Summary A major obstacle to tuberculosis (TB) control is the problem of chronic TB infection (CTBI). NOS2-/-and gp91phox -/-mice given aminoguanidine [to suppress the effects of nitric oxide synthase 2 (NOS2)] -indicating minimal metabolic effect on the cysH mutant survival in these mice. The cysH mutant was also susceptible to peroxynitrite and hydrogen peroxide in vitro . These results show that CysH is important for Mtb protection during the chronic infection phase, and that resistance to nitrosative and oxidative stress may be the mechanism of this protection. Thus, this metabolic gene of an intracellular pathogen could have a secondary role in protection against the host immune response. Finally the lack of an endogenous human orthologue of cysH and its possible role in defence against adaptive immunity renders CysH an attractive enzyme for further studies as a target for therapeutics active against CTBI.

Bacterial sulfate assimilation pathways provide for activation of inorganic sulfur for the biosynthesis of cysteine and methionine, through either adenosine 5-phosphosulfate (APS) or 3-phosphoadenosine 5-phosphosulfate (PAPS) as intermediates. PAPS is also the substrate for sulfotransferases that produce sulfolipids, putative virulence factors, in Mycobacterium tuberculosis such as SL-1. In this report, genetic complementation using Escherichia coli mutant strains deficient in APS kinase and PAPS reductase was used to define the M. tuberculosis and Mycobacterium smegmatis CysH enzymes as APS reductases. Consequently, the sulfate assimilation pathway of M. tuberculosis proceeds from sulfate through APS, which is acted on by APS reductase in the first committed step toward cysteine and methionine. Thus, M. tuberculosis most likely produces PAPS for the sole use of this organism's sulfotransferases. Deletion of CysH from M. smegmatis afforded a cysteine and methionine auxotroph consistent with a metabolic branch point centered on APS. In addition, we have redefined the substrate specificity of the B. subtilis CysH, formerly designated a PAPS reductase, as an APS reductase, based on its ability to complement a mutant E. coli strain deficient in APS kinase. Together, these studies show that two conserved sequence motifs, CCXXRKXXPL and SXGCXXCT, found in the C termini of all APS reductases, but not in PAPS reductases, may be used to predict the substrate specificity of these enzymes. A functional domain of the M. tuberculosis CysC protein was cloned and expressed in E. coli, confirming the ability of this organism to make PAPS. The expression of recombinant M. tuberculosis APS kinase provides a means for the discovery of inhibitors of this enzyme and thus of the biosynthesis of SL-1.

Mycobacteria contain high levels of the disaccharide trehalose in free form as well as within various immunologically relevant glycolipids such as cord factor and sulfolipid-1. By contrast, most bacteria use trehalose solely as a general osmoprotectant or thermoprotectant. Mycobacterium tuberculosis and Mycobacterium smegmatis possess three pathways for the synthesis of trehalose. Most bacteria possess only one trehalose biosynthesis pathway and do not elaborate the disaccharide into more complex metabolites, suggesting a distinct role for trehalose in mycobacteria. We disabled key enzymes required for each of the three pathways in M. smegmatis by allelic replacement. The resulting trehalose biosynthesis mutant was unable to proliferate and enter stationary phase unless supplemented with trehalose. At elevated temperatures, however, the mutant was unable to proliferate even in the presence of trehalose. Genetic complementation experiments showed that each of the three pathways was able to recover the mutant in the absence of trehalose, even at elevated temperatures. From a panel of trehalose analogs, only those with the native ␣,␣-(1,1) anomeric stereochemistry rescued the mutant, whereas alternate stereoisomers and general osmo-and thermoprotectants were inactive. These findings suggest a dual role for trehalose as both a thermoprotectant and a precursor of critical cell wall metabolites.Trehalose (␣-D-glucopyranosyl-(1,1)-␣-D-glucopyranoside, Fig.

