mymAoperon ofMycobacterium tuberculosis: its regulation and importance in the cell envelope

Functional genetic diversity among Mycobacterium tuberculosis complex clinical isolates: delineation of conserved core and lineage-specific transcriptomes during intracellular survival

Tuberculosis exerts a tremendous burden on global health, with approximately 9 million new infections and approximately 2 million deaths annually. The Mycobacterium tuberculosis complex (MTC) was initially regarded as a highly homogeneous population; however, recent data suggest the causative agents of tuberculosis are more genetically and functionally diverse than appreciated previously. The impact of this natural variation on the virulence and clinical manifestations of the pathogen remains largely unknown. This report examines the effect of genetic diversity among MTC clinical isolates on global gene expression and survival within macrophages. We discovered lineage-specific transcription patterns in vitro and distinct intracellular growth profiles associated with specific responses to host-derived environmental cues. Strain comparisons also facilitated delineation of a core intracellular transcriptome, including genes with highly conserved regulation across the global panel of clinical isolates. This study affords new insights into the genetic information that M. tuberculosis has conserved under selective pressure during its long-term interactions with its human host.

Mycobacterium tuberculosis protein kinase K confers survival advantage during early infection in mice and regulates growth in culture and during persistent infection: implications for immune modulation

Mycobacterium tuberculosis serine/threonine protein kinases (STPKs) are key regulators of growth and metabolism; however, evidence for their roles in virulence is limited. In a preliminary screen based on comparative expression between strains H37Rv and H37Ra, six STPK genes, pknD, pknG, pknH, pknJ, pknK and pknL, showed higher expression in H37Rv. In the second screen, STPK expression was analysed in H37Rv-infected human macrophages. Interestingly, significant expression of pknK was detected only at 18 h post-infection, suggesting its involvement in early infection events. We have investigated the roles of PknK in vitro and in vivo. PknK levels were induced under stationary phase and deletion of pknK resulted in increased resistance of the mutant to acidic pH, hypoxia, oxidative and stationary-phase stresses in vitro. These results, together with the increased survival of the ΔpknK strain during persistent infection in mice, reveal a role for PknK in adaptive mechanisms that slow the growth of mycobacteria. A novel finding of this study was the inhibition of growth of ΔpknK strain during acute infection in mice that correlated with the significant upregulation of tumour necrosis factor as well as the simultaneous downregulation of interleukin-12p40, interferon-γ and induced nitric oxide synthase transcripts. Finally, we provide evidence for the localization of PknK during infection and discuss its implications in pathogenesis.

Mycobacterium tuberculosis transcriptional adaptation, growth arrest and dormancy phenotype development is triggered by vitamin C

Background

Tubercle bacilli are thought to persist in a dormant state during latent tuberculosis (TB) infection. Although little is known about the host factors that induce and maintain Mycobacterium tuberculosis (M. tb) within latent lesions, O₂ depletion, nutrient limitation and acidification are some of the stresses implicated in bacterial dormancy development/growth arrest. Adaptation to hypoxia and exposure to NO/CO is implemented through the DevRS/DosT two-component system which induces the dormancy regulon.

Methodology/Principal Findings

Here we show that vitamin C (ascorbic acid/AA) can serve as an additional signal to induce the DevR regulon. Physiological levels of AA scavenge O₂ and rapidly induce the DevR regulon at an estimated O₂ saturation of <30%. The kinetics and magnitude of the response suggests an initial involvement of DosT and a sustained DevS-mediated response during bacterial adaptation to increasing hypoxia. In addition to inducing DevR regulon mechanisms, vitamin C induces the expression of selected genes previously shown to be responsive to low pH and oxidative stress, triggers bacterial growth arrest and promotes dormancy phenotype development in M. tb grown in axenic culture and intracellularly in THP-1 cells.

Conclusions/Significance

Vitamin C mimics multiple intracellular stresses and has wide-ranging regulatory effects on gene expression and physiology of M. tb which leads to growth arrest and a ‘dormant’ drug-tolerant phenotype, but in a manner independent of the DevRS/DosT sytem. The ‘AA-dormancy infection model’ offers a potential alternative to other models of non-replicating persistence of M. tb and may be useful for investigating host-‘dormant’ M. tb interactions. Our findings offer a new perspective on the role of nutritional factors in TB and suggest a possible role for vitamin C in TB.

