Distribution of thiols in microorganisms: mycothiol is a major thiol in most actinomycetes

A coupled spectrophotometric assay for l-cysteine:1-D-myo-inosityl 2-amino-2-deoxy-alpha-D-glucopyranoside ligase and its application for inhibitor screening

Most actinomycetes, including Mycobacterium tuberculosis, do not produce glutathione but make an alternative thiol, mycothiol, which has functions similar to those of glutathione. A key step in mycothiol biosynthesis is the ATP-dependent ligation of Cys to GlcN-Ins catalyzed by MshC to produce Cys-GlcN-Ins, AMP, and PP(i). MshC is essential for growth of M. tuberculosis and is therefore a potential target for drugs directed against tuberculosis. A coupled-enzyme assay for MshC was developed using pyrophosphatase to convert pyrophosphate to phosphate and spectrophotometric detection of the latter via the phosphomolybdate complex with malachite green. The assay was readily adapted for use in a 96-well microtiter plate format. A secondary high-performance liquid chromatography assay measuring Cys-GlcN-Ins production was used to validate potential hits. Preliminary testing on a library of 2,024 compounds predicted to inhibit ATP-dependent enzymes identified many promiscuous and pyrophosphatase inhibitors of MshC and a single validated inhibitor with IC(50) approximately 100 microM.

Global control of cysteine metabolism by CymR in Bacillus subtilis

YrzC has previously been identified as a repressor controlling ytmI expression via its regulation of YtlI activator synthesis in Bacillus subtilis. We identified YrzC as a master regulator of sulfur metabolism. Gene expression profiles of B. subtilis delta yrzC mutant and wild-type strains grown in minimal medium with sulfate as the sole sulfur source were compared. In the mutant, increased expression was observed for 24 genes previously identified as repressed in the presence of sulfate. Since several genes involved in the pathways leading to cysteine formation were found, we propose to rename YrzC CymR, for "cysteine metabolism repressor." A CymR-dependent binding to the promoter region of the ytlI, ssuB, tcyP, yrrT, yxeK, cysK, or ydbM gene was demonstrated using gel shift experiments. A potential CymR target site, TAAWNCN2ANTWNAN3ATMGGAATTW, was found in the promoter region of these genes. In a DNase footprint experiment, the protected region in the ytlI promoter region contained this consensus sequence. Partial deletion or introduction of point mutations in this sequence confirmed its involvement in ytlI, yrrT, and yxeK regulation. The addition of O-acetylserine in gel shift experiments prevented CymR-dependent binding to DNA for all of the targets characterized. Transcriptome analysis of a delta cymR mutant and the wild-type strain also brought out significant changes in the expression level of a large set of genes related to stress response or to transition toward anaerobiosis.

Predicted highly expressed genes in Nocardia farcinica and the implication for its primary metabolism and nocardial virulence

Nocardia farcinica is a Gram positive, filamentous bacterium, and is considered an opportunistic pathogen. In this study, the highly expressed genes in N. farcinica were predicted using the codon adaptation index (CAI) as a numerical estimator of gene expressivity. Using ribosomal protein (RP) genes as references, the top approximately approximately 10% of the genes were predicted to be the predicted highly expressed (PHX) genes in N. farcinica using a CAI cutoff of greater than 0.73. Consistent with earlier analysis of Streptomyces genomes, most of the PHX genes in N. farcinica were involved in various 'house-keeping' functions important for cell growth. However, 15 genes putatively involved in nocardial virulence were predicted as PHX genes in N. farcinica, which included genes encoding four Mce proteins, cyclopropane fatty acid synthase which is involved in the modification of cell wall which may be important for nocardia virulence, polyketide synthase PKS13 for mycolic acid synthesis and a non-ribosomal peptide synthetase involved in biosynthesis of a mycobactin-related siderophore. In addition, multiple genes involved in defense against reactive oxygen species (ROS) produced by the phagocyte were predicted with high expressivity, which included alkylhydroperoxide reductase (ahpC), catalase (katG), superoxide dismutase (sodF), thioredoxin, thioredoxin reductase, glutathione peroxidase, and peptide methionine sulfoxide reductase, suggesting that combating against ROS is essential for survival of N. farcinica in host cells. The study also showed that the distribution of PHX genes in the N. farcinica circular chromosome was uneven, with more PHX genes located in the regions close to replication initiation site. The results provided the first estimates of global gene expression patterns in N. farcinica, which will be useful in guiding experimental design for further investigations.

The gene ncgl2918 encodes a novel maleylpyruvate isomerase that needs mycothiol as cofactor and links mycothiol biosynthesis and gentisate assimilation in Corynebacterium glutamicum

Data mining of the Corynebacterium glutamicum genome identified 4 genes analogous to the mshA, mshB, mshC, and mshD genes that are involved in biosynthesis of mycothiol in Mycobacterium tuberculosis and Mycobacterium smegmatis. Individual deletion of these genes was carried out in this study. Mutants mshC- and mshD- lost the ability to produce mycothiol, but mutant mshB- produced mycothiol as the wild type did. The phenotypes of mutants mshC- and mshD- were the same as the wild type when grown in LB or BHIS media, but mutants mshC- and mshD- were not able to grow in mineral medium with gentisate or 3-hydroxybenzoate as carbon sources. C. glutamicum assimilated gentisate and 3-hydroxybenzoate via a glutathione-independent gentisate pathway. In this study it was found that the maleylpyruvate isomerase, which catalyzes the conversion of maleylpyruvate into fumarylpyruvate in the glutathione-independent gentisate pathway, needed mycothiol as a cofactor. This mycothiol-dependent maleylpyruvate isomerase gene (ncgl2918) was cloned, actively expressed, and purified from Escherichia coli. The purified mycothiol-dependent isomerase is a monomer of 34 kDa. The apparent Km and Vmax values for maleylpyruvate were determined to be 148.4 +/- 11.9 microM and 1520 +/- 57.4 micromol/min/mg, respectively (mycothiol concentration, 2.5 microM). Previous studies had shown that mycothiol played roles in detoxification of oxidative chemicals and antibiotics in streptomycetes and mycobacteria. To our knowledge, this is the first demonstration that mycothiol is essential for growth of C. glutamicum with gentisate or 3-hydroxybenzoate as carbon sources and the first characterization of a mycothiol-dependent maleylpyruvate isomerase.

Characterization of the Bifunctional γ-Glutamate-cysteine Ligase/Glutathione Synthetase (GshF) of Pasteurella multocida

Glutamate-cysteine ligase (gamma-ECL) and glutathione synthetase (GS) are the two unrelated ligases that constitute the glutathione biosynthesis pathway in most eukaryotes, purple bacteria, and cyanobacteria. gamma-ECL is a member of the glutamine synthetase family, whereas GS enzymes group together with highly diverse carboxyl-to-amine/thiol ligases, all characterized by the so-called two-domain ATP-grasp fold. This generalized scheme toward the formation of glutathione, however, is incomplete, as functional steady-state levels of intracellular glutathione may also accumulate solely by import, as has been reported for the Pasteurellaceae member Haemophilus influenzae, as well as for certain Gram-positive enterococci and streptococci, or by the action of a bifunctional fusion protein (termed GshF), as has been reported recently for the Gram-positive firmicutes Streptococcus agalactiae and Listeria monocytogenes. Here, we show that yet another member of the Pasteurellaceae family, Pasteurella multocida, acquires glutathione both by import and GshF-driven biosynthesis. Domain architecture analysis shows that this P. multocida GshF bifunctional ligase contains an N-terminal gamma-proteobacterial gamma-ECL-like domain followed by a typical ATP-grasp domain, which most closely resembles that of cyanophycin synthetases, although it has no significant homology with known GS ligases. Recombinant P. multocida GshF overexpresses as an approximately 85-kDa protein, which, on the basis of gel-sizing chromatography, forms dimers in solution. The gamma-ECL activity of GshF is regulated by an allosteric type of glutathione feedback inhibition (K(i) = 13.6 mM). Furthermore, steady-state kinetics, on the basis of which we present a novel variant of half-of-the-sites reactivity, indicate intimate domain-domain interactions, which may explain the bifunctionality of GshF proteins.

