Biosynthesis and functions of mycothiol, the unique protective thiol of Actinobacteria

Mycothiol synthesis by an anomerization reaction through endocyclic cleavage

Mycothiol is found in Gram-positive bacteria, where it helps in maintaining a reducing intracellular environment and it plays an important role in protecting the cell from toxic chemicals. The inhibition of the mycothiol biosynthesis is considered as a treatment for tuberculosis. Mycothiol contains an α-aminoglycoside, which is difficult to prepare stereoselectively by a conventional glycosylation reaction. In this study, mycothiol was synthesized by an anomerization reaction from an easily prepared β-aminoglycoside through endocyclic cleavage.

Copper homeostasis in Mycobacterium tuberculosis

Copper (Cu) is a trace element essential for the growth and development of almost all organisms, including bacteria. However, Cu overload in most systems is toxic. Studies show Cu accumulates in macrophage phagosomes infected with bacteria, suggesting Cu provides an innate immune mechanism to combat invading pathogens. To counteract the host-supplied Cu, increasing evidence suggests that bacteria have evolved Cu resistance mechanisms to facilitate their pathogenesis. In particular, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, has evolved multiple pathways to respond to Cu. Here, we summarize what is currently known about Cu homeostasis in Mtb and discuss potential sources of Cu encountered by this and other pathogens in a mammalian host.

Resistance to Isoniazid and Ethionamide in Mycobacterium tuberculosis: Genes, Mutations, and Causalities

Isoniazid (INH) is the cornerstone of tuberculosis (TB) chemotherapy, used for both treatment and prophylaxis of TB. The antimycobacterial activity of INH was discovered in 1952, and almost as soon as its activity was published, the first INH-resistant Mycobacterium tuberculosis strains were reported. INH and its structural analog and second-line anti-TB drug ethionamide (ETH) are pro-drugs. INH is activated by the catalase-peroxidase KatG, while ETH is activated by the monooxygenase EthA. The resulting active species reacts with NAD+ to form an INH-NAD or ETH-NAD adduct, which inhibits the enoyl ACP reductase InhA, leading to mycolic acid biosynthesis inhibition and mycobacterial cell death. The major mechanism of INH resistance is mutation in katG, encoding the activator of INH. One specific KatG variant, S315T, is found in 94% of INH-resistant clinical isolates. The second mechanism of INH resistance is a mutation in the promoter region of inhA (c-15t), which results in inhA overexpression and leads to titration of the drug. Mutations in the inhA open reading frame and promoter region are also the major mechanism of resistance to ETH, found more often in ETH-resistant clinical isolates than mutations in the activator of ETH. Other mechanisms of resistance to INH and ETH include expression changes of the drugs' activators, redox alteration, drug inactivation, and efflux pump activation. In this article, we describe each known mechanism of resistance to INH and ETH and its importance in M. tuberculosis clinical isolates.

Bacterial GCN5-Related N-Acetyltransferases: From Resistance to Regulation

The GCN5-related N-acetyltransferases family (GNAT) is an important family of proteins that includes more than 100000 members among eukaryotes and prokaryotes. Acetylation appears as a major regulatory post-translational modification and is as widespread as phosphorylation. N-Acetyltransferases transfer an acetyl group from acetyl-CoA to a large array of substrates, from small molecules such as aminoglycoside antibiotics to macromolecules. Acetylation of proteins can occur at two different positions, either at the amino-terminal end (αN-acetylation) or at the ε-amino group (εN-acetylation) of an internal lysine residue. GNAT members have been classified into different groups on the basis of their substrate specificity, and in spite of a very low primary sequence identity, GNAT proteins display a common and conserved fold. This Current Topic reviews the different classes of bacterial GNAT proteins, their functions, their structural characteristics, and their mechanism of action.