Dissecting the role of critical residues and substrate preference of a Fatty Acyl-CoA Synthetase (FadD13) of Mycobacterium tuberculosis

Newly emerging multi-drug resistant strains of Mycobacterium tuberculosis (M.tb) severely limit the treatment options for tuberculosis (TB); hence, new antitubercular drugs are urgently needed. The mymA operon is essential for the virulence and intracellular survival of M.tb and thus represents an attractive target for the development of new antitubercular drugs. This study is focused on the structure-function relationship of Fatty Acyl-CoA Synthetase (FadD13, Rv3089) belonging to the mymA operon. Eight site-directed mutants of FadD13 were designed, constructed and analyzed for the structural-functional integrity of the enzyme. The study revealed that mutation of Lys⁴⁸⁷ resulted in ∼95% loss of the activity thus demonstrating its crucial requirement for the enzymatic activity. Comparison of the kinetic parameters showed the residues Lys¹⁷² and Ala³⁰² to be involved in the binding of ATP and Ser⁴⁰⁴ in the binding of CoenzymeA. The influence of mutations of the residues Val²⁰⁹ and Trp³⁷⁷ emphasized their importance in maintaining the structural integrity of FadD13. Besides, we show a synergistic influence of fatty acid and ATP binding on the conformation and rigidity of FadD13. FadD13 represents the first Fatty Acyl-CoA Synthetase to display biphasic kinetics for fatty acids. FadD13 exhibits a distinct preference for C₂₆/C₂₄ fatty acids, which in the light of earlier reported observations further substantiates the role of the mymA operon in remodeling the cell envelope of intracellular M.tb under acidic conditions. A three-dimensional model of FadD13 was generated; the docking of ATP to the active site verified its interaction with Lys¹⁷², Ala³⁰² and Lys⁴⁸⁷ and corresponded well with the results of the mutational studies. Our study provides a significant understanding of the FadD13 protein including the identification of residues important for its activity as well as in the maintenance of structural integrity. We believe that the findings of this study will provide valuable inputs in the development of inhibitors against the mymA operon, an important target for the development of antitubercular drugs.

Mycobacterial subversion of chemotherapeutic reagents and host defense tactics: challenges in tuberculosis drug development

Recent worldwide emergence of multidrug-resistant and extensively drug-resistant tuberculosis is threatening to destabilize tuberculosis control programs and urging global attention to the development of alternative tuberculosis therapies. Major roadblocks limiting the development and effectiveness of new drugs to combat tuberculosis are the profound innate resistance of Mycobacterium tuberculosis to host defense mechanisms as well as its intrinsic tolerance to chemotherapeutic reagents. The triangle of interactions among the pathogen, the host responses, and the drugs used to cure the disease are critical for the outcome of tuberculosis. We must better understand this three-way interaction in order to develop drugs that are able to kill the bacillus in the most effective way and minimize the emergence of drug resistance. Here we review our recent understanding of the molecular basis underlying intrinsic antibiotic resistance and survival tactics of M. tuberculosis. This knowledge may help to reveal current targets for the development of novel antituberculosis drugs.

The Mycobacterium tuberculosis protein kinase K modulates activation of transcription from the promoter of mycobacterial monooxygenase operon through phosphorylation of the transcriptional regulator VirS

