Deletion of two-component regulatory systems increases the virulence of Mycobacterium tuberculosis

Tuberculosis: a problem with persistence

Mycobacterium tuberculosis is one of most successful pathogens of mankind, infecting one-third of the global population and claiming two million lives every year. The ability of the bacteria to persist in the form of a long-term asymptomatic infection, referred to as latent tuberculosis, is central to the biology of the disease. The persistence of bacteria in superficially normal tissue was recognized soon after the discovery of the tubercle bacillus, and much of our knowledge about persistent populations of M. tuberculosis dates back to the first half of the last century. Recent advances in microbial genetics and host immunity provide an opportunity for renewed investigation of this persistent threat to human health.

Two sensor kinases contribute to the hypoxic response of Mycobacterium tuberculosis

Current estimates indicate that nearly a third of the world's population is latently infected with Mycobacterium tuberculosis. Reduced oxygen tension and nitric oxide exposure are two conditions encountered by bacilli in vivo that may promote latency. In vitro exposure to hypoxia or nitric oxide results in bacterial stasis with concomitant induction of a 47-gene regulon controlled by the transcription factor DosR. In this report we demonstrate that both the dosS gene adjacent to dosR and another gene, dosT (Rv2027c), encode sensor kinases, each of which can autophosphorylate at a conserved histidine and then transfer phosphate to an aspartate residue of DosR. Mutant bacteria lacking both sensors are unable to activate expression of DosR-regulated genes. These data indicate that DosR/DosS/DosT comprise a two-component signaling system that is required for the M. tuberculosis genetic response to hypoxia and nitric oxide, two conditions that produce reversible growth arrest in vitro and may contribute to latency in vivo.

Disruption of response regulator gene,devR, leads to attenuation in virulence ofMycobacterium tuberculosis

The devR-devS two-component system of Mycobacterium tuberculosis was identified earlier and partially characterized in our laboratory. A devR::kan mutant of M. tuberculosis was constructed by allelic exchange. The devR mutant strain showed reduced cell-to-cell adherence in comparison to the parental strain in laboratory culture media. This phenotype was reversed on complementation with a wild-type copy of devR. The devR mutant and parental strains grew at equivalent rates within human monocytes either in the absence or in the presence of lymphocytic cells. The expression of DevR was not modulated upon entry of M. tuberculosis into human monocytes. However, guinea pigs infected with the mutant strain showed a significant decrease in gross lesions in lung, liver and spleen; only mild pathological changes in liver and lung; and a nearly 3 log lower bacterial burden in spleen compared to guinea pigs infected with the parental strain. Our results suggest that DevR is required for virulence in guinea pigs but is not essential for entry, survival and multiplication of M. tuberculosis within human monocytes in vitro. The attenuation in virulence of the devR mutant in guinea pigs together with DevR-DevS being a bona fide signal transduction system indicates that DevR plays a critical and regulatory role in the adaptation and survival of M. tuberculosis within tissues.

The knockout of the lprG-Rv1410 operon produces strong attenuation of Mycobacterium tuberculosis

P27 lipoprotein was previously described as an antigen in the Mycobacterium tuberculosis complex, encoded by the lprG gene, also named Rv1411 in the TubercuList (http://genolist.pasteur.fr/TubercuList) gene bank. It forms an operon with Rv1410 that encodes for an efflux pump, P55. A mutant of the H37Rv strain of M. tuberculosis not producing P27 (strain DeltaP27) was obtained by two-step mutagenesis using the counterselectable marker sacB and a thermosensitive origin of replication in the shuttle plasmid pPR27. By RT-PCR, we observed no lprG or Rv1410 mRNA in the DeltaP27 mutant strain compared with the wild type and complemented strains. Western blot experiments using anti-P27 polyclonal sera showed that the P27 protein was present both in the parental and in a complemented strain, in which the entire lprG-Rv1410 operon was reintroduced, but absent in the mutant strain. The three strains showed similar growth kinetics and characteristics in culture broth. To study the effect of the lprG mutation on M. tuberculosis virulence, BALB/c mice were inoculated to determine bacterial loads in spleens. At days 15 and 35 after infection, decreases of 1.5 and 2.5 logs in the bacterial load were found, respectively, in animals inoculated with the DeltaP27 mutant strain or with the wild type. This attenuation was reverted in the complemented strain. These results demonstrated that lprG gene is required for growth of M. tuberculosis in immunocompetent mice. The reversion of attenuation in the complemented strain indicates that the attenuated phenotype resulted from disruption of the lprG-Rv1410 operon.