Complete genome sequence of a psychotrophic Arthrobacter strain A3 (CGMCC 1.8987), a novel long-chain hydrocarbons producer

Arthrobacter strain A3, a psychotrophic bacterium isolated from the Tian Shan Mountain of China, can degrade the cellulose and synthesis the long-chain hydrocarbons efficiently in low temperature. Here we report the complete genome sequence of this bacterium. The complete genome sequence of Arthrobacter strain A3, consisting of a cycle chromosome with a size of 4.26 Mbp and a cycle plasmid with a size of 194kbp. In this genome, a hydrocarbon biosynthesis gene cluster (oleA, oleB/oleC and oleD) was identified. To resistant the extreme environment, this strain contains a unique mycothiol-biosynthetic pathway (mshA-D), which has not been found in other Arthrobacter species before. The availability of this genome sequence allows us to investigate the genetic basis of adaptation to growth in a nutrient-poor permafrost environment and to evaluate of the biofuel-synthetic potential of this species.

Synthesis of mycothiol conjugate analogues and evaluation of their antimycobacterial activity

Drug-resistant Mycobacterium tuberculosis is a growing health problem. As proof of principle that the bacterial-specific metabolite mycothiol could be used as a delivery agent for antimycobacterial agents, simplified analogues of mycothiol were synthesised containing an S-trichloroethenyl substituted cysteine residue. It was envisaged that uptake of the mycothiol analogue would be followed by release of the known cytotoxin S-trichloroethenyl cysteine by the action of mycothiol S-conjugate amidase or its paralog, mycothiol deacetylase MshB. Promising activity was displayed against model Mycobacteria, although further development will be required to improve selectivity.

Mass spectrometry-based assay for the rapid detection of thiol-containing natural products

To expedite discovery of thiol-containing compounds, we devised a selective solid-supported reagent for their immobilization, followed by cleavage of a photocleavable linker to yield stable natural product conjugates for direct detection by mass spectrometry.

Genome sequence comparisons of serial multi-drug-resistant Mycobacterium tuberculosis isolates over 21 years of infection in a single patient

We report a case of chronic pulmonary multi-drug-resistant tuberculosis. Despite 14 years of treatment, Mycobacterium tuberculosis was persistently isolated from sputum. Following treatment cessation the patient remained well, although M. tuberculosis was isolated from sputum for a further 8 years. Genome sequencing of eight serial M. tuberculosis isolates cultured between 1991 and 2011 revealed 17 mutations (0.8 mutations per genome year^- 1). Eight of these were persisting mutations and only two mutations were detected in the 7 years following cessation of treatment in 2004. In four isolates there were mixed alleles, suggesting the likely presence of bacterial subpopulations. The initial 1991 isolate demonstrated genotypic resistance to isoniazid (katG W91R), rifampicin (rpoB S531L), ethambutol (embB M306V), streptomycin (gidB L16R), quinolones (gyrA S95T) and P-aminosalicylic acid (thyA T202A). Subsequent resistance mutations developed for pyrazinamide (pncA I31F) and ethionamide (ethA frameshift). Such information might have been instructive when developing a treatment regimen. In retrospect and with the benefit of high-resolution genomic hindsight we were able to determine that the patient received only one or two active anti-tuberculous agents for most of their treatment. Additionally, mutations in bacA and Rv2326c were detected, which may have contributed to the persistent but mild disease course. BacA is likely to be associated with maintenance of chronic infection and Rv2326c with a decreased bacterial metabolic state. These results expand our understanding of M. tuberculosis evolution during human infection and underline the link between antibiotic resistance and clinical persistence.

Polyketide Quinones Are Alternate Intermediate Electron Carriers during Mycobacterial Respiration in Oxygen-Deficient Niches

Mycobacterium tuberculosis (Mtb) adaptation to hypoxia is considered crucial to its prolonged latent persistence in humans. Mtb lesions are known to contain physiologically heterogeneous microenvironments that bring about differential responses from bacteria. Here we exploit metabolic variability within biofilm cells to identify alternate respiratory polyketide quinones (PkQs) from both Mycobacterium smegmatis (Msmeg) and Mtb. PkQs are specifically expressed in biofilms and other oxygen-deficient niches to maintain cellular bioenergetics. Under such conditions, these metabolites function as mobile electron carriers in the respiratory electron transport chain. In the absence of PkQs, mycobacteria escape from the hypoxic core of biofilms and prefer oxygen-rich conditions. Unlike the ubiquitous isoprenoid pathway for the biosynthesis of respiratory quinones, PkQs are produced by type III polyketide synthases using fatty acyl-CoA precursors. The biosynthetic pathway is conserved in several other bacterial genomes, and our study reveals a redox-balancing chemicocellular process in microbial physiology.

