Transient requirement of the PrrA-PrrB two-component system for early intracellular multiplication of Mycobacterium tuberculosis

Structural and functional aspects of the sensor histidine kinase PrrB from Mycobacterium tuberculosis

We describe the solution structures of two- and three-domain constructs of the sensor histidine kinase PrrB from Mycobacterium tuberculosis, which allow us to locate the HAMP linker relative to the ATP binding and dimerization domains. We show that the three-domain construct is active both for autophosphorylation and for phosphotransfer to the cognate response regulator, PrrA. We also describe the high-resolution crystal structure of the catalytic domain alone, and we show that, in solution, it binds ATP. The conformational flexibility of this domain is discussed and related to other structural information.

The Structural Basis of Signal Transduction for the Response Regulator PrrA from Mycobacterium tuberculosis

The structure of the two-domain response regulator PrrA from Mycobacterium tuberculosis shows a compact structure in the crystal with a well defined interdomain interface. The interface, which does not include the interdomain linker, makes the recognition helix and the trans-activation loop of the effector domain inaccessible for interaction with DNA. Part of the interface involves hydrogen-bonding interactions of a tyrosine residue in the receiver domain that is believed to be involved in signal transduction, which, if disrupted, would destabilize the interdomain interface, allowing a more extended conformation of the molecule, which would in turn allow access to the recognition helix. In solution, there is evidence for an equilibrium between compact and extended forms of the protein that is far toward the compact form when the protein is inactivated but moves toward a more extended form when activated by the cognate sensor kinase PrrB.

The Mycobacterium tuberculosis PhoPR two-component system regulates genes essential for virulence and complex lipid biosynthesis

Two-component signal transduction systems (2-CS) play an important role in bacterial pathogenesis. In the work presented here, we have studied the effects of a mutation in the Mycobacterium tuberculosis (Mtb) PhoPR 2-CS on the pathogenicity, physiology and global gene expression of this bacterial pathogen. Disruption of PhoPR causes a marked attenuation of growth in macrophages and mice and prevents growth in low-Mg2+ media. The inability to grow in THP-1 macrophages can be partially overcome by the addition of excess Mg2+ during infection. Global transcription assays demonstrate PhoP is a positive transcriptional regulator of several genes, but do not support the hypothesis that the Mtb PhoPR system is sensing Mg2+ starvation, as is the case with the Salmonella typhimurium PhoPQ 2-CS. The genes that were positively regulated include those found in the pks2 and the msl3 gene clusters that encode enzymes for the biosynthesis of sulphatides and diacyltrehalose and polyacyltrehalose respectively. Complementary biochemical studies, in agreement with recent results from another group, indicate that these complex lipids are also absent from the phoP mutant, and the lack of these components in its cell envelope may indirectly cause the mutant's high-Mg2+ growth requirement. The experiments reported here provide functional evidence for the PhoPR 2-CS involvement in Mtb pathogenesis, and they suggest that a major reason for the attenuation observed in the phoP mutant is the absence of certain complex lipids that are known to be important for virulence.

Regulation of bacterial virulence by two-component systems

In bacteria, two-component systems (TCS) are widely used signal transduction devices which are engaged in a multitude of gene regulatory systems that respond to changing growth conditions. Many pathogenic bacteria encounter different microenvironments during their infectious cycle and their ability to efficiently adapt to different niches inside and outside of their host organisms is frequently mediated by TCSs, which can, therefore, be considered as an essential prerequisite for their pathogenicity. Although significant progress has been made in the elucidation of basic principles of the signal transduction process itself, in many pathogens the contribution of TCS to bacterial virulence is insufficiently recognized.

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.

A two-component signal transduction system with a PAS domain-containing sensor is required for virulence of Mycobacterium tuberculosis in mice

Mycobacterium tuberculosis, the causative organism of tuberculosis, encounters oxidative stress during phagocytosis by the macrophage and following macrophage activation during an acquired immune response, and also from internally generated sources of radical oxygen intermediates through intermediary metabolism. We have identified the SenX3 protein, a sensor in 1 of the 11 complete pairs of two-component signal transduction systems in M. tuberculosis, as a possible orthologue of the Mak2p protein from the fission yeast Schizosaccharomyces pombe that is known to sense peroxide stress. Moreover, the SenX3-RegX3 two-component system was the top scoring hit in a homology search with the Escherichia coli ArcB-ArcA global control system of aerobic genes. Using structural modelling techniques we have determined that SenX3 contains a PAS-like domain found in a variety of prokaryotic and eukaryotic sensors of oxygen and redox. Mutants with knock-outs of senX3 or of the accompanying transcriptional regulator regX3 were constructed and found to have reduced virulence in a mouse model of tuberculosis infection, the mutant bacteria persisting for up to 4 months post-infection; complemented mutants had regained virulence confirming that it was mutations of this two-component system that were responsible for the avirulent phenotype. This work identifies the PAS domain as a possible drug target for tuberculosis and mutations in the senX3-regX signal transduction system as potentially useful components of live vaccine strains.

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.