Mycobacterium tuberculosis encodes for 11 eukaryotic-like serine/threonine protein kinases. Genetic and biochemical studies show that these kinases regulate various cellular processes including cell shape and morphology, glucose and glutamine transport, phagosome-lysosome fusion and the expression, and/or activity of transcription factors. PknK is the largest predicted serine/threonine protein kinase in M. tuberculosis. Here, we have cloned, overexpressed, and purified protein kinase K (PknK) to near homogeneity and show that its ability to phosphorylate proteins is dependent on the invariant lysine (Lys(55)), and on two conserved threonine residues present in its activation loop. Despite being devoid of any apparent transmembrane domain, PknK is localized to the cell wall fraction, suggesting probable anchoring of the kinase to the cell membrane region. The pknK gene is located in the vicinity of the virS gene, which is known to regulate the expression of the mycobacterial monooxygenase (mymA) operon. We report here for the first time that VirS is in fact a substrate of PknK. In addition, four of the proteins encoded by mymA operon are also found to be substrates of PknK. Results show that VirS is a bona fide substrate of PknK in vivo, and PknK-mediated phosphorylation of VirS increases its affinity for mym promoter DNA. Reporter assays reveal that PknK modulates VirS-mediated stimulation of transcription from the mym promoter. These findings suggest that the expression of mymA operon genes is regulated through PknK-mediated phosphorylation of VirS.

Does the lipR gene of tubercle bacilli have a role in tuberculosis transmission and pathogenesis?

Mycobacterium tuberculosis lipases, a diverse class of enzymes involved in lipid metabolism, may have an important role in tuberculosis (TB) pathogenesis. We explored the association of large sequence polymorphism (LSP) in one of the M. tuberculosis lipase-encoding genes, lipR (Rv3084), with patient characteristics using a population-based sample of clinical isolates to elucidate the potential role of lipR in TB pathogenesis. LSP in lipR was found in 104 (15.6%) of 665 isolates, of which 96% belonged to principal genetic group 3. When linkage by molecular type and epidemiologic evidence were compared, molecularly clustered cases infected with a lipR LSP isolate were more often epidemiologically linked than clustered cases infected with a lipR wild-type isolate. Further epidemiologic and functional studies are necessary to determine if the association between this lipR LSP and recent transmission we identified in this population reflects a functional role of lipR in TB transmission and pathogenesis or other unidentified mechanisms.

The genetics of cell wall biosynthesis in Mycobacterium tuberculosis

Despite an available vaccine and effective antibiotics, Mycobacterium tuberculosis is still the causative agent of almost 2 million deaths every year. The cell wall of M. tuberculosis is composed of sugars and lipids of exotic structure, many of which contribute to its pathogenicity. The majority of the enzymes responsible for building this structure are essential. However, they share very little homology with well-characterized enzymes, which makes their identification in the genome difficult. Despite this, our knowledge of the structure of the cell wall of M. tuberculosis is fairly complete and an increasing number of genes have been identified that are involved in its biosynthesis. By contrast, data concerning regulation of the expression of these genes and control of the cell wall composition are restricted. This review summarizes current information on the genetics of cell wall biosynthesis in M. tuberculosis, incorporating available data on gene organization and regulation.

Differential fadE28 expression associated with phenotypic virulence of Mycobacterium tuberculosis

Ability to persist in human macrophages is central to the virulence of Mycobacterium tuberculosis and is not invariable among various strains. Differential gene expression that is associated with phenotypic virulence may provide additional information of virulent genes involved in the pathogenesis of M. tuberculosis, which is not fully elucidated. Three hypervirulent strains of M. tuberculosis isolated from patients suffering with tuberculous meningitis were shown to grow more rapidly inside human macrophages in a previous study. In the current investigation, expression of 7 mycobacterial genes (fadE28, mce1A, mymA, acr, sigA, sugC, and Rv3723) of these strains during ex vivo macrophage challenge and in vitro acid shock was quantified by real-time PCR. Using rrs gene as a normalisation gene, fadE28 gene exhibited differential gene expression that is associated with phenotypic virulence, whereas the other 6 genes showed indistinguishable expression patterns. Up-regulation of fadE28 gene in the hypervirulent strains may account for virulence by increasing the efficiency of beta-oxidation, which is important for the persistence in macrophages as M. tuberculosis uses fatty acids preferably inside phagosome of macrophages. The fadE28 gene, together with its adjacent genes may also be critical in the process of lipid modification that could facilitate parasitism in human macrophages.