The versatile low-molecular-weight thiols: Beyond cell protection

Low-molecular-weight (LMW) thiols are extensively involved in the maintenance of cellular redox potentials and the protection of cells from a variety of reactive chemical and electrophilic species. However, we recently found that the metabolic coupling of two LMW thiols - mycothiol (MSH) and ergothioneine (EGT) - programs the biosynthesis of the anti-infective agent lincomycin A. Remarkably, such a constructive role of the thiols in the biosynthesis of natural products has so far received relatively little attention. We speculate that the unusual thiol EGT might function as a chiral thiolation carrier (for modification) and a novel activator (for glycosylation) of sugar. Additionally, we examine recent evidence for LMW thiols (MSH and others) as sulfur donors of sulfur-containing natural products. Clearly, the LMW thiols have more diverse activities beyond cell protection, and more attention should be paid to the correlation of their functions with thiol-dependent enzymes.

Deacetylation of mycothiol-derived 'waste product' triggers the last biosynthetic steps of lincosamide antibiotics

Two homologous pyridoxal 5′-phosphate-dependent enzymes LmbF and CcbF transform the deacetylated S-cysteinyl residue of related intermediates in the biosynthesis of lincomycin/celesticetin in different ways.

The immediate post-condensation steps in lincomycin biosynthesis are reminiscent of the mycothiol-dependent detoxification system of actinomycetes. This machinery provides the last proven lincomycin intermediate, a mercapturic acid derivative, which formally represents the ‘waste product’ of the detoxification process. We identified and purified new lincomycin intermediates from the culture broth of deletion mutant strains of Streptomyces lincolnensis and tested these compounds as substrates for proteins putatively involved in lincomycin biosynthesis. The results, based on LC-MS, in-source collision-induced dissociation mass spectrometry and NMR analysis, revealed the final steps of lincomycin biosynthesis, i.e. conversion of the mercapturic acid derivative to lincomycin. Most importantly, we show that deacetylation of the N′-acetyl-S-cysteine residue of the mercapturic acid derivative is required to ‘escape’ the detoxification-like system and proceed towards completion of the biosynthetic pathway. Additionally, our results, supported by l-cysteine-¹³C₃, ¹⁵N incorporation experiments, give evidence that a different type of reaction catalysed by the homologous pair of pyridoxal-5′-phosphate-dependent enzymes, LmbF and CcbF, forms the branch point in the biosynthesis of lincomycin and celesticetin, two related lincosamides.

Physiological roles of bacillithiol in intracellular metal processing

Glutathione (GSH) is an abundantly produced low-molecular-weight (LMW) thiol in many organisms. However, a number of Gram-positive bacteria do not produce GSH, but instead produce bacillithiol (BSH) as one of the major LMW thiols. Similar to GSH, studies have found that BSH has various roles in the cell, including protection against hydrogen peroxide, hypochlorite and disulfide stress. BSH also participates in the detoxification of thiol-reactive antibiotics and the electrophilic metabolite methylglyoxal. Recently, a number of studies have highlighted additional roles for BSH in the processing of intracellular metals. Herein, we examine the potential functions of BSH in the biogenesis of Fe-S clusters, cytosolic metal buffering and the prevention of metal intoxication.