Disulfiram irreversibly aggregates betaine aldehyde dehydrogenase--a potential target for antimicrobial agents against Pseudomonas aeruginosa

In the human pathogen Pseudomonas aeruginosa, betaine aldehyde dehydrogenase (PaBADH) may play the dual role of assimilating carbon and nitrogen from choline or choline precursors--abundant at infection sites--and producing glycine betaine, which protects the bacterium against the high-osmolality stress prevalent in the infected tissues. This tetrameric enzyme contains four cysteine residues per subunit and is a potential drug target. In our search for specific inhibitors, we mutated the catalytic Cys286 to alanine and chemically modified the recombinant wild-type and the four Cys-->Ala single mutants with thiol reagents. The small methyl-methanethiosulfonate inactivated the enzymes without affecting their stability while the bulkier dithionitrobenzoic acid (DTNB) and bis[diethylthiocarbamyl] disulfide (disulfiram) induced enzyme dissociation--at 23 degrees C--and irreversible aggregation--at 37 degrees C. Of the four Cys-->Ala mutants only C286A retained its tetrameric structure after DTNB or disulfiram treatments, suggesting that steric constraints arising upon the covalent attachment of a bulky group to C286 resulted in distortion of the backbone configuration in the active site region followed by a severe decrease in enzyme stability. Since neither NAD(P)H nor betaine aldehyde prevented disulfiram-induced PaBADH inactivation or aggregation, and reduced glutathione was unable to restore the activity of the modified enzyme, we propose that disulfiram could be a useful drug to combat infection by P. aeruginosa.

Cloning and preliminary characterization of lh3 gene encoding a putative acetyltransferase from a rifamycin SV-producing strain Amycolatopsis mediterranei

An ORF located immediately downstream of glnR gene was cloned from Amycolatopsis mediterranei U32 and was named lh3. Sequence analysis revealed that lh3 encodes a putative acetyltransferase, which shows high amino acid sequence similarities to the mycothiol synthase (MshD) from other actinomycetes. For functional analysis, mutation in lh3 gene was generated by gene replacement with an apramycin resistance gene through homologous recombination. Compared with the wild type strain, the resulting mutant was more sensitive to H2O2, apramycin and erythromycin by two- to three-fold. These results suggest that the lh3 gene plays an important role in the course of detoxification in A. mediterranei U32.

The substrate-induced conformational change of Mycobacterium tuberculosis mycothiol synthase

The structure of the ternary complex of mycothiol synthase from Mycobacterium tuberculosis with bound desacetylmycothiol and CoA was determined to 1.8 A resolution. The structure of the acetyl-CoA-binary complex had shown an active site groove that was several times larger than its substrate. The structure of the ternary complex reveals that mycothiol synthase undergoes a large conformational change in which the two acetyltransferase domains are brought together through shared interactions with the functional groups of desacetylmycothiol, thereby decreasing the size of this large central groove. A comparison of the binary and ternary structures illustrates many of the features that promote catalysis. Desacetylmycothiol is positioned with its primary amine in close proximity and in the proper orientation for direct nucleophilic attack on the si-face of the acetyl group of acetyl-CoA. Glu-234 and Tyr-294 are positioned to act as a general base and general acid to promote acetyl transfer. In addition, this structure provides further evidence that the N-terminal acetyltransferase domain no longer has enzymatic activity and is vestigial in nature.

A mycothiol synthase mutant of Mycobacterium smegmatis produces novel thiols and has an altered thiol redox status

Mycobacteria and other actinomycetes do not produce glutathione but make mycothiol (MSH; AcCys-GlcN-Ins) that has functions similar to those of glutathione and is essential for growth of Mycobacterium tuberculosis. Mycothiol synthase (MshD) catalyzes N acetylation of Cys-GlcN-Ins to produce MSH in Mycobacterium smegmatis mc2155, and Cys-GlcN-Ins is maintained at a low level. The mycothiol synthase mutant, the mshD::Tn5 mutant, produces high levels of Cys-GlcN-Ins along with two novel thiols, N-formyl-Cys-GlcN-Ins and N-succinyl-Cys-GlcN-Ins, and a small amount of MSH. The nonenzymatic reaction of acyl-coenzyme A (CoA) with Cys-GlcN-Ins to produce acyl-Cys-GlcN-Ins is a facile reaction under physiologic conditions, with succinyl-CoA being an order of magnitude more reactive than acetyl-CoA. The uncatalyzed reaction rates are adequate to account for the observed production of N-succinyl-Cys-GlcN-Ins and MSH under physiologic conditions. It was shown that the N-acyl-Cys-GlcN-Ins compounds are maintained in a substantially reduced state in the mutant but that Cys-GlcN-Ins exists in disulfide forms at 5 to 40% at different stages of growth. MSH was able to facilitate reduction of N-succinyl-Cys-GlcN-Ins disulfide through thiol-disulfide exchange, but N-formyl-Cys-GlcN-Ins was ineffective. The oxidized state of Cys-GlcN-Ins in cells appears to result from a high susceptibility to autoxidation and a low capacity of the cell to reduce its disulfide forms. The mutant exhibited no enhanced sensitivity to hydrogen peroxide, tert-butyl hydroperoxide, or cumene hydroperoxide relative to the parent strain, suggesting that the most abundant thiol, N-formyl-Cys-GlcN-Ins, functions as a substitute for MSH.

Thiosugars: new perspectives regarding availability and potential biochemical and medicinal applications

Thiosugars, containing a sulfur atom as heteroatom or a disaccharide linked via a sulfur bridge, possess unique physicochemical properties such as water solubility, which differs from conventional functionalized monosaccharides. The differences in biological activities between thiosugars and their oxygen analogs depend on geometric, conformational, and flexibility differences. They depend also on their electronic differences, the sulfide function being less electronegative and more polarizable than the ethereal moiety. Many functionalized thiosugars occur naturally and are potential targets for the development of carbohydrate-based therapeutics. Among the few new examples of the potential new targets are salacinol and kotalanol, tagetitoxin, thiolactomycin and analogues, mycothiol and analogues, and S-nitrosothiols. These new developments and representative examples of functionalized thiosugar prototypes as potential new targets are presented in this mini review.

Determination of coenzyme A levels in Pyrococcus furiosus and other Archaea: implications for a general role for coenzyme A in thermophiles

Physiologically significant levels of intracellular coenzyme A were identified in Pyrococcus furiosus, Thermococcus litoralis, and Sulfolobus solfataricus, suggesting a role for CoA as an important low molecular mass thiol in the thermophilic Archaea. In P. furiosus, cells grown in the presence of sulfur showed significantly higher levels of oxidized CoA compared with those grown in the absence of S(0). T. litoralis showed strikingly similar CoA levels, although with low disulfide levels in both the presence and absence of S(0). S. solfataricus showed similarly high levels of CoA thiol, with correspondingly low levels of the CoA disulfide. These results are consistent with the identification of a coenzyme A disulfide reductase (CoADR) in P. furiosus and horikoshii as well as the presence of CoADR homologues in the genomes of S. solfataricus and T. kodakaraensis.