Global transcriptional profile of Mycobacterium tuberculosis during THP-1 human macrophage infection

During lung infection, Mycobacterium tuberculosis resides in macrophages and subverts the bactericidal mechanisms of these professional phagocytes. Comprehension of this host-pathogen relationship is fundamental for the development of new therapies to cure and prevent tuberculosis. In this work, we analyzed the transcriptional profile of M. tuberculosis infecting human macrophage-like THP-1 cells in order to identify putative bacterial pathogenic factors that can be relevant for the intracellular survival of M. tuberculosis. We compared the gene expression profile of M. tuberculosis H37Rv after 4 h and 24 h of infection of human macrophage-like THP-1 cells with the gene expression profile of the strain growing exponentially in broth cultures. We found 585 genes expressed differentially by intracellular M. tuberculosis. An analysis of the gene expression profile of M. tuberculosis inside THP-1 cells suggests the perturbation of the cell envelope as a major intracellular stress inside THP-1 macrophages.

Comparative transcriptomics reveals key gene expression differences between the human and bovine pathogens of the Mycobacterium tuberculosis complex

Members of the Mycobacterium tuberculosis complex show distinct host preferences, yet the molecular basis for this tropism is unknown. Comparison of the M. tuberculosis and Mycobacterium bovis genome sequences revealed no unique genes in the bovine pathogen per se, indicating that differences in gene expression may play a significant role in host predilection. To define the key gene expression differences between M. tuberculosis and M. bovis we have performed transcriptome analyses of cultures grown under steady-state conditions in a chemostat. This revealed that the human and bovine pathogens show differential expression of genes encoding a range of functions, including cell wall and secreted proteins, transcriptional regulators, PE/PPE proteins, lipid metabolism and toxin-antitoxin pairs. Furthermore, we probed the gene expression response of M. tuberculosis and M. bovis to an acid-shock perturbation which triggered a notably different expression response in the two strains. Through these approaches we have defined a core gene set that shows differential expression between the human and bovine tubercle bacilli, and the biological implications are discussed.

The DNA-binding domain as a functional indicator: the case of the AraC/XylS family of transcription factors

The AraC/XylS family of transcription factors, which include proteins that are involved in the regulation of diverse biological processes, has been of considerable interest recently and has been constantly expanding by means of in silico predictions and experimental analysis. In this work, using a HMM based on the DNA binding domain of 58 experimentally characterized proteins from the AraC/XylS (A/X), 1974 A/X proteins were found in 149 out of 212 bacterial genomes. This domain was used as a template to generate a phylogenetic tree and as a tool to predict the putative regulatory role of the new members of this family based on their proximity to a particular functional cluster in the tree. Based on this approach we assigned a functional regulatory role for 75% of the TFs dataset. Of these, 33.7% regulate genes involved in carbon-source catabolism, 9.6% global metabolism, 8.3% nitrogen metabolism, 2.9% adaptation responses, 8.9% stress responses, and 11.7% virulence. The abundance of TFs involved in the regulation of metabolic processes indicates that bacteria have optimized their regulatory systems to control energy uptake. In contrast, the lower percentage of TFs required for stress, adaptation and virulence regulation reflects the specialization acquired by each subset of TFs associated with those processes. This approach would be useful in assigning regulatory roles to uncharacterized members of other transcriptional factor families and it might facilitate their experimental analysis.

A predicted operon map for Mycobacterium tuberculosis

The prediction of operons in Mycobacterium tuberculosis (MTB) is a first step toward understanding the regulatory network of this pathogen. Here we apply a statistical model using logistic regression to predict operons in MTB. As predictors, our model incorporates intergenic distance and the correlation of gene expression calculated for adjacent gene pairs from over 474 microarray experiments with MTB RNA. We validate our findings with known examples from the literature and experimentation. From this model, we rank each potential operon pair by the strength of evidence for cotranscription, choose a classification threshold with a true positive rate of over 90% at a false positive rate of 9.1%, and use it to construct an operon map for the MTB genome.