Bacillithiol: a key protective thiol in Staphylococcus aureus

Bacillithiol is a low-molecular-weight thiol analogous to glutathione and is found in several Firmicutes, including Staphylococcus aureus. Since its discovery in 2009, bacillithiol has been a topic of interest because it has been found to contribute to resistance during oxidative stress and detoxification of electrophiles, such as the antibiotic fosfomycin, in S. aureus. The rapid increase in resistance of methicillin-resistant Staphylococcus aureus (MRSA) to available therapeutic agents is a great health concern, and many research efforts are focused on identifying new drugs and targets to combat this organism. This review describes the discovery of bacillithiol, studies that have elucidated the physiological roles of this molecule in S. aureus and other Bacilli, and the contribution of bacillithiol to S. aureus fitness during pathogenesis. Additionally, the bacillithiol biosynthesis pathway is evaluated as a novel drug target that can be utilized in combination with existing therapies to treat S. aureus infections.

Efficient Calculation of Enzyme Reaction Free Energy Profiles Using a Hybrid Differential Relaxation Algorithm: Application to Mycobacterial Zinc Hydrolases

Determination of the free energy profile for an enzyme reaction mechanism is of primordial relevance, paving the way for our understanding of the enzyme's catalytic power at the molecular level. Although hybrid, mostly DFT-based, QM/MM methods have been extensively applied to this type of studies, achieving accurate and statistically converged results at a moderate computational cost is still an open challenge. Recently, we have shown that accurate results can be achieved in less computational time, combining Jarzynski's relationship with a hybrid differential relaxation algorithm (HyDRA), which allows partial relaxation of the solvent during the nonequilibrium steering of the reaction. In this work, we have applied this strategy to study two mycobacterial zinc hydrolases. Mycobacterium tuberculosis infections are still a worldwide problem and thus characterization and validation of new drug targets is an intense field of research. Among possible drug targets, recently two essential zinc hydrolases, MshB (Rv1170) and MA-amidase (Rv3717), have been proposed and structurally characterized. Although possible mechanisms have been proposed by analogy to the widely studied human Zn hydrolases, several key issues, particularly those related to Zn coordination sphere and its role in catalysis, remained unanswered. Our results show that mycobacterial Zn hydrolases share a basic two-step mechanism. First, the attacking water becomes deprotonated by the conserved base and establishes the new C-O bond leading to a tetrahedral intermediate. The intermediate requires moderate reorganization to allow for proton transfer to the amide N and C-N bond breaking to occur in the second step. Zn ion plays a key role in stabilizing the tetrahedral intermediate and balancing the negative charge of the substrate during hydroxide ion attack. Finally, comparative analysis of other Zn hydrolases points to a convergent mechanistic evolution.

High-Resolution Mass Spectrometry Based Proteomic Analysis of the Response to Vancomycin-Induced Cell Wall Stress in Streptomyces coelicolor A3(2)

Understanding how bacteria survive periods of cell wall stress is of fundamental interest and can help generate ideas for improved antibacterial treatments. In this study we use tandem mass tagging to characterize the proteomic response of vancomycin resistant Streptomyces coelicolor to the exposure to sublethal levels of the antibiotic. A common set of 804 proteins were identified in triplicate experiments. Contrasting changes in the abundance of proteins closely associated with the cytoplasmic membrane with those taking place in the cytosol identified aspects of protein spatial localization that are associated with the response to vancomycin. Enzymes for peptidoglycan precursor, mycothiol, ectoine and menaquinone biosynthesis together with a multisubunit nitrate reductase were recruited to the membrane following vancomycin treatment. Many proteins with regulatory functions (including sensor protein kinases) also exhibited significant changes in abundance exclusively in the membrane-associated protein fraction. Several enzymes predicted to be involved in extracellular peptidoglycan crossbridge formation became significantly depleted from the membrane. A comparison with data previously acquired on the changes in gene transcription following vancomycin treatment identified a common high-confidence set of changes in gene expression. Generalized changes in protein abundance indicate roles for proteolysis, the pentose phosphate pathway and a reorganization of amino acid biosynthesis in the stress response.