Functional analysis of the Mycobacterium tuberculosis MprAB two-component signal transduction system

The mechanisms utilized by Mycobacterium tuberculosis to establish, maintain, or reactivate from latent infection in the host are largely unknown but likely include genes that mediate adaptation to conditions encountered during persistence. Previously, a two-component signal transduction system, mprAB, was found to be required in M. tuberculosis for establishment and maintenance of persistent infection in a tissue- and stage-specific fashion. To begin to characterize the role of this system in M. tuberculosis physiology and virulence, a functional analysis of the mprA and mprB gene products was initiated. Here, evidence is presented demonstrating that sensor kinase MprB and response regulator MprA function as an intact signal-transducing pair in vitro and in vivo. Sensor kinase MprB can be autophosphorylated, can donate phosphate to MprA, and can act as a phospho-MprA phosphatase in vitro. Correspondingly, response regulator MprA can accept phosphate from MprB or from small phosphodonors including acetyl phosphate. Mutagenesis of residues His249 in MprB and Asp48 in MprA abolished the ability of these proteins to be phosphorylated in vitro. Introduction of these alleles into Mycobacterium bovis BCG attenuated virulence in macrophages in vivo. Together, these results support a role for the mprAB two-component system in M. tuberculosis physiology and pathogenesis. Characterization of two-component signal transduction systems will enhance our understanding of processes regulated by M. tuberculosis during acute and/or persistent infection in the host.

The Mycobacterium tuberculosis dosRS two-component system is induced by multiple stresses

Induction of the Mycobacterium tuberculosis dosR gene, which is known to respond to hypoxia, was measured using RTq-PCR following exposure to different stresses. Increased expression was seen after exposure to S-nitrosoglutathione (GSNO), ethanol and (to a lesser extent) H2O2, but not heat- or cold-shock. We also demonstrated that hspX, which is dependent on dosR for expression, is induced when cultures are left standing for 30 min, while significant but minor induction was seen following a 10 min centrifugation. Microarray analysis was used to compare gene expression in wild-type and deltadosR strains following 30 min standing. Fifty-two genes were significantly up-regulated, and 19 genes were down-regulated. These included genes that had previously been reported as being part of the dosR regulon, and also some novel ones.

Hypervirulent mutant of Mycobacterium tuberculosis resulting from disruption of the mce1 operon

An estimated one-third of the world's population is latently infected with Mycobacterium tuberculosis, the etiologic agent of tuberculosis. Here, we demonstrate that, unlike wild-type M. tuberculosis, a strain of M. tuberculosis disrupted in the mce1 operon was unable to enter a stable persistent state of infection in mouse lungs. Instead, the mutant continued to replicate and killed the mice more rapidly than did the wild-type strain. Histological examination of mouse lungs infected with the mutant strain revealed diffusely organized granulomas with aberrant inflammatory cell migration. Murine macrophages infected ex vivo with the mutant strain were reduced in their ability to produce tumor necrosis factor alpha, IL-6, monocyte chemoattractant protein 1, and nitric oxide (NO), but not IL-4. The mce1 mutant strain complemented with the mce1 genes stimulated tumor necrosis factor alpha and NO production by murine macrophages at levels stimulated by the wild-type strain. These observations indicate that the mce1 operon mutant is unable to stimulate T helper 1-type immunity in mice. The hypervirulence of the mutant strain may have resulted from its inability to stimulate a proinflammatory response that would otherwise induce organized granuloma formation and control the infection without killing the organism. The mce1 operon of M. tuberculosis may be involved in modulating the host inflammatory response in such a way that the bacterium can enter a persistent state without being eliminated or causing disease in the host.