NO-mediated cytoprotection: instant adaptation to oxidative stress in bacteria

Numerous sophisticated systems have been described that protect bacteria from increased levels of reactive oxygen species. Although indispensable during prolonged oxidative stress, these response systems depend on newly synthesized proteins, and are hence both time and energy consuming. Here, we describe an "express" cytoprotective system in Bacillus subtilis which depends on nitric oxide (NO). We show that NO immediately protects bacterial cells from reactive oxygen species by two independent mechanisms. NO transiently suppresses the enzymatic reduction of free cysteine that fuels the damaging Fenton reaction. In addition, NO directly reactivates catalase, a major antioxidant enzyme that has been inhibited in vivo by endogenous cysteine. Our data also reveal a critical role for bacterial NO-synthase in adaptation to oxidative stress associated with fast metabolic changes, and suggest a possible role for NO in defending pathogens against immune oxidative attack.

Genetic and physiological responses of Bacillus subtilis to metal ion stress

Metal ion homeostasis is regulated principally by metalloregulatory proteins that control metal ion uptake, storage and efflux genes. We have used transcriptional profiling to survey Bacillus subtilis for genes that are rapidly induced by exposure to high levels of metal ions including Ag(I), Cd(II), Cu(II), Ni(II) and Zn(II) and the metalloid As(V). Many of the genes affected by metal stress were controlled by known metalloregulatory proteins (Fur, MntR, PerR, ArsR and CueR). Additional metal-induced genes are regulated by two newly defined metal-sensing ArsR/SmtB family repressors: CzrA and AseR. CzrA represses the CadA efflux ATPase and the cation diffusion facilitator CzcD and this repression is alleviated by Zn(II), Cd(II), Co(II), Ni(II) and Cu. CadA is the major determinant for Cd(II) resistance, while CzcD protects the cell against elevated levels of Zn(II), Cu, Co(II) and Ni(II). AseR negatively regulates itself and AseA, an As(III) efflux pump which contributes to arsenite resistance in cells lacking a functional ars operon. Our results extend the range of identified effectors for the As(III)-sensor ArsR to include Cd(II) and Ag(I) and for the Cu-sensor CueR to include Ag(I) and, weakly, Cd(II) and Zn(II). In addition to systems dedicated to metal homeostasis, specific metal stresses also strongly induced pathways related to cysteine, histidine and arginine metabolism.

A master regulator σBgoverns osmotic and oxidative response as well as differentiation via a network of sigma factors inStreptomyces coelicolor

The differentiating bacterium Streptomyces coelicolor harbours some 66 sigma factors, which support its complex life cycle. sigma(B), a functional homologue of sigma(S) from Escherichia coli, controls both osmoprotection and differentiation in S. coelicolor A3(2). Microarray analysis revealed sigma(B)-dependent induction of more than 280 genes by 0.2 M KCl. These genes encode several sigma factors, oxidative defence proteins, chaperones, systems to provide osmolytes, cysteine, mycothiol, and gas vesicle. sigma(B) controlled induction of itself and its two paralogues (sigma(L) and sigma(M)) in a hierarchical order of sigma(B)-->sigma(L)-->sigma(M), as revealed by S1 mapping and Western blot analyses. The phenotype of each sigma mutant suggested a sequential action in morphological differentiation; sigma(B) in forming aerial mycelium, sigma(L) in forming spores and sigma(M) for efficient sporulation. sigma(B) was also responsible for the increase in cysteine and mycothiol, the major thiol buffer in actinomycetes, upon osmotic shock, revealing an overlap between protections against osmotic and oxidative stresses. Proteins in sigB mutant were more oxidized (carbonylated) than the wild type. These results support a hypothesis that sigma(B) serves as a master regulator that triggers other related sigma factors in a cascade, and thus regulates differentiation and osmotic and oxidative response in S. coelicolor.

A multidomain fusion protein in Listeria monocytogenes catalyzes the two primary activities for glutathione biosynthesis

Glutathione is the predominant low-molecular-weight peptide thiol present in living organisms and plays a key role in protecting cells against oxygen toxicity. Until now, glutathione synthesis was thought to occur solely through the consecutive action of two physically separate enzymes, gamma-glutamylcysteine ligase and glutathione synthetase. In this report we demonstrate that Listeria monocytogenes contains a novel multidomain protein (termed GshF) that carries out complete synthesis of glutathione. Evidence for this comes from experiments which showed that in vitro recombinant GshF directs the formation of glutathione from its constituent amino acids and the in vivo effect of a mutation in GshF that abolishes glutathione synthesis, results in accumulation of the intermediate gamma-glutamylcysteine, and causes hypersensitivity to oxidative agents. We identified GshF orthologs, consisting of a gamma-glutamylcysteine ligase (GshA) domain fused to an ATP-grasp domain, in 20 gram-positive and gram-negative bacteria. Remarkably, 95% of these bacteria are mammalian pathogens. A plausible origin for GshF-dependent glutathione biosynthesis in these bacteria was the recruitment by a GshA ancestor gene of an ATP-grasp gene and the subsequent spread of the fusion gene between mammalian hosts, most likely by horizontal gene transfer.

Glutathione Synthesis in Streptococcus agalactiae

Gamma-glutamylcysteine synthetase (gamma-GCS) and glutathione synthetase (GS), distinct enzymes that together account for glutathione (GSH) synthesis, have been isolated and characterized from several Gram-negative prokaryotes and from numerous eukaryotes including mammals, amphibians, plants, yeast, and protozoa. Glutathione synthesis is relatively uncommon among the Gram-positive bacteria, and, to date, neither the genes nor the proteins involved have been identified. In the present report, we show that crude extracts of Streptococcus agalactiae catalyze the gamma-GCS and GS reactions and can synthesize GSH from its constituent amino acids. The putative gene for S. agalactiae gamma-GCS was identified and cloned, and the corresponding protein was expressed and purified. Surprisingly, it was found that the isolated enzyme catalyzes both the ATP-dependent synthesis of L-gamma-glutamyl-L-cysteine from L-glutamate and L-cysteine and the ATP-dependent synthesis of GSH from L-gamma-glutamyl-L-cysteine and glycine. This novel bifunctional enzyme, referred to as gamma-GCS-GS, has been characterized in terms of catalytic activity, substrate specificity, and inhibition by GSH, cystamine, and transition state analog sulfoximines. The N-terminal 518 amino acids of gamma-GCS-GS (total M(r) 85,000) show 32% identity and 43% similarity with E. coli gamma-GCS (M(r) 58,000), but the C-terminal putative GS domain (remaining 202 amino acids) of gamma-GCS-GS shows no significant homology with known GS sequences. The C terminus (360 amino acids) is, however, homologous to D-Ala, D-Ala ligase (24% identity; 38% similarity), an enzyme having the same protein fold as known GS proteins. These results are discussed in terms of the evolution of GSH synthesis and the possible occurrence of a similar bifunctional GSH synthesis enzyme in other bacterial species.

Ab initio studies of the properties of intracellular thiols ergothioneine and ovothiol

Intracellular naturally occurring aromatic thiols such as ergothioneine and the ovothiols have been shown to play a variety of roles in cellular function. A detailed ab initio electronic structure analysis of these thiols is reported evaluating the thermodynamics of the reactions of these intracellular thiols with alkyl thiols, HO*, H2O2, ascorbate and their disulfides.