Protective role of bacillithiol in superoxide stress and Fe-S metabolism in Bacillus subtilis

Glutathione (GSH) serves as the prime thiol in most organisms as its depletion increases antibiotic and metal toxicity, impairs oxidative stress responses, and affects Fe and Fe–S cluster metabolism. Many gram-positive bacteria lack GSH, but instead produce other structurally unrelated yet functionally equivalent thiols. Among those, bacillithiol (BSH) has been recently identified in several low G+C gram-positive bacteria. In this work, we have explored the link between BSH and Fe–S metabolism in Bacillus subtilis. We have identified that B. subtilis lacking BSH is more sensitive to oxidative stress (paraquat), and metal toxicity (Cu(I) and Cd(II)), but not H₂O₂. Furthermore, a slow growth phenotype of BSH null strain in minimal medium was observed, which could be recovered upon the addition of selected amino acids (Leu/Ile and Glu/Gln), supplementation of iron, or chemical complementation with BSH disulfide (BSSB) to the growth medium. Interestingly, Fe–S cluster containing isopropylmalate isomerase (LeuCD) and glutamate synthase (GOGAT) showed decreased activities in BSH null strain. Deficiency of BSH also resulted in decreased levels of intracellular Fe accompanied by increased levels of manganese and altered expression levels of Fe–S cluster biosynthetic SUF components. Together, this study is the first to establish a link between BSH and Fe–S metabolism in B. subtilis.

Functional characterization of a mycothiol peroxidase in Corynebacterium glutamicum that uses both mycoredoxin and thioredoxin reducing systems in the response to oxidative stress

Previous studies have identified a putative mycothiol peroxidase (MPx) in Corynebacterium glutamicum that shared high sequence similarity to sulfur-containing Gpx (glutathione peroxidase; CysGPx). In the present study, we investigated the MPx function by examining its potential peroxidase activity using different proton donors. The MPx degrades hydrogen peroxide and alkyl hydroperoxides in the presence of either the thioredoxin/Trx reductase (Trx/TrxR) or the mycoredoxin 1/mycothione reductase/mycothiol (Mrx1/Mtr/MSH) regeneration system. Mrx1 and Trx employ different mechanisms in reducing MPx. For the Mrx1 system, the catalytic cycle of MPx involves mycothiolation/demycothiolation on the Cys(36) sulfenic acid via the monothiol reaction mechanism. For the Trx system, the catalytic cycle of MPx involves formation of an intramolecular disulfide bond between Cys(36) and Cys(79) that is pivotal to the interaction with Trx. Both the Mrx1 pathway and the Trx pathway are operative in reducing MPx under stress conditions. Expression of mpx markedly enhanced the resistance to various peroxides and decreased protein carbonylation and intracellular reactive oxygen species (ROS) accumulation. The expression of mpx was directly activated by the stress-responsive extracytoplasmic function-σ (ECF-σ) factor [SigH]. Based on these findings, we propose that the C. glutamicum MPx represents a new type of GPx that uses both mycoredoxin and Trx systems for oxidative stress response.

Redox regulation by reversible protein S-thiolation in bacteria

Low molecular weight (LMW) thiols function as thiol-redox buffers to maintain the reduced state of the cytoplasm. The best studied LMW thiol is the tripeptide glutathione (GSH) present in all eukaryotes and Gram-negative bacteria. Firmicutes bacteria, including Bacillus and Staphylococcus species utilize the redox buffer bacillithiol (BSH) while Actinomycetes produce the related redox buffer mycothiol (MSH). In eukaryotes, proteins are post-translationally modified to S-glutathionylated proteins under conditions of oxidative stress. S-glutathionylation has emerged as major redox-regulatory mechanism in eukaryotes and protects active site cysteine residues against overoxidation to sulfonic acids. First studies identified S-glutathionylated proteins also in Gram-negative bacteria. Advances in mass spectrometry have further facilitated the identification of protein S-bacillithiolations and S-mycothiolation as BSH- and MSH-mixed protein disulfides formed under oxidative stress in Firmicutes and Actinomycetes, respectively. In Bacillus subtilis, protein S-bacillithiolation controls the activities of the redox-sensing OhrR repressor and the methionine synthase MetE in vivo. In Corynebacterium glutamicum, protein S-mycothiolation was more widespread and affected the functions of the maltodextrin phosphorylase MalP and thiol peroxidase (Tpx). In addition, novel bacilliredoxins (Brx) and mycoredoxins (Mrx1) were shown to function similar to glutaredoxins in the reduction of BSH- and MSH-mixed protein disulfides. Here we review the current knowledge about the functions of the bacterial thiol-redox buffers glutathione, bacillithiol, and mycothiol and the role of protein S-thiolation in redox regulation and thiol protection in model and pathogenic bacteria.