Cuticular wax biosynthesis in petunia petals: cloning and characterization of an alcohol-acyltransferase that synthesizes wax-esters

The surface of plants is covered by cuticular wax, which contains a mixture of very long-chain fatty acid (VLCFA) derivatives. This wax surface provides a hydrophobic barrier which reduces non-stomatal water loss. One component of the cuticular wax is the alkyl esters, which typically contain a VLCFA esterified to an alcohol of a similar length. As part of an EST project, we recently identified an acyltransferase with 19% sequence identity (amino acid) to a bacterial 'bifunctional' wax-ester synthase/diacylglycerol acyltransferase (WS/DGAT). Northern analysis revealed that this petunia homologue was expressed predominantly within the petals. The cDNA encoding the WS/DGAT homologue was introduced into a yeast strain deficient in triacylglycerol biosynthesis. The expressed protein failed to restore triacylglycerol biosynthesis, indicating that it lacked DGAT activity. However, isoamyl esters of fatty acids were detected, which suggested that the petunia cDNA encoded a wax-synthase. Waxes were extracted from petunia petals and leaves. The petal wax extract was rich in VLCFA esters of methyl, isoamyl, and short-to-medium straight chain alcohols (C4-C12). These low molecular weight wax-esters were not present in leaf wax. In-vitro enzymes assays were performed using the heterologously expressed protein and 14C-labelled substrates. The expressed protein was membrane bound, and displayed a preference for medium chain alcohols and saturated very long-chain acyl-CoAs. In fact, the activity would be sufficient to produce most of the low molecular wax-esters present in petals, with methyl-esters being the exception. This work is the first characterization of a eukaryotic protein from the WS/DGAT family.

Determination of the minimal acid-inducible promoter region of the lipF gene from Mycobacterium tuberculosis

The Mycobacterium tuberculosis gene lipF, Rv3487c, is transcriptionally upregulated by exposure to acidic growth media. We previously identified a 477 base pair (bp) region of DNA 147 bp upstream of lipF that is transcriptionally upregulated by exposure to growth media at pH 4.5 [Saviola, B., Woolwine, S., Bishai, W. R., 2003. Isolation of acid-inducible genes of Mycobacterium tuberculosis with the use of recombinase-based in vivo expression technology. Infect. Immun. 71, 1379-1388]. In this study we truncate the lipF promoter region first from the 3' DNA end and then from the 5' DNA end. The truncated promoter regions were placed upstream of the gene for the green fluorescent protein (gfp) and each promoter region was analyzed in Mycobacterium smegmatis for its ability to undergo transcriptional upregulation in response to acid stress. A minimal acid-inducible promoter region was identified and is located between -515 bp and -573 bp with respect to the start site of translation of lipF. The 59 bp minimal promoter region is a defined DNA sequence that confers full promoter activity that is transcriptionally upregulated in response to acid stress. Primer extension analysis was performed on acid-induced M. smegmatis bearing the minimal promoter region fused to gfp and revealed a start site of transcription specifically upregulated by acid stress corresponding to -511 bp upstream of lipF with respect to the start of translation.

Serine threonine protein kinases of mycobacterial genus: phylogeny to function

Serine/threonine protein kinases (STPKs) are known to act as sensors of environmental signals that thereby regulate developmental changes and host pathogen interactions. In this study, we carried out comparative genome analysis of six completely sequenced pathogenic and nonpathogenic mycobacterial species to systematically characterize the STPK complement of mycobacterium. Our analysis revealed that while Mycobacterium tuberculosis strains have 11 conserved kinases, this number varies from 4 to 24 in other mycobacterial species. pknA, an essential STPK encoding gene, was found to be truncated in the initial analysis of M. avium subsp. paratuberculosis (Map) and M. tuberculosis C genomes. However, resequencing of pknA gene in Map confirmed that the truncation was due to a sequencing error. The conservation of division and cell wall gene cluster involved in cell envelope biosynthesis and cell division, in the vicinity of pknL locus, implicates a possible role of PknL in cell division and envelop biosynthesis. We identified a cyclophilin domain as part of a mycobacterial kinase in Map that suggests a plausible regulation of cyclophilins by phosphorylation. The co-inheritance of pknA, pknB, pknG, and pknL loci across genomes and some unique repertoire of pathogen-specific kinases such as pknI and pknJ of Mtb complex suggest similitude and divergence between pathogenic and nonpathogenic signaling. This study would add another dimension toward identification of physiological substrates and thereby function, while resolving the existing complexities in signaling network between the two domains of life, pathogen and nonpathogen.

