. In most cases the mutated codon has a similar overall codon preference to the WT one. Initial findings suggest a change in mRNA structure leading to change in CcdB: CcdA ratio, thereby causing cell death. These observations have important implications, because TA systems are ubiquitous, highly regulated and are known to be involved in multiple functions including drug tolerance. However a role for RNA structure in their regulation has not been shown previously. Appendix–I lists the mutational sensitivity scores for the CcdB mutants. Phenotypes for CcdA mutants obtained through deep sequencing have been tabulated in Appendix-II. Overall, we provide extensive datasets for mutational sensitivities of a globular (CcdB) and an intrinsically disordered protein (CcdA). Exploration of the molecular determinants of these mutant phenotypes not only provides interesting insights into CcdAB operon function but is also useful in understanding various aspects of protein stability, folding and activity as well as regulation of gene expression in bacteria.

Bacillus subtilis is a gram positive soil bacterium that is surrounded by many other microorganisms its environment. That is why it is necessary for the bacterium to be able to fight with these microorganisms for the nutrients and living space. B. subtilis contains the modules in its genetic make-up that improve its ability to compete. These modules are called the toxin-antitoxin systems. This Diploma Thesis is trying to identify yet undescribed extracellular toxins produced by the wild type BSB1 strain of B. subtilis. The related microorganism Bacillus megaterium was used as a competing bacterium. The contact-dependent or independent manner of killing the competing bacterium was demonstrated using this model. By deletion analysis and comparisons of the genomes of the various strains of B. subtilis, the SPβ prophage was first identified as a region containing an unknown toxin(s). Analysis of the extracellular proteome of B. subtilis subsequently revealed an unknown toxin (or toxin complex, respectively) of the molecular weight exceeding 100 kDa. Even more fascinating was the finding that such a large protein molecule is resistant to the pancreatic protease, trypsin. Subsequent non-enzymatic cyanogen bromide cleavage of the extracellular proteins and their analysis by mass spectrometry revealed...