Roles of thioredoxin reductase during the aerobic life of Lactococcus lactis

Thiol-disulfide bond balance is generally maintained in bacteria by thioredoxin reductase-thioredoxin and/or glutathione-glutaredoxin systems. Some gram-positive bacteria, including Lactococcus lactis, do not produce glutathione, and the thioredoxin system is presumed to be essential. We constructed an L. lactis trxB1 mutant. The mutant was obtained under anaerobic conditions in the presence of dithiothreitol (DTT). Unexpectedly, the trxB1 mutant was viable without DTT and under aerated static conditions, thus disproving the essentiality of this system. Aerobic growth of the trxB1 mutant did not require glutathione, also ruling out the need for this redox maintenance system. Proteomic analyses showed that known oxidative stress defense proteins are induced in the trxB1 mutant. Two additional effects of trxB1 were not previously reported in other bacteria: (i) induction of proteins involved in fatty acid or menaquinone biosynthesis, indicating that membrane synthesis is part of the cellular response to a redox imbalance, and (ii) alteration of the isoforms of the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GapB). We determined that the two GapB isoforms in L. lactis differed by the oxidation state of catalytic-site cysteine C152. Unexpectedly, a decrease specific to the oxidized, inactive form was observed in the trxB1 mutant, possibly because of proteolysis of oxidized GapB. This study showed that thioredoxin reductase is not essential in L. lactis and that its inactivation triggers induction of several mechanisms acting at the membrane and metabolic levels. The existence of a novel redox function that compensates for trxB1 deficiency is suggested.

Expression, purification and X-ray crystallographic analysis of thioredoxin from Streptomyces coelicolor

Thioredoxins are ubiquitous proteins that serve as reducing agents and general protein disulfide reductases. In turn, they are reduced by electrons obtained from the NADPH-containing thioredoxin reductase. Thioredoxins have been isolated and characterized from a large number of organisms. The Gram-positive bacterium Streptomyces coelicolor contains three thioredoxins that are involved in unknown biological processes. trxA from S. coelicolor was cloned and expressed in Escherichia coli and the protein purified and crystallized using the hanging-drop method of vapour diffusion. The crystal structure of thioredoxin A has been determined at 1.5 A resolution using a synchrotron-radiation source. The protein reveals a thioredoxin-like fold with a typical CXXC active site. The crystal exhibits the symmetry of space group P2(1)2(1)2, with unit-cell parameters a = 43.6, b = 71.8, c = 33.2 A.

A comparison of the effects of ocular preservatives on mammalian and microbial ATP and glutathione levels

The aim of this study was to investigate the mechanism of action of the preservative sodium chlorite (NaClO2), and the relationship with intracellular glutathione depletion. A detailed comparison of the dose responses of two cultured ocular epithelial cell types and four species of microorganism was carried out, and comparisons were also made with the quaternary ammonium compound benzalkonium chloride (BAK), and the oxidant hydrogen peroxide (H2O2). The viability of mammalian and microbial cells was assessed in the same way, by the measurement of intracellular ATP using a bioluminescence method. Intracellular total glutathione was measured by reaction with 5,5'-dithiobis-2-nitrobenzoic acid in a glutathione reductase-dependent recycling assay. BAK and H2O2 caused complete toxicity to conjunctival and corneal epithelial cells at approximately 25 ppm, in contrast to NaClO2, where > 100 ppm was required. The fungi Candida albicans and Alternaria alternata had a higher resistance to NaClO2 than the bacteria Staphyloccus aureus and Pseudomonas aeruginosa, but the bacteria were extremely resistant to H2O2. NaClO2 caused substantial depletion of intracellular glutathione in all cell types, at concentrations ranging from < 10 ppm in Pseudomonas, 25-100 ppm in epithelial cells, to > 500 ppm in fungal cells. The mechanisms of cytotoxicity of NaClO2, H2O2 and BAK all appeared to differ. NaClO2 was found to have the best balance of high antibacterial toxicity with low ocular toxicity. The lower toxicity of NaClO2 to the ocular cells, compared with BAK and H2O2, is in agreement with fewer reported adverse effects of application in the eye.

A pantothenate kinase from Staphylococcus aureus refractory to feedback regulation by coenzyme A

The key regulatory step in CoA biosynthesis in bacteria and mammals is pantothenate kinase (CoaA), which governs the intracellular concentration of CoA through feedback regulation by CoA and its thioesters. CoaA from Staphylococcus aureus (SaCoaA) has a distinct primary sequence that is more similar to the mammalian pantothenate kinases than the prototypical bacterial CoaA of Escherichia coli. In contrast to all known pantothenate kinases, SaCoaA activity is not feedback-regulated by CoA or CoA thioesters. Metabolic labeling of S. aureus confirms that CoA levels are not controlled by CoaA or at steps downstream from CoaA. The pantothenic acid antimetabolite N-heptylpantothenamide (N7-Pan) possesses potent antimicrobial activity against S. aureus and has multiple cellular targets. N7-Pan is a substrate for SaCoaA and is converted to the inactive butyldethia-CoA analog by the downstream pathway enzymes. The analog is also incorporated into acyl carrier protein and D-alanyl carrier protein, the prosthetic groups of which are derived from CoA. The inactivation of acyl carrier protein and the cessation of fatty acid synthesis are the most critical causes of growth inhibition by N7-Pan because the toxicity of the drug is ameliorated by supplementing the growth medium with fatty acids. The absence of feedback regulation at the pantothenate kinase step allows the accumulation of high concentrations of intracellular CoA, consistent with the physiology of S. aureus, which lacks glutathione and relies on the CoA/CoA disulfide reductase redox system for protection from oxidative damage.

Lipoproteins ofMycobacterium tuberculosis: an abundant and functionally diverse class of cell envelope components

Mycobacterium tuberculosis remains the predominant bacterial scourge of mankind. Understanding of its biology and pathogenicity has been greatly advanced by the determination of whole genome sequences for this organism. Bacterial lipoproteins are a functionally diverse class of membrane-anchored proteins. The signal peptides of these proteins direct their export and post-translational lipid modification. These signal peptides are amenable to bioinformatic analysis, allowing the lipoproteins encoded in whole genomes to be catalogued. This review applies bioinformatic methods to the identification and functional characterisation of the lipoproteins encoded in the M. tuberculosis genomes. Ninety nine putative lipoproteins were identified and so this family of proteins represents ca. 2.5% of the M. tuberculosis predicted proteome. Thus, lipoproteins represent an important class of cell envelope proteins that may contribute to the virulence of this major pathogen.

Antimicrobial reactive oxygen and nitrogen species: concepts and controversies

Phagocyte-derived reactive oxygen and nitrogen species are of crucial importance for host resistance to microbial pathogens. Decades of research have provided a detailed understanding of the regulation, generation and actions of these molecular mediators, as well as their roles in resisting infection. However, differences of opinion remain with regard to their host specificity, cell biology, sources and interactions with one another or with myeloperoxidase and granule proteases. More than a century after Metchnikoff first described phagocytosis, and more than four decades after the discovery of the burst of oxygen consumption that is associated with microbial killing, the seemingly elementary question of how phagocytes inhibit, kill and degrade microorganisms remains controversial. This review updates the reader on these concepts and the topical questions in the field.