Sulfolipid-1 (SL-1) is an abundant sulfated glycolipid and potential virulence factor found in Mycobacterium tuberculosis. SL-1 consists of a trehalose-2-sulfate (T2S) disaccharide elaborated with four lipids. We identified and characterized a conserved mycobacterial sulfotransferase, Stf0, which generates the T2S moiety of SL-1. Biochemical studies demonstrated that the enzyme requires unmodified trehalose as substrate and is sensitive to small structural perturbations of the disaccharide. Disruption of stf0 in Mycobacterium smegmatis and M. tuberculosis resulted in the loss of T2S and SL-1 formation, respectively. The structure of Stf0 at a resolution of 2.6 A reveals the molecular basis of trehalose recognition and a unique dimer configuration that encloses the substrate into a bipartite active site. These data provide strong evidence that Stf0 carries out the first committed step in the biosynthesis of SL-1 and establish a system for probing the role of SL-1 in M. tuberculosis infection.

The Mycobacterium tuberculosis genome contains four copies of an operon called mce (mce1-4). Previously we reported that M. tuberculosis disrupted in the mce1 operon is more virulent than wild-type M. tuberculosis in mice. We generated single deletion mutants in mce3 (Dmce3) and mce4 (Dmce4) operons and a double deletion mutant (Dmce3/4). Similar doubling times and growth characteristics were observed for all mutants and the wild-type (parent) M. tuberculosis H37Rv strain in culture and in macrophages. In addition, similar bacterial burdens were detected in organs from mice infected with Dmce3 and the parent strain. However, the bacterial burdens of mice infected with Dmce4 and Dmce 3/4 were less than those of mice infected with the parent strain. The median survival times of mice infected with wild-type M. tuberculosis, Dmce3, Dmce4 and Dmce3/4 were 40.5, 46, 58 and 62 weeks, respectively. Histopathological examination of lungs at 15 weeks post-infection showed that the extent of the lung lesions was less prominent in mice infected with Dmce4 and Dmce 3/4 mutants than in mice infected with the other two strains. These observations suggest that the mce3 and mce4 operons have a role distinct from that of mce1 for in vivo survival of M. tuberculosis.

The study of the metabolome presents numerous challenges, first among them being the cataloging of its constituents. A step in this direction will be the development of tools to identify metabolites that share common structural features. The importance of sulfated molecules in cell-cell communication motivated us to develop a rapid two-step method for identifying these metabolites in microorganisms, particularly in pathogenic mycobacteria. Sulfurcontaining molecules were initially identified by mass spectral analysis of cell extracts from bacteria labeled metabolically with a stable sulfur isotope ( 34 SO 4 2؊ ). To differentiate sulfated from reduced-sulfur-containing molecules, we employed a mutant lacking the reductive branch of the sulfate assimilation pathway. In these sulfur auxotrophs, heavy sulfate is channeled exclusively into sulfated metabolites. The method was applied to the discovery of several new sulfated molecules in Mycobacterium tuberculosis and Mycobacterium smegmatis. Because a sulfur auxotrophic strain is the only requirement of the approach, many microorganisms can be studied in this manner. Such genetic engineering in combination with stable isotopic labeling can be applied to various metabolic pathways and their products.T he modification of primary and secondary metabolites by the addition or removal of sulfate can have a profound influence on their biological properties (1-5). Typically, sulfated molecules are directed outside the cell, where they serve as modulators of cell-cell interactions. As a notable example pertinent to human health, sulfation of the glycans of endothelial CD34 engenders high-affinity binding with the leukocyte adhesion molecule L-selectin, facilitating an interaction that eventually leads to the recruitment of lymphocytes into peripheral lymph nodes (6). Similarly, sulfation of tyrosyl residues found on the chemokine receptor CCR5 is a modification required for binding of HIV gp120 and therefore efficient viral entry (7).The roles of sulfated compounds in prokaryotes and other microbes are less clear. In one well-characterized case, however, sulfation acts as a modulator of cell-cell communication, similar to its role in eukaryotes. The nitrogen-fixing bacterium Sinorhizobium meliloti utilizes a sulfated glycolipid as a secondary messenger to induce root nodulation in its plant host alfalfa (4,8). Mutants lacking the sulfotransferase that installs this sulfate ester are unable to induce root nodulation in alfalfa but gain the ability to colonize the roots of vetch. That sulfation of a single glycolipid plays such a vital role in nitrogen fixation has far-reaching implications for the agricultural community and presents a possible target for chemical or genetic engineering.Sulfation may also be relevant to the process of bacterial pathogenesis (9). Several mycobacteria, including the human pathogens Mycobacterium tuberculosis and Mycobacterium avium, are known to produce sulfated compounds. One example is Sulfatide-1 (SL-1, Fig. 1A), a sulfated glycolipid from M....