Functional profiling of a human cytomegalovirus genome

Human cytomegalovirus (HCMV), a ubiquitous herpesvirus, causes a lifelong subclinical infection in healthy adults but leads to significant morbidity and mortality in neonates and immunocompromised individuals. Its ability to grow in different cell types is responsible for HCMV-associated diseases, including mental retardation and retinitis, and vascular disorders. To globally assess viral gene function for replication in cells, we determined the genomic sequence of a bacterial artificial chromosome (BAC)-based clone of HCMV Towne strain and used this information to delete each of its 162 unique ORFs and generate a collection of viral mutants. The growth of these mutants in different cultured cells was examined to systematically investigate the necessity of each ORF for replication. Our results showed that 45 ORFs are essential for viral replication in fibroblasts and 117 are nonessential. Some genes were found to be required for viral replication in retinal pigment epithelial cells and microvascular endothelial cells, but not in fibroblasts, indicating their role as tropism factors. Interestingly, several viral mutants grew 10- to 500-fold better than the parental strain in different cell types, suggesting that the deleted ORFs encode replication temperance or repressing functions. Thus, HCMV encodes supportive and suppressive growth regulators for optimizing its replication in human fibroblasts, epithelial, and endothelial cells. Suppression of viral replication by virus-encoded temperance factors represents a novel mechanism for regulating the growth of an animal virus, and may contribute to HCMV's optimal infection of different tissues and successful proliferation among the human population.

Mycobacterium tuberculosis Rv1395 is a class III transcriptional regulator of the AraC family involved in cytochrome P450 regulation

Rv1395 is annotated as a potential transcriptional regulator of the AraC family. The Rv1395 insertional mutant was identified in a signature tag mutagenesis study in Mycobacterium tuberculosis and was shown to be attenuated in the lungs of mice. Here, we used comparative genomics and biochemical methods to show that Rv1395 is unique to the M. tuberculosis complex and that it encodes a protein that binds the region between two divergent genes, a member of the cytochrome P450 family (Rv1394c or cyp132) and Rv1395 itself. Rv1395 binds to this DNA region by its helix-turn-helix-containing C-terminal domain, and it recognizes two sites with different affinity. We identified the transcriptional start points (TSP) of Rv1394c and Rv1395: both genes have two TSPs, three of which are located in the intergenic region. We constructed and compared various transcriptional fusions consisting of the promoter regions and a reporter gene in Mycobacterium smegmatis: this showed that Rv1395 induces the expression of the cytochrome P450 gene (Rv1394c) and represses its own transcription. This was confirmed in M. tuberculosis when the wild type and a Rv1395-overexpressing strain were used as hosts for the fusions. Site-directed mutagenesis showed that Rv1395 binds to the two sites in a co-operative manner and that binding to both sites is required for Rv1395 optimal activity. A model describing the potential mode of action of Rv1395 is discussed.

The senX3-regX3 two-component regulatory system of Mycobacterium tuberculosis is required for virulence

Two-component regulatory systems have been widely implicated in bacterial virulence. To investigate the role of one such system in Mycobacterium tuberculosis, a strain was constructed in which the senX3-regX3 system was deleted by homologous recombination. The mutant strain (Tame15) showed a growth defect after infection of macrophages and was attenuated in both immunodeficient and immunocompetent mice. Competitive hybridization of total RNA from the wild-type and mutant strains to a whole-genome microarray was used to identify changes in gene expression resulting from the deletion. One operon was highly up-regulated in the mutant, indicating that regX3 probably has a role as a repressor of this operon. Other genes which were up- or down-regulated were also identified. Many of the genes showing down-regulation are involved in normal growth of the bacterium, indicating that the mutant strain is subject to some type of growth slow-down or stress. Genes showing differential expression were further grouped according to their pattern of gene expression under other stress conditions. From this analysis 50 genes were identified which are the most likely to be controlled by RegX3. Most of these genes are of unknown function and no obvious motifs were found upstream of the genes identified. Thus, it has been demonstrated that the senX3-regX3 two-component system is involved in the virulence of M. tuberculosis and a number of genes controlled by this system have been identified.