Hypervirulent mutants of Mycobacterium tuberculosis, whose growth rates are higher in vivo, have now been reported to have mutations in both regulatory and structural genes, but the basis for this unusual phenotype is not understood. One hypervirulence gene, dosR (devR, Rv2031c), activates transcription of approximately 50 genes in this pathogen in response to hypoxia and nitric oxide stress. The most dramatic activation (ϳ80-fold) is activation of the hspX (acr, Rv2031c) gene, which encodes a 16-kDa ␣-crystallin-like protein that is a major antigen. In this study we found that a ⌬acr mutant exhibited increased growth following infection of BALB/c mice in vivo and in both resting and activated macrophages in vitro (as measured by the number of CFU). The increased growth in macrophages was equal to that of a ⌬dosR mutant, while introduction of a constitutively expressed hspX gene reduced the ⌬dosR virulence to wild-type levels. These results suggest that the increased number of CFU of the ⌬dosR mutant was largely due to loss of hspX expression. We also confirmed that constitutive expression of hspX slows growth in vitro, and we propose that hspX plays an active role in slowing the growth of M. tuberculosis in vivo immediately following infection.

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SummaryPathogenic mycobacteria have the ability to persist in phagocytic cells and to suppress the immune system. The glycolipid lipoarabinomannan (LAM), in particular its mannose cap, has been shown to inhibit phagolysosome fusion and to induce immunosuppressive IL-10 production via interaction with the mannose receptor or DC-SIGN. Hence, the current paradigm is that the mannose cap of LAM is a crucial factor in mycobacterial virulence. However, the above studies were performed with purified LAM, never with live bacteria. Here we evaluate the biological properties of capless mutants of Mycobacterium marinum and M. bovis BCG, made by inactivating homologues of Rv1635c. We show that its gene product is an undecaprenyl phosphomannose-dependent mannosyltransferase. Compared with parent strain, capless M. marinum induced slightly less uptake by and slightly more phagolysosome fusion in infected macrophages but this did not lead to decreased survival of the bacteria in vitro, nor in vivo in zebra fish. Loss of caps in M. bovis BCG resulted in a sometimes decreased binding to human dendritic cells or DC-SIGN-transfected Raji cells, but no differences in IL-10 induction were observed. In mice, capless M. bovis BCG did not survive less well in lung, spleen or liver and induced a similar cytokine profile. Our data contradict the current paradigm and demonstrate that mannose-capped LAM does not dominate the Mycobacterium-host interaction.

Inositol is utilized by Mycobacterium tuberculosis in the production of its major thiol and of essential cell wall lipoglycans. We have constructed a mutant lacking the gene encoding inositol-1-phosphate synthase (ino1), which catalyses the first committed step in inositol synthesis. This mutant is only viable in the presence of extremely high levels of inositol. Mutant bacteria cultured in inositol-free medium for four weeks showed a reduction in levels of mycothiol, but phosphatidylinositol mannoside, lipomannan and lipoarabinomannan levels were not altered. The ino1 mutant was attenuated in resting macrophages and in SCID mice. We used site-directed mutagenesis to alter four putative active site residues; all four alterations resulted in a loss of activity, and we demonstrated that a D310N mutation caused loss of the active site Zn2+ ion and a conformational change in the NAD+ cofactor.

Phosphatidylinositol (PI) is essential for Mycobacterium tuberculosis viability and the enzymes involved in the PI biosynthetic pathway are potential antimycobacterial agents for which little structural information is available. The rate-limiting step in the pathway is the production of (L)-myo-inositol 1-phosphate from (D)-glucose 6-phosphate, a complex reaction catalyzed by the enzyme inositol 1-phosphate synthase. We have determined the crystal structure of this enzyme from Mycobacterium tuberculosis (tbINO) at 1.95 A resolution, bound to the cofactor NAD+. The active site is located within a deep cleft at the junction between two domains. The unexpected presence of a zinc ion here suggests a mechanistic difference from the eukaryotic inositol synthases, which are stimulated by monovalent cations, that may be exploitable in developing selective inhibitors of tbINO.

SummaryThe recA gene of Mycobacterium smegmatis has been cloned and sequenced. The amino acid sequence of the RecA protein is highly homologous to other RecA proteins. Three other potential open reading frames were identified. One of these showed extensive homology to a protein, HypB, involved in the incorporation of nickel into hydrogenases. Another, found downstream of and overlapping recA, was similar to a gene, recX, which has been proposed to play a regulatory role related to recA function. The homology between the M. smegmatis sequence and that of Mycobacterium tuberculosis extended upstream of the recA coding region for 140 bp including a motif identical to the Cheo-box consensus sequence which has been shown to bind LexA. In addition, the transcriptional start sites were found to be identical to those identified previously for M. tuberculosis. Transcriptional fusions to the reporter gene chloramphenicol acetyltransferase (CAT) revealed that recA was DNA-damage inducible and that expression required sequences at some distance from the mapped transcriptional start sites. Although a motif with only one mismatch to the Cheo box was found in the intergenic region between orf1 and orf2 these open reading frames were not DNA-damage inducible, nor was this motif required for regulation of recA expression. Gel retardation assays revealed that the reason for this was that LexA did not bind to this sequence containing a mismatch. Reverse transcription/polymerase chain reaction analysis of M. smegmatis RNA demonstrated that recA and orf3 (recX ) are within the same trancriptional unit.

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