Mycobacteria are known to acquire resistance to the antituberculous drug pyrazinamide (PZA) through mutations in the gene encoding pyrazinamidase (PZase), an enzyme that converts PZA into pyrazinoic acid, the presumed active form of PZA against bacteria. Additional mechanisms of resistance to the drug are known to exist but have not been fully investigated. Among these is the non-uptake of the pro-drug, a possibility investigated in the present study. The uptake mechanism of PZA, a requisite step for the activation of the pro-drug, was studied in Mycobacterium tuberculosis. The incorporation of [14C]PZA by the bacilli was followed in both neutral and acidic environments since PZA activity is known to be optimal a t acidic pH. By using a protonophore (carbonyl cyanide m-chlorophenylhydrazone; CCCP), valinomycin, arsenate and low temperature, it was shown that an ATP-dependent transport system is involved in the uptake of PZA. Whilst the structurally analogous compound nicotinamide inhibited the transport system of PZA, other structurally related compounds such as pyrazinoic acid, isoniazid and cytosine did not. Acidic conditions were also without effect. Based on diffusion experiments in liposomes, it was found that PZA diffuses rapidly through membrane bilayers, faster than glycerol, whilst the presence of OmpATb, the porin-like protein of M. tuberculosis, in proteoliposomes slightly increased the diffusion of the drug. This finding may explain why the cell wall mycolate hydrophobic layer does not represent the limiting step in the diffusion of PZA, as judged from comparative experiments using a M. tuberculosis strain and its isogenic mutant elaborating 40 O/ O less covalently linked mycolates. PZase activity, and PZA uptake and susceptibility in different mycobacterial species were compared. M. tuberculosis, a naturally PZA-susceptible species, was the only species that exhibited both PZase activity and PZA uptake; no such correlation was observed with the four naturally resistant species examined. Mycobacterium smegmatis possessed a functional PZase but did not take up PZA; the reverse was true for the PZase-negative strain of Mycobacterium avium used, with PZA uptake comparable to that of M. tuberculosis. Mycobacterium bovis BCG and Mycobacterium kansasii exhibited neither a PZase activity nor PZA uptake. These data clearly demonstrate that one of the mechanisms of resistance to PZA resides in the failure of strains to take up the drug, indicating that susceptibility to PZA in mycobacteria requires both the presence of a functional PZase and a PZA transport system. No correlation was observed between the occurrence and cellular location of PZase and of nicotinamidase in the strains examined, suggesting that one or both amides can be hydrolysed by other mycobacterial amidases.

One key adaptation that Mycobacterium tuberculosis established to survive long term in vivo is a reliance on lipids as an energy source. M. tuberculosis H37Rv has 36 fadD genes annotated as putative fatty acyl-CoA synthetase genes, which encode enzymes that activate fatty acids for metabolism. One such gene, fadD5 (Rv0166), is located within the mce1 operon, a cluster of genes associated with M. tuberculosis persistence. We disrupted the putative fatty acid binding site of fadD5 in H37Rv M. tuberculosis. No significant differences were found in the growth of the mutant and wild-type strains in vitro in nutrient-rich broth or in activated RAW264.7 cells. However, the fadD5 mutant was diminished in growth in minimal medium containing mycolic acid, but not other long-chain fatty acids. C57BL/6 mice infected with the fadD5 mutant survived significantly longer than those infected with wild-type, and the mutant never attained the plateau phase of infection in the mouse lungs. The steady-state infection phase was maintained for up to 168 days at a level one to two logs less than that shown by wild-type. These observations raise a rather intriguing possibility that FadD5 may serve to recycle mycolic acids for the long-term survival of the tubercle bacilli.