Using Lactococcus lactis for glutathione overproduction

Glutathione and gamma-glutamylcysteine were produced in Lactococcus lactis using a controlled expression system and the genes gshA and gshB from Escherichia coli encoding the enzymes gamma-glutamylcysteine synthetase and glutathione synthetase. High levels of gamma-glutamylcysteine were found in strains growing on chemically defined medium and expressing either gshA alone or both gshA and gshB. As anticipated, glutathione was found in a strain expressing gshA and gshB. The level of glutathione production could be increased by addition of the precursor amino acid cysteine to the medium. The addition of cysteine led to an increased activity of glutathione synthetase, which is remarkable because the amino acid is not a substrate of this enzyme. The final intracellular glutathione concentration attained was 358 nmol mg(-1) protein, which is the highest concentration reported for a bacterium, demonstrating the suitability of engineered L. lactis for fine-chemical production and as a model for studies of the impact of glutathione on flavour formation and other properties of food.

Targeted mutagenesis of the Mycobacterium smegmatis mca gene, encoding a mycothiol-dependent detoxification protein

Mycothiol (MSH), a functional analogue of glutathione (GSH) that is found exclusively in actinomycetes, reacts with electrophiles and toxins to form MSH-toxin conjugates. Mycothiol S-conjugate amidase (Mca) then catalyzes the hydrolysis of an amide bond in the S conjugates, producing a mercapturic acid of the toxin, which is excreted from the bacterium, and glucosaminyl inositol, which is recycled back to MSH. In this study, we have generated and characterized an allelic exchange mutant of the mca gene of Mycobacterium smegmatis. The mca mutant accumulates the S conjugates of the thiol-specific alkylating agent monobromobimane and the antibiotic rifamycin S. Introduction of M. tuberculosis mca epichromosomally or introduction of M. smegmatis mca integratively resulted in complementation of Mca activity and reduced levels of S conjugates. The mutation in mca renders the mutant strain more susceptible to electrophilic toxins, such as N-ethylmalemide, iodoacetamide, and chlorodinitrobenzene, and to several oxidants, such as menadione and plumbagin. Additionally we have shown that the mca mutant is also more susceptible to the antituberculous antibiotic streptomycin. Mutants disrupted in genes belonging to MSH biosynthesis are also more susceptible to streptomycin, providing further evidence that Mca detoxifies streptomycin in the mycobacterial cell in an MSH-dependent manner.

Oxidative stress triggers thiol oxidation in the glyceraldehyde-3-phosphate dehydrogenase of Staphylococcus aureus

The high-resolution two-dimensional protein gel electrophoresis technique combined with matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used to analyse the oxidative stress response in Staphylococcus aureus COL. Exponentially growing cells were supplemented with 100 mM H2O2 leading to a growth arrest lasting 30 min. The comparison of the two-dimensional pattern of cytoplasmic protein extracts of stressed and unstressed cells revealed only a few changes in the protein synthesis profile. However, the isoelectric points of Gap (glyceraldehyde-3-phosphate dehydrogenase), AhpC (alkylhydroperoxide reductase) and MvaS (HMG-CoA-synthase) changed strikingly. For analysis of the modification of Gap, tandem hybrid mass spectrometry (Q-Star) was used. The observed pI shift resulted from the oxidation to sulphonic acid of cysteine 151, which is crucial for catalytic activity. A drop in ATP and a complete inactivation of Gap was accompanied by the growth arrest. About 30 min after the addition of H2O2, the damaged Gap was still present, but a new protein spot at the original location became visible, representing the newly synthesized enzyme that is active again. This is accompanied by the restoration of Gap enzyme activity, ATP levels and recovery of growth. There is a strong correlation between growth, ATP level and Gap activity under oxidative stress conditions, indicating that the H2O2-triggered Gap inactivation might be one reason for growth arrest under these conditions. Our data indicate that the damaged Gap protein was not repaired.

First total synthesis of mycothiol and mycothiol disulfide

[reaction: see text] The first total synthesis of mycothiol and mycothiol disulfide was achieved by linking D-2,3,4,5,6-penta-O-acetyl-myo-inositol, O-(3,4,6-tri-O-acetyl)-2-azido-2-deoxy-alpha,beta-D-glucopyranosyl) trichloroacetimidate, and N,S-diacetyl-L-cysteine and deprotecting peracetylated mycothiol. The first full spectral characterization is reported for underivatized mycothiol. The structure of mycothiol was confirmed by spectral analysis of the known bimane derivative.

The Mycobacterium tuberculosis ino1 gene is essential for growth and virulence

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.

Role of a cysteine synthase in Staphylococcus aureus

The gram-positive human pathogen Staphylococcus aureus is often isolated with media containing potassium tellurite, to which it has a higher level of resistance than Escherichia coli. The S. aureus cysM gene was isolated in a screen for genes that would increase the level of tellurite resistance of E. coli DH5alpha. The protein encoded by S. aureus cysM is sequentially and functionally homologous to the O-acetylserine (thiol)-lyase B family of cysteine synthase proteins. An S. aureus cysM knockout mutant grows poorly in cysteine-limiting conditions, and analysis of the thiol content in cell extracts showed that the cysM mutant produced significantly less cysteine than wild-type S. aureus SH1000. S. aureus SH1000 cannot use sulfate, sulfite, or sulfonates as the source of sulfur in cysteine biosynthesis, which is explained by the absence of genes required for the uptake and reduction of these compounds in the S. aureus genome. S. aureus SH1000, however, can utilize thiosulfate, sulfide, or glutathione as the sole source of sulfur. Mutation of cysM caused increased sensitivity of S. aureus to tellurite, hydrogen peroxide, acid, and diamide and also significantly reduced the ability of S. aureus to recover from starvation in amino acid- or phosphate-limiting conditions, indicating a role for cysteine in the S. aureus stress response and survival mechanisms.

Transcriptome and proteome analysis of Bacillus subtilis gene expression in response to superoxide and peroxide stress

The Gram-positive soil bacterium Bacillus subtilis responds to oxidative stress by the activation of different cellular defence mechanisms. These are composed of scavenging enzymes as well as protection and repair systems organized in highly sophisticated networks. In this study, the peroxide and the superoxide stress stimulons of B. subtilis were characterized by means of transcriptomics and proteomics. The results demonstrate that oxidative-stress-responsive genes can be classified into two groups. One group encompasses genes which show similar expression patterns in the presence of both reactive oxygen species. Examples are members of the PerR and the Fur regulon which were induced by peroxide and superoxide stress. Similarly, both kinds of stress stimulated the activation of the stringent response. The second group is composed of genes primarily responding to one stimulus, like the members of the SOS regulon which were particularly upregulated in the presence of peroxide, and many genes involved in sulfate assimilation and methionine biosynthesis which were only induced by superoxide. Several genes encoding proteins of unknown function could be assigned to one of these groups.

Multiple thioredoxin-mediated routes to detoxify hydroperoxides in Mycobacterium tuberculosis

Drug resistance and virulence of Mycobacterium tuberculosis are in part related to the pathogen's antioxidant defense systems. KatG(-) strains are resistant to the first line tuberculostatic isoniazid but need to compensate their catalase deficiency by alternative peroxidase systems to stay virulent. So far, only NADH-driven and AhpD-mediated hydroperoxide reduction by AhpC has been implicated as such virulence-determining mechanism. We here report on two novel pathways which underscore the importance of the thioredoxin system for antioxidant defense in M. tuberculosis: (i) NADPH-driven hydroperoxide reduction by AhpC that is mediated by thioredoxin reductase and thioredoxin C and (ii) hydroperoxide reduction by the atypical peroxiredoxin TPx that equally depends on thioredoxin reductase but can use both, thioredoxin B and C. Kinetic analyses with different hydroperoxides including peroxynitrite qualify the redox cascade comprising thioredoxin reductase, thioredoxin C, and TPx as the most efficient system to protect M. tuberculosis against oxidative and nitrosative stress in situ.