The acid-induced operon Rv3083–Rv3089 is required for growth of Mycobacterium tuberculosis in macrophages

The Rv3083-Rv3089 operon of Mycobacterium tuberculosis has been shown to be induced 17-33-fold when tubercle bacilli were exposed in vitro to acidic conditions which may mimic those that the bacilli encounter early during the infection and it is induced during growth in macrophages. To understand the role of this operon in intracellular survival, we constructed a knockout of the operon in the M. tuberculosis H37Rv strain. No differences were observed in the growth of mutant and wild-type mycobacteria on axenic media. Though the uptake of mutant and wild-type bacteria by eukaryotic cells was similar, the mutant failed to grow subsequently. By 192h post-infection, the fold differences between the wild-type and mutant bacteria were significant thus leading to the conclusion that the mutant is defective for intracellular growth in these cell lines. Complementation of the knockout restored intracellular growth to wild-type levels. During the first 24-48h post-infection, mutant bacteria also stimulated production of significantly less IL-1beta, IL-6, IL-8, RANTES, and MCP-1 by THP-1 cells than wild-type bacteria. Overall, the data indicate that the operon plays an important role in the ability of M. tuberculosis to grow inside host cells.

In vitro and intra-macrophage gene expression by Rhodococcus equi strain 103

Rhodococcus equi is a facultative intracellular respiratory pathogen of foals that persists and multiplies within macrophages. In foals, virulence is associated with 80-90 kb plasmids, which include a pathogenicity island (PI) containing the virulence-associated protein (vap) gene family, but detailed understanding of the basis of virulence is still poor. A 60 spot-based DNA microarray was developed containing eight PI genes and 42 chromosomal putative virulence or virulence-associated genes selected from a recent partial genome sequence in order to study transcription of these genes by R. equi grown inside macrophages and under in vitro conditions thought to simulate those of macrophages. In addition to seven PI genes, nine chromosomal genes involved in fatty acid and lipid metabolism (choD, fadD13, fbpB), heme biosynthesis (hemE), iron utilization (mbtF), heat shock resistance and genes encoding chaperones (clpB, groEL), a sigma factor (sigK), and a transcriptional regulator (moxR) were significantly induced in R. equi growing inside macrophages. The pattern of R. equi chromosomal genes significantly transcribed inside macrophages largely differed from those transcribed under in vitro conditions (37 degrees C, pH 5.0 or 50mM H2O2 for 30 min). This study has identified genes, other than those of the virulence plasmid, the transcription of which is enhanced within equine macrophages. These genes should be investigated further to improve understanding of how this organism survives intracellularly.

Deciphering the genes involved in pathogenesis of Mycobacterium tuberculosis

We had previously reported identification of the virS gene of Mycobacterium tuberculosis (Rv3082c) belonging to the AraC family of transcriptional regulators. The 7 genes (Rv3083-Rv3089) which are present divergently to virS (Rv3082c) constitute an operon designated as the mymA operon. Further investigation on the regulation of this operon showed that transcription of the mymA operon is dependent on the presence of VirS protein. A 4-fold induction of the mymA operon promoter occurs specifically in the wild type M. tuberculosis and not in the virS mutant of M. tuberculosis (M.tbDeltavirS) when exposed to acidic pH. Expression of the mymA operon was also induced in infected macrophages by ten-fold over a six-day period. Bioinformatic analysis suggested the involvement of these proteins in the modification of fatty acids required for cell envelope. This was supported by altered colony morphology and cell envelope structure and increased accumulation of C24/C26 fatty acids by M.tbDeltavirS in comparison to the wild type strain. Thus, induction of mymA operon can play an important role in remodeling the envelope of intracellular M. tuberculosis under acidic conditions. Genomic analysis of M. tuberculosis revealed the presence of two tyrosine phosphatase genes--mptpA (Rv2234) and mptpB (Rv0153c). We have characterized both the tyrosine phosphatases of M. tuberculosis. To evaluate the role of MptpB in the pathogenesis of M. tuberculosis we have disrupted mptpB in the genome of M. tuberculosis. The wild type as well as mptpB mutant strain were comparable in their ability to infect and survive in the resting macrophages. However, the mptpB mutant strain was more sensitive to killing as compared to the wild type strain by IFN-gamma activated macrophages. In guinea pig model of tuberculosis an approximately 70-fold reduced bacillary load was observed in the spleen of the animals infected with mptpB mutant strain as compared to the bacillary load in animals infected with the wild type strain at 6 weeks post-infection. These results suggest that mymA operon as well as mptpB gene of M. tuberculosis play an important role in the survival of the pathogen in the host.