Lincosamide synthetase--a unique condensation system combining elements of nonribosomal peptide synthetase and mycothiol metabolism

In the biosynthesis of lincosamide antibiotics lincomycin and celesticetin, the amino acid and amino sugar units are linked by an amide bond. The respective condensing enzyme lincosamide synthetase (LS) is expected to be an unusual system combining nonribosomal peptide synthetase (NRPS) components with so far unknown amino sugar related activities. The biosynthetic gene cluster of celesticetin was sequenced and compared to the lincomycin one revealing putative LS coding ORFs shared in both clusters. Based on a bioassay and production profiles of S. lincolnensis strains with individually deleted putative LS coding genes, the proteins LmbC, D, E, F and V were assigned to LS function. Moreover, the newly recognized N-terminal domain of LmbN (LmbN-CP) was also assigned to LS as a NRPS carrier protein (CP). Surprisingly, the homologous CP coding sequence in celesticetin cluster is part of ccbZ gene adjacent to ccbN, the counterpart of lmbN, suggesting the gene rearrangement, evident also from still active internal translation start in lmbN, and indicating the direction of lincosamide biosynthesis evolution. The in vitro test with LmbN-CP, LmbC and the newly identified S. lincolnensis phosphopantetheinyl transferase Slp, confirmed the cooperation of the previously characterized NRPS A-domain LmbC with a holo-LmbN-CP in activation of a 4-propyl-L-proline precursor of lincomycin. This result completed the functional characterization of LS subunits resembling NRPS initiation module. Two of the four remaining putative LS subunits, LmbE/CcbE and LmbV/CcbV, exhibit low but significant homology to enzymes from the metabolism of mycothiol, the NRPS-independent system processing the amino sugar and amino acid units. The functions of particular LS subunits as well as cooperation of both NRPS-based and NRPS-independent LS blocks are discussed. The described condensing enzyme represents a unique hybrid system with overall composition quite dissimilar to any other known enzyme system.

Corynebacterium glutamicum methionine sulfoxide reductase A uses both mycoredoxin and thioredoxin for regeneration and oxidative stress resistance

Oxidation of methionine leads to the formation of the S and R diastereomers of methionine sulfoxide (MetO), which can be reversed by the actions of two structurally unrelated classes of methionine sulfoxide reductase (Msr), MsrA and MsrB, respectively. Although MsrAs have long been demonstrated in numerous bacteria, their physiological and biochemical functions remain largely unknown in Actinomycetes. Here, we report that a Corynebacterium glutamicum methionine sulfoxide reductase A (CgMsrA) that belongs to the 3-Cys family of MsrAs plays important roles in oxidative stress resistance. Deletion of the msrA gene in C. glutamicum resulted in decrease of cell viability, increase of ROS production, and increase of protein carbonylation levels under various stress conditions. The physiological roles of CgMsrA in resistance to oxidative stresses were corroborated by its induced expression under various stresses, regulated directly by the stress-responsive extracytoplasmic-function (ECF) sigma factor SigH. Activity assays performed with various regeneration pathways showed that CgMsrA can reduce MetO via both the thioredoxin/thioredoxin reductase (Trx/TrxR) and mycoredoxin 1/mycothione reductase/mycothiol (Mrx1/Mtr/MSH) pathways. Site-directed mutagenesis confirmed that Cys56 is the peroxidatic cysteine that is oxidized to sulfenic acid, while Cys204 and Cys213 are the resolving Cys residues that form an intramolecular disulfide bond. Mrx1 reduces the sulfenic acid intermediate via the formation of an S-mycothiolated MsrA intermediate (MsrA-SSM) which is then recycled by mycoredoxin and the second molecule of mycothiol, similarly to the glutathione/glutaredoxin/glutathione reductase (GSH/Grx/GR) system. However, Trx reduces the Cys204-Cys213 disulfide bond in CgMsrA produced during MetO reduction via the formation of a transient intermolecular disulfide bond between Trx and CgMsrA. While both the Trx/TrxR and Mrx1/Mtr/MSH pathways are operative in reducing CgMsrA under stress conditions in vivo, the Trx/TrxR pathway alone is sufficient to reduce CgMsrA under normal conditions. Based on these results, a catalytic model for the reduction of CgMsrA by Mrx1 and Trx is proposed.