Glutathione protects Lactococcus lactis against oxidative stress

Glutathione was found in several dairy Lactococcus lactis strains grown in M17 medium. None of these strains was able to synthesize glutathione. In chemically defined medium, L. lactis subsp. cremoris strain SK11 was able to accumulate up to approximately 60 mM glutathione when this compound was added to the medium. Stationary-phase cells of strain SK11 grown in chemically defined medium supplemented with glutathione showed significantly increased resistance (up to fivefold increased resistance) to treatment with H2O2 compared to the resistance of cells without intracellular glutathione. The resistance to H2O2 treatment was found to be dependent on the accumulation of glutathione in 16 strains of L. lactis tested. We propose that by taking up glutathione, L. lactis might activate a glutathione-glutathione peroxidase-glutathione reductase system in stationary-phase cells, which catalyzes the reduction of H2O2. Glutathione reductase, which reduces oxidized glutathione, was detectable in most strains of L. lactis, but the activities of different strains were very variable. In general, the glutathione reductase activities of L. lactis subsp. lactis are higher than those of L. lactis subsp. cremoris, and the activities were much higher when strains were grown aerobically. In addition, glutathione peroxidase is detectable in strain SK11, and the level was fivefold greater when the organism was grown aerobically than when the organism was grown anaerobically. Therefore, the presence of glutathione in L. lactis could result in greater stability under storage conditions and quicker growth upon inoculation, two important attributes of successful starter cultures.

Transcriptional regulation of the Staphylococcus aureus thioredoxin and thioredoxin reductase genes in response to oxygen and disulfide stress

In this report we describe the cloning, organization, and promoter analysis of the Staphylococcus aureus thioredoxin (trxA) and thioredoxin reductase (trxB) genes and their transcription in response to changes in oxygen concentration and to oxidative stress compounds. Northern analysis showed that the S. aureus trxA and trxB genes were transcribed equally well in aerobic and anaerobic conditions. Several oxidative stress compounds were found to rapidly induce transcription of the trxA and trxB genes. The most pronounced effects were seen with diamide, a thiol-specific oxidant that promotes disulfide bond formation; menadione, a redox cycling agent; and tau-butyl hydroperoxide, an organic peroxide. In each case the induction was independent of the general stress sigma factor sigma(B). These studies show that the S. aureus trxA and trxB genes are upregulated following exposure to these oxidative stress agents, resulting in increased disulfide bond formation. In contrast, no effect of hydrogen peroxide on induction of the trxA and trxB genes was seen. We also show that the S. aureus thioredoxin reductase appears to be essential for growth. This observation, coupled with structural differences between the bacterial and mammalian thioredoxin reductases, suggests that it may serve as a target for the development of new antimicrobials.

Measurement of glutathione in activated sludges

Thermal, electric, mechanical or oxidative stress seem a promising way to reduce the production of excess activated sludge during biological wastewater treatment. However, the adaptation and the resistance of the sludge microbial ecosystem to stress conditions is a major question as it may definitively limit the effect of some treatments. Defence mechanisms developed by aerobic organisms, in particular, in response to oxidative stress involve various antioxidant activities and compounds such as glutathione. An HPLC method was developed for measuring reduced and total glutathione (GSH and GSHt) in perchloric acid sludge extracts. The method was sensitive, highly specific and validated for linearity, precision and recovery. Considering the extraction yield and the oxidation of GSH during extract storage, the measured GSH concentration was estimated to represent 60% of the GSH content from activated sludges. GSHt ranged from 0.32 to 3.34micromolg(-1) volatile solids and the GSH/GSHt ratio ranged from 32% to 91%. Measurements performed on sludges stressed in precise conditions selected to reach a reduction of sludge production showed a decrease of GSH and GSHt concentrations with thermal, mechanical, electric and ozone stress.

Crystal Structure of MshB from Mycobacterium tuberculosis , a Deacetylase Involved in Mycothiol Biosynthesis

All living species require protection against the damaging effects of the reactive oxygen species that are a natural by-product of aerobic life. In most organisms, glutathione is a critical component of these defences, maintaining a reducing environment inside cells. Some bacteria, however, including pathogenic mycobacteria, use an alternative low molecular mass thiol compound called mycothiol (MSH) for this purpose. Enzymes that synthesize MSH are attractive candidates for the design of novel anti-TB drugs because of the importance of MSH for mycobacterial life and the absence of such enzymes in humans. We have determined the three-dimensional structure of MshB (Rv1170), a metal-dependent deacetylase from Mycobacterium tuberculosis that catalyses the second step in MSH biosynthesis. The structure, determined at 1.9A resolution by X-ray crystallography (R=19.0%, R(free)=21.4%), reveals an alpha/beta fold in which helices pack against a seven-stranded mostly parallel beta-sheet. Large loops emanating from the C termini of the beta-strands enclose a deep cavity, which is the location of the putative active site. At the bottom of this cavity is a metal-binding site associated with a sequence motif AHPDDE that is invariant in all homologues. An adventitiously bound beta-octylglucoside molecule, used in crystallization, enables us to model the binding of the true substrate and propose a metal-dependent mechanistic model for deacetylation. Sequence comparisons indicate that MshB is representative of a wider family of enzymes that act on substituted N-acetylglucosamine residues, including a deacetylase involved in the biosynthesis of glycosylphosphatidylinositol (GPI) anchors in eukaryotes.

Glutathione, Altruistic Metabolite in Fungi

Glutathione (GSH; gamma-L-glutamyl-L-cysteinyl-glycine), a non-protein thiol with a very low redox potential (E'0 = 240 mV for thiol-disulfide exchange), is present in high concentration up to 10 mM in yeasts and filamentous fungi. GSH is concerned with basic cellular functions as well as the maintenance of mitochondrial structure, membrane integrity, and in cell differentiation and development. GSH plays key roles in the response to several stress situations in fungi. For example, GSH is an important antioxidant molecule, which reacts non-enzymatically with a series of reactive oxygen species. In addition, the response to oxidative stress also involves GSH biosynthesis enzymes, NADPH-dependent GSH-regenerating reductase, glutathione S-transferase along with peroxide-eliminating glutathione peroxidase and glutaredoxins. Some components of the GSH-dependent antioxidative defence system confer resistance against heat shock and osmotic stress. Formation of protein-SSG mixed disulfides results in protection against desiccation-induced oxidative injuries in lichens. Intracellular GSH and GSH-derived phytochelatins hinder the progression of heavy metal-initiated cell injuries by chelating and sequestering the metal ions themselves and/or by eliminating reactive oxygen species. In fungi, GSH is mobilized to ensure cellular maintenance under sulfur or nitrogen starvation. Moreover, adaptation to carbon deprivation stress results in an increased tolerance to oxidative stress, which involves the induction of GSH-dependent elements of the antioxidant defence system. GSH-dependent detoxification processes concern the elimination of toxic endogenous metabolites, such as excess formaldehyde produced during the growth of the methylotrophic yeasts, by formaldehyde dehydrogenase and methylglyoxal, a by-product of glycolysis, by the glyoxalase pathway. Detoxification of xenobiotics, such as halogenated aromatic and alkylating agents, relies on glutathione S-transferases. In yeast, these enzymes may participate in the elimination of toxic intermediates that accumulate in stationary phase and/or act in a similar fashion as heat shock proteins. GSH S-conjugates may also form in a glutathione S-transferases-independent way, e.g. through chemical reaction between GSH and the antifugal agent Thiram. GSH-dependent detoxification of penicillin side-chain precursors was shown in Penicillium sp. GSH controls aging and autolysis in several fungal species, and possesses an anti-apoptotic feature.