Intracellular autoregulation of the Mycobacterium tuberculosis PrrA response regulator

Two-component systems are major regulatory systems for bacterial adaptation to environmental changes. During the infectious cycle of Mycobacterium tuberculosis, adaptation to an intracellular environment is critical for multiplication and survival of the micro-organism within the host. The M. tuberculosis prrA gene, encoding the regulator of the two-component system PrrA-PrrB, has been shown to be induced upon macrophage phagocytosis and to be transiently required for the early stages of macrophage infection. In order to study the mechanisms of regulation of the PrrA-PrrB two-component system, PrrA and the cytoplasmic part of the PrrB histidine kinase were produced and purified as hexahistidine-tagged recombinant proteins. Electrophoretic mobility shift assays indicated that PrrA specifically binds to the promoter of its own operon, with increased affinity upon phosphorylation. Moreover, induction of fluorescence was observed after phagocytosis of a wild-type M. tuberculosis strain containing the gfp reporter gene under the control of the prrA-prrB promoter, while this induction was not seen in a prrA/B mutant strain containing the same construct. These results indicate that the early intracellular induction of prrA depends on the autoregulation of this two-component system.

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.

Rapid and efficient transposon mutagenesis of Bartonella henselae by transposome technology

Molecular genetics are difficult to perform in Bartonella henselae, the causative agent of cat scratch disease and the vasculoproliferative disorders bacillary angiomatosis and bacillary peliosis. To elucidate the underlying bacterial pathogenic mechanisms, genetic manipulation of B. henselae is the method of choice. We describe how to perform transposon mutagenesis in B. henselae using transposome technology. B. henselae mutants revealed by this technique showed random transpositional insertion into the chromosome. In contrast to transposon mutagenesis by conjugational transfer, transposome technology allows transposon mutagenesis of early passaged Bartonella spp. with approximately 100-fold higher efficiency. The results show that transposome technique is a rapid, efficient and simple method to generate transposon mutants of B. henselae.

Mycobacterium tuberculosis pathogenesis and molecular determinants of virulence

Tuberculosis (TB), one of the oldest known human diseases. is still is one of the major causes of mortality, since two million people die each year from this malady. TB has many manifestations, affecting bone, the central nervous system, and many other organ systems, but it is primarily a pulmonary disease that is initiated by the deposition of Mycobacterium tuberculosis, contained in aerosol droplets, onto lung alveolar surfaces. From this point, the progression of the disease can have several outcomes, determined largely by the response of the host immune system. The efficacy of this response is affected by intrinsic factors such as the genetics of the immune system as well as extrinsic factors, e.g., insults to the immune system and the nutritional and physiological state of the host. In addition, the pathogen may play a role in disease progression since some M. tuberculosis strains are reportedly more virulent than others, as defined by increased transmissibility as well as being associated with higher morbidity and mortality in infected individuals. Despite the widespread use of an attenuated live vaccine and several antibiotics, there is more TB than ever before, requiring new vaccines and drugs and more specific and rapid diagnostics. Researchers are utilizing information obtained from the complete sequence of the M. tuberculosis genome and from new genetic and physiological methods to identify targets in M. tuberculosis that will aid in the development of these sorely needed antitubercular agents.

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.

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

Two-component regulatory signal transduction systems are widely distributed among bacteria and enable the organisms to make coordinated changes in gene expression in response to a variety of environmental stimuli. The genome sequence of Mycobacterium tuberculosis contains 11 complete two-component systems, four isolated homologous regulators, and three isolated homologous sensors. We have constructed defined mutations in six of these genes and measured virulence in a SCID mouse model. Mice infected with four of the mutants (deletions of devR, tcrXY, trcS, and kdpDE) died more rapidly than those infected with wild-type bacteria. The other two mutants (narL and Rv3220c) showed no change compared to the wild-type H37Rv strain. The most hypervirulent mutant (devRdelta) also grew more rapidly in the acute stage of infection in immunocompetent mice and in gamma interferon-activated macrophages. These results define a novel class of genes in this pathogen whose presence slows down its multiplication in vivo or increases its susceptibility to host killing mechanisms. Thus, M. tuberculosis actively maintains a balance between its own survival and that of the host.

Molecular mechanisms regulating persistent Mycobacterium tuberculosis infection

Establishing persistent infection and resisting elimination by the host's immune system are key factors contributing to latent infection by Mycobacterium tuberculosis. Recently, bacterial determinants regulating these processes have been identified. Here, we review molecular mechanisms regulating persistent infection and discuss the highly dynamic interaction of M. tuberculosis with the host.

Interaction of the sensor module of Mycobacterium tuberculosis H37Rv KdpD with members of the Lpr family

The genetic and biochemical mechanisms by which Mycobacterium tuberculosis senses and responds to the complex environment that it encounters during infection and persistence within the host remain unknown. In a number of bacterial species, the Kdp signal transduction pathway appears to be the primary response to environmental osmotic stress, which is primarily mediated by K+ concentration in bacteria. We show that kdp encodes for components of a mycobacterial signalling pathway by demonstrating the K+ dependence of kdpFABC expression in both M. tuberculosis H37Rv and Mycobacterium smegmatis. To identify proteins of M. tuberculosis that participate in this signalling pathway, we used the N-terminal sensing module of the histidine kinase KdpD as bait in a yeast two-hybrid screen. We show that the sensing domain of KdpD interacts specifically with two membrane lipoproteins, LprJ (Rv1690) and LprF (Rv1368). Overexpression of lprF and lprJ alleles in mycobacterial kdpF-lacZ reporter strains enabled us to identify alleles that modulate kdpFABC expression. By exploiting the yeast three-hybrid system, we have found that the histidine kinase domain of KdpD forms ternary complexes with LprF and LprJ and the sensing module of KdpD. Our results establish a role for membrane proteins in the Kdp signalling pathway and suggest that LprF and LprJ function as accessory or ligand-binding proteins that communicate directly with the sensing domain of KdpD to modulate kdp expression.