Requirement of the mymA operon for appropriate cell wall ultrastructure and persistence of Mycobacterium tuberculosis in the spleens of guinea pigs

We had recently reported that the mymA operon (Rv3083 to Rv3089) of Mycobacterium tuberculosis is regulated by AraC/XylS transcriptional regulator VirS (Rv3082c) and is important for the cell envelope of M. tuberculosis. In this study, we further show that a virS mutant (MtbdeltavirS) and a mymA mutant (Mtbmym::hyg) of M. tuberculosis exhibit reduced contents and altered composition of mycolic acids along with the accumulation of saturated C24 and C26 fatty acids compared to the parental strain. These mutants were markedly more susceptible to major antitubercular drugs at acidic pH and also showed increased sensitivity to detergent (sodium dodecyl sulfate) and to acidic stress than the parental strain. We show that disruption of virS and mymA genes impairs the ability of M. tuberculosis to survive in activated macrophages, but not in resting macrophages, suggesting the importance of the mymA operon in protecting the bacterium against harsher conditions. Infection of guinea pigs with MtbdeltavirS, Mtbmym::hyg, and the parental strain resulted in an approximately 800-fold-reduced bacillary load of the mutant strains compared with the parental strain in spleens, but not in the lungs, of animals at 20 weeks postinfection. Phenotypic traits were fully complemented upon reintroduction of the virS gene into MtbdeltavirS. These observations show the important role of the mymA operon in the pathogenesis of M. tuberculosis at later stages of the disease.

The AraC family transcriptional regulator Rv1931c plays a role in the virulence of Mycobacterium tuberculosis

A Mycobacterium tuberculosis strain disrupted in the AraC homologue Rv1931c was isolated. The mutant strain exhibited reduced survival both in macrophages and in a mouse infection model, with survival being restored on complementation with the Rv1931c gene. These results suggest that Rv1931c regulates genes important for virulence of M. tuberculosis.

Induction of a novel class of diacylglycerol acyltransferases and triacylglycerol accumulation in Mycobacterium tuberculosis as it goes into a dormancy-like state in culture

Mycobacterium tuberculosis enters the host by inhalation of an infectious aerosol and replicates in the alveolar macrophages until the host's immune defense causes bacteriostasis, which leads the pathogen to go into nonreplicative drug-resistant dormancy. The dormant pathogen can survive for decades till the host's immune system is weakened and active tuberculosis develops. Even though fatty acids are thought to be the major energy source required for the persistence phase, the source of fatty acids used is not known. We postulate that the pathogen uses triacylglycerol (TG) as a storage form of fatty acids. Little is known about the biosynthesis of TG in M. tuberculosis. We show that 15 mycobacterial genes that we identified as putative triacylglycerol synthase (tgs) when expressed in Escherichia coli showed TGS activity, and we report some basic catalytic characteristics of the most active enzymes. We show that several tgs genes are induced when the pathogen goes into the nonreplicative drug-resistant state caused by slow withdrawal of O(2) and also by NO treatment, which is known to induce dormancy-associated genes. The gene (Rv3130c) that shows the highest TGS activity when expressed in E. coli shows the highest induction by hypoxia and NO treatment. Biochemical evidence shows that TG synthesis and accumulation occur under both conditions. We conclude that TG may be a form of energy storage for use during long-term dormancy. Therefore, TG synthesis may be an appropriate target for novel antilatency drugs that can prevent the organism from surviving dormancy and thus assist in the control of tuberculosis.