Flavoprotein Disulfide Reductases: Advances in Chemistry and Function

The flavoprotein disulfide reductases represent a family of enzymes that show high sequence and structural homology. They catalyze the pyridine-nucleotide-dependent reduction of a variety of substrates, including disulfide-bonded substrates (lipoamide dehydrogenase, glutathione reductase and functional homologues, thioredoxin reductase, and alkylhydroperoxide reductase), mercuric ion (mercuric ion reductase), hydrogen peroxide (NADH peroxidase), molecular oxygen (NADH oxidase), and the reductive cleavage of a carbonyl-activated carbon-sulfur bond followed by carboxylation (2-ketopropyl-coenzyme-M carboxylase?oxidoreductase). They use at least one nonflavin redox center to transfer electrons from reduced pyridine nucleotide to their substrate through flavin adenine dinucleotide. The nature of the nonflavin redox center located adjacent to the flavin varies and three types have been identified: an enzymic disulfide (most commonly), an enzymic cysteine sulfenic acid (NADH peroxidase and NADH oxidase), and a mixed Cys-S-S-CoA disulfide (coenzyme A disulfide reductase). Selection of the particular nonflavin redox center and utilization of a second, or even a third, nonflavin redox center in some cases presumably represents the most efficient strategy for reduction of the individual substrate.

Mycothiol is essential for growth of Mycobacterium tuberculosis Erdman

Mycothiol (MSH) is the major low-molecular-mass thiol in mycobacteria and is associated with the protection of Mycobacterium tuberculosis from toxic oxidants and antibiotics. The biosynthesis of MSH is a multistep process, with the enzymatic reaction designated MshC being the ligase step in MSH production. A targeted disruption of the native mshC gene in M. tuberculosis Erdman produced no viable clones possessing either a disrupted mshC gene or reduced levels of MSH. However, when a second copy of the mshC gene was incorporated into the chromosome prior to the targeted disruption, multiple clones having the native gene disrupted and the second copy of mshC intact were obtained. These clones produced normal levels of MSH. These results demonstrate that the mshC gene and, more generally, the production of MSH are essential for the growth of M. tuberculosis Erdman under laboratory conditions.

Thiol-Based Regulatory Switches

Thiol-based regulatory switches play central roles in cellular responses to oxidative stress, nitrosative stress, and changes in the overall thiol-disulfide redox balance. Protein sulfhydryls offer a great deal of flexibility in the different types of modification they can undergo and the range of chemical signals they can perceive. For example, recent work on OhrR and OxyR has clearly established that disulfide bonds are not the only cysteine oxidation products that are likely to be relevant to redox sensing in vivo. Furthermore, different stresses can result in distinct modifications to the same protein; in OxyR it seems that distinct modifications can occur at the same cysteine, and in Yap1 a partner protein ensures that the disulfide bond induced by peroxide stress is different from the disulfide bond induced by other stresses. These kinds of discoveries have also led to the intriguing suggestion that different modifications to the same protein can create multiple activation states and thus deliver discrete regulatory outcomes. In this review, we highlight these issues, focusing on seven well-characterized microbial proteins controlled by thiol-based switches, each of which exhibits unique regulatory features.

The crystal structure of 1-D-myo-inosityl 2-acetamido-2-deoxy-alpha-D-glucopyranoside deacetylase (MshB) from Mycobacterium tuberculosis reveals a zinc hydrolase with a lactate dehydrogenase fold

Mycothiol (1-D-myo-inosityl 2-(N-acetyl-L-cysteinyl)amido-2-deoxy-alpha-D-glucopyranoside, MSH or AcCys-GlcN-inositol (Ins)) is the major reducing agent in actinomycetes, including Mycobacterium tuberculosis. The biosynthesis of MSH involves a deacetylase that removes the acetyl group from the precursor GlcNAc-Ins to yield GlcN-Ins. The deacetylase (MshB) corresponds to Rv1170 of M. tuberculosis with a molecular mass of 33,400 Da. MshB is a Zn2+ metalloprotein, and the deacetylase activity is completely dependent on the presence of a divalent metal cation. We have determined the x-ray crystallographic structure of MshB, which reveals a protein that folds in a manner resembling lactate dehydrogenase in the N-terminal domain and a C-terminal domain consisting of two beta-sheets and two alpha-helices. The zinc binding site is in the N-terminal domain occupying a position equivalent to that of the NAD+ co-factor of lactate dehydrogenase. The Zn2+ is 5 coordinate with 3 residues from MshB (His-13, Asp-16, His-147) and two water molecules. One water would be displaced upon binding of substrate (GlcNAc-Ins); the other is proposed as the nucleophilic water assisted by the general base carboxylate of Asp-15. In addition to the Zn2+ providing electrophilic assistance in the hydrolysis, His-144 imidazole could form a hydrogen bond to the oxyanion of the tetrahedral intermediate. The extensive sequence identity of MshB, the deacetylase, with mycothiol S-conjugate amidase, an amide hydrolase that mediates detoxification of mycothiol S-conjugate xenobiotics, has allowed us to construct a faithful model of the catalytic domain of mycothiol S-conjugate amidase based on the structure of MshB.

Listeria monocytogenes sigma B regulates stress response and virulence functions

While the stress-responsive alternative sigma factor sigma(B) has been identified in different species of Bacillus, Listeria, and Staphylococcus, the sigma(B) regulon has been extensively characterized only in B. subtilis. We combined biocomputing and microarray-based strategies to identify sigma(B)-dependent genes in the facultative intracellular pathogen Listeria monocytogenes. Hidden Markov model (HMM)-based searches identified 170 candidate sigma(B)-dependent promoter sequences in the strain EGD-e genome sequence. These data were used to develop a specialized, 208-gene microarray, which included 166 genes downstream of HMM-predicted sigma(B)-dependent promoters as well as selected virulence and stress response genes. RNA for the microarray experiments was isolated from both wild-type and Delta sigB null mutant L. monocytogenes cells grown to stationary phase or exposed to osmotic stress (0.5 M KCl). Microarray analyses identified a total of 55 genes with statistically significant sigma(B)-dependent expression under the conditions used in these experiments, with at least 1.5-fold-higher expression in the wild type over the sigB mutant under either stress condition (51 genes showed at least 2.0-fold-higher expression in the wild type). Of the 55 genes exhibiting sigma(B)-dependent expression, 54 were preceded by a sequence resembling the sigma(B) promoter consensus sequence. Rapid amplification of cDNA ends-PCR was used to confirm the sigma(B)-dependent nature of a subset of eight selected promoter regions. Notably, the sigma(B)-dependent L. monocytogenes genes identified through this HMM/microarray strategy included both stress response genes (e.g., gadB, ctc, and the glutathione reductase gene lmo1433) and virulence genes (e.g., inlA, inlB, and bsh). Our data demonstrate that, in addition to regulating expression of genes important for survival under environmental stress conditions, sigma(B) also contributes to regulation of virulence gene expression in L. monocytogenes. These findings strongly suggest that sigma(B) contributes to L. monocytogenes gene expression during infection.

The metabolism of nitrosothiols in the Mycobacteria: identification and characterization of S-nitrosomycothiol reductase

When grown in culture Mycobacterium smegmatis metabolized S-nitrosoglutathione to oxidized glutathione and nitrate, which suggested a possible involvement of an S-nitrosothiol reductase and mycobacterial haemoglobin. The mycothiol-dependent formaldehyde dehydrogenase from M. smegmatis was purified by a combination of Ni2+-IMAC (immobilized metal ion affinity chromatography), hydrophobic interaction, anion-exchange and affinity chromatography. The enzyme had a subunit molecular mass of 38263 kDa. Steady-state kinetic studies indicated that the enzyme catalyses the NAD+-dependent conversion of S-hydroxymethylmycothiol into formic acid and mycothiol by a rapid-equilibrium ordered mechanism. The enzyme also catalysed an NADH-dependent decomposition of S-nitrosomycothiol (MSNO) by a sequential mechanism and with an equimolar stoichiometry of NADH:MSNO, which indicated that the enzyme reduces the nitroso group to the oxidation level of nitroxyl. Vmax for the MSNO reductase reaction indicated a turnover per subunit of approx. 116700 min(-1), which was 76-fold faster than the formaldehyde dehydrogenase activity. A gene, Rv2259, annotated as a class III alcohol dehydrogenase in the Mycobacterium tuberculosis genome was cloned and expressed in M. smegmatis as the C-terminally His6-tagged product. The purified recombinant enzyme from M. tuberculosis also catalysed both activities. M. smegmatis S-nitrosomycothiol reductase converted MSNO into the N -hydroxysulphenamide, which readily rearranged to mycothiolsulphinamide. In the presence of MSNO reductase, M. tuberculosis HbN (haemoglobin N) was converted with low efficiency into metHbN [HbN(Fe3+)] and this conversion was dependent on turnover of MSNO reductase. These observations suggest a possible route in vivo for the dissimilation of S-nitrosoglutathione.

Crystal structure of the conserved protein TT1542 from Thermus thermophilus HB8

The TT1542 protein from Thermus thermophilus HB8 is annotated as a conserved hypothetical protein, and belongs to the DUF158 family in the Pfam database. A BLAST search revealed that homologs of TT1542 are present in a wide range of organisms. The TT1542 homologs in eukaryotes, PIG-L in mammals, and GPI12 in yeast and protozoa, have N-acetylglucosaminylphosphatidylinositol (GlcNAc-PI) de-N-acetylase activity. Although most of the homologs in prokaryotes are hypothetical and have no known function, Rv1082 and Rv1170 from Mycobacterium tuberculosis are enzymes involved in the mycothiol detoxification pathway. Here we report the crystal structure of the TT1542 protein at 2.0 A resolution, which represents the first structure for this superfamily of proteins. The structure of the TT1542 monomer consists of a twisted beta-sheet composed of six parallel beta-strands and one antiparallel beta-strand (with the strand order 3-2-1-4-5-7-6) sandwiched between six alpha-helices. The N-terminal five beta-strands and four alpha-helices form an incomplete Rossmann fold-like structure. The structure shares some similarity to the sugar-processing enzymes with Rossmann fold-like domains, especially those of the GPGTF (glycogen phosphorylase/glycosyl transferase) superfamily, and also to the NAD(P)-binding Rossmann fold domains. TT1542 is a homohexamer in the crystal and in solution, the six monomers forming a cylindrical structure. Putative active sites are suggested by the structure and conserved amino acid residues.

Synthesis of 2-deoxy-2-C-alkylglucosides of myo-inositol as possible inhibitors of a N-deacetylase enzyme in the biosynthesis of mycothiol

Two new analogues of 1-D-1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-myo-inositol, a biosynthetic intermediate in the production of mycothiol in the Mycobacteria have been synthesized. Both the 2-deoxy-2-C-(2'-hydroxypropyl)-D-glucoside 5, and the 2-deoxy-2-C-(2'-oxopropyl)-D-glucoside 6, are derived from fully benzylated 1-D-1-O-(2-C-allyl-2-deoxy)-D-glucopyranosyl)-myo-inositol 20, readily assembled via a protected 2-C-allyl-2-deoxyglucosyl fluoride. Both 5 and 6 inhibit the incorporation of [3H]inositol by whole cells of Mycobacterium smegmatis into a number of metabolites which contain inositol.

Bacterial mercury resistance from atoms to ecosystems

Bacterial resistance to inorganic and organic mercury compounds (HgR) is one of the most widely observed phenotypes in eubacteria. Loci conferring HgR in Gram-positive or Gram-negative bacteria typically have at minimum a mercuric reductase enzyme (MerA) that reduces reactive ionic Hg(II) to volatile, relatively inert, monoatomic Hg(0) vapor and a membrane-bound protein (MerT) for uptake of Hg(II) arranged in an operon under control of MerR, a novel metal-responsive regulator. Many HgR loci encode an additional enzyme, MerB, that degrades organomercurials by protonolysis, and one or more additional proteins apparently involved in transport. Genes conferring HgR occur on chromosomes, plasmids, and transposons and their operon arrangements can be quite diverse, frequently involving duplications of the above noted structural genes, several of which are modular themselves. How this very mobile and plastic suite of proteins protects host cells from this pervasive toxic metal, what roles it has in the biogeochemical cycling of Hg, and how it has been employed in ameliorating environmental contamination are the subjects of this review.

The glycosyltransferase gene encoding the enzyme catalyzing the first step of mycothiol biosynthesis (mshA)

Mycothiol is the major thiol present in most actinomycetes and is produced from the pseudodisaccharide 1D-myo-inosityl 2-acetamido-2-deoxy-alpha-D-glucopyranoside (GlcNAc-Ins). A transposon mutant of Mycobacterium smegmatis shown to be GlcNAc-Ins and mycothiol deficient was sequenced to identify a putative glycosyltransferase gene designated mshA. The ortholog in Mycobacterium tuberculosis, Rv0486, was used to complement the mutant phenotype.

Copper homeostasis in Enterococcus hirae

Copper is an essential component of life because of its convenient redox potential of 200-800 mV when bound to protein. Extensive insight into copper homeostasis has only emerged in the last decade and Enterococcus hirae has served as a paradigm for many aspects of the process. The cop operon of E. hirae regulates copper uptake, availability, and export. It consists of four genes that encode a repressor, CopY, a copper chaperone, CopZ, and two CPx-type copper ATPases, CopA and CopB. Most of these components have been conserved across the three evolutionary kingdoms. The four Cop proteins have been studied in vivo as well as in vitro and their function is understood in some detail.

Inactivation of mshB, a key gene in the mycothiol biosynthesis pathway in Mycobacterium smegmatis

The mshB gene encoding N-acetyl-1-D-myo-inosityl-2-amino-2-deoxy-alpha-D-glucopyranoside deacetylase (MshB) is a key enzyme in mycothiol biosynthesis. Disruption of mshB in Mycobacterium smegmatis resulted in decreased production of mycothiol (5-10 % of the parent strain mc(2)155) but did not abolish mycothiol synthesis completely. Complementation of the MshB(-) mutants with the mshB gene resulted in increased mycothiol production towards the exponential and stationary phases of the bacterial growth cycle. These results suggest that another enzyme is capable of mycothiol biosynthesis by providing N-acetylglucosaminylinositol deacetylation activity in the absence of MshB. One of the candidate enzymes capable of carrying out such reactions is the MshB orthologue mycothiol amide hydrolase, MCA. However, epichromosomal expression of mca in the MshB(-) mutants did not restore mycothiol levels to the level of the parent strain. Unlike other mutants, which have little or no detectable levels of mycothiol, the MshB(-) mutant did not exhibit increased resistance to isoniazid. However, the MshB(-) mutant was resistant to ethionamide. Phenotypic analysis of other mutants lacking mycothiol revealed that MshA(-) mutants also exhibit ethionamide resistance but that a MshC(-)mutant was sensitive to ethionamide, suggesting that mycothiol or its early intermediates influence ethionamide activation.