Glutathionylspermidine in the modification of protein SH groups: the enzymology and its application to study protein glutathionylation

Cysteine is very susceptible to reactive oxygen species. In response; posttranslational thiol modifications such as reversible disulfide bond formation have arisen as protective mechanisms against undesired in vivo cysteine oxidation. In Gram-negative bacteria a major defense mechanism against cysteine overoxidation is the formation of mixed protein disulfides with low molecular weight thiols such as glutathione and glutathionylspermidine. In this review we discuss some of the mechanistic aspects of glutathionylspermidine in prokaryotes and extend its potential use to eukaryotes in proteomics and biochemical applications through an example with tissue transglutaminase and its S-glutathionylation.

Metabolic coupling of two small-molecule thiols programs the biosynthesis of lincomycin A

Low-molecular-mass thiols in organisms are well known for their redox-relevant role in protection against various endogenous and exogenous stresses. In eukaryotes and Gram-negative bacteria, the primary thiol is glutathione (GSH), a cysteinyl-containing tripeptide. In contrast, mycothiol (MSH), a cysteinyl pseudo-disaccharide, is dominant in Gram-positive actinobacteria, including antibiotic-producing actinomycetes and pathogenic mycobacteria. MSH is equivalent to GSH, either as a cofactor or as a substrate, in numerous biochemical processes, most of which have not been characterized, largely due to the dearth of information concerning MSH-dependent proteins. Actinomycetes are able to produce another thiol, ergothioneine (EGT), a histidine betaine derivative that is widely assimilated by plants and animals for variable physiological activities. The involvement of EGT in enzymatic reactions, however, lacks any precedent. Here we report that the unprecedented coupling of two bacterial thiols, MSH and EGT, has a constructive role in the biosynthesis of lincomycin A, a sulfur-containing lincosamide (C8 sugar) antibiotic that has been widely used for half a century to treat Gram-positive bacterial infections. EGT acts as a carrier to template the molecular assembly, and MSH is the sulfur donor for lincomycin maturation after thiol exchange. These thiols function through two unusual S-glycosylations that program lincosamide transfer, activation and modification, providing the first paradigm for EGT-associated biochemical processes and for the poorly understood MSH-dependent biotransformations, a newly described model that is potentially common in the incorporation of sulfur, an element essential for life and ubiquitous in living systems.

Functional characterization of Corynebacterium glutamicum mycothiol S-conjugate amidase

The present study focuses on the genetic and biochemical characterization of mycothiol S-conjugate amidase (Mca) of Corynebacterium glutamicum. Recombinant C. glutamicum Mca was heterologously expressed in Escherichia coli and purified to apparent homogeneity. The molecular weight of native Mca protein determined by gel filtration chromatography was 35 kDa, indicating that Mca exists as monomers in the purification condition. Mca showed amidase activity with mycothiol S-conjugate of monobromobimane (MSmB) in vivo while mca mutant lost the ability to cleave MSmB. In addition, Mca showed limited deacetylase activity with N-acetyl-D-glucosamine (GlcNAc) as substrate. Optimum pH for amidase activity was between 7.5 and 8.5, while the highest activity in the presence of Zn²⁺ confirmed Mca as a zinc metalloprotein. Amino acid residues conserved among Mca family members were located in C. glutamicum Mca and site-directed mutagenesis of these residues indicated that Asp14, Tyr137, His139 and Asp141 were important for activity. The mca deletion mutant showed decreased resistance to antibiotics, alkylating agents, oxidants and heavy metals, and these sensitive phenotypes were recovered in the complementary strain to a great extent. The physiological roles of Mca in resistance to various toxins were further supported by the induced expression of Mca in C. glutamicum under various stress conditions, directly under the control of the stress-responsive extracytoplasmic function-sigma (ECF-σ) factor SigH.

Characterization of p-hydroxycinnamate catabolism in a soil Actinobacterium

p-Hydroxycinnamates, such as ferulate and p-coumarate, are components of plant cell walls and have a number of commercial applications. Rhodococcus jostii RHA1 (RHA1) catabolizes ferulate via vanillate and the β-ketoadipate pathway. Here, we used transcriptomics to identify genes in RHA1 that are upregulated during growth on ferulate versus benzoate. The upregulated genes included three transcriptional units predicted to encode the uptake and β-oxidative deacetylation of p-hydroxycinnamates: couHTL, couNOM, and couR. Neither ΔcouL mutants nor ΔcouO mutants grew on p-hydroxycinnamates, but they did grow on vanillate. Among several p-hydroxycinnamates, CouL catalyzed the thioesterification of p-coumarate and caffeate most efficiently (k(cat)/K(m) = ∼ 400 mM(-1) s(-1)). p-Coumarate was also RHA1's preferred growth substrate, suggesting that CouL is a determinant of the pathway's specificity. CouL did not catalyze the activation of sinapate, in similarity to two p-coumaric acid:coenzyme A (CoA) ligases from plants, and contains the same bulged loop that helps determine substrate specificity in the plant homologues. The couO mutant accumulated 4-hydroxy-3-methoxyphenyl-β-ketopropionate in the culture supernatant when incubated with ferulate, supporting β-oxidative deacetylation. This phenotype was not complemented with a D257N variant of CouO, consistent with the predicted role of Asp257 as a metal ligand in this amidohydrolase superfamily member. These data suggest that CouO functionally replaces the β-ketothiolase and acyl-CoA thioesterase that occur in canonical β-oxidative pathways. Finally, the transcriptomics data suggest the involvement of two distinct formaldehyde detoxification pathways in vanillate catabolism and identify a eugenol catabolic pathway. The results of this study augment our understanding of the bacterial catabolism of aromatics from renewable feedstocks.

Engineered coryneform bacteria as a bio-tool for arsenic remediation

Despite current remediation efforts, arsenic contamination in water sources is still a major health problem, highlighting the need for new approaches. In this work, strains of the nonpathogenic and highly arsenic-resistant bacterium Corynebacterium glutamicum were used as inexpensive tools to accumulate inorganic arsenic, either as arsenate (As(V)) or arsenite (As(III)) species. The assays made use of "resting cells" from these strains, which were assessed under well-established conditions and compared with C. glutamicum background controls. The two mutant As(V)-accumulating strains were those used in a previously published study: (i) ArsC1/C2, in which the gene/s encoding the mycothiol-dependent arsenate reductases is/are disrupted, and (ii) MshA/C mutants unable to produce mycothiol, the low molecular weight thiol essential for arsenate reduction. The As(III)-accumulating strains were either those lacking the arsenite permease activities (Acr3-1 and Acr3-2) needed in As(III) release or recombinant strains overexpressing the aquaglyceroporin genes (glpF) from Corynebacterium diphtheriae or Streptomyces coelicolor, to improve As(III) uptake. Both genetically modified strains accumulated 30-fold more As(V) and 15-fold more As(III) than the controls. The arsenic resistance of the modified strains was inversely proportional to their metal accumulation ability. Our results provide the basis for investigations into the use of these modified C. glutamicum strains as a new bio-tool in arsenic remediation efforts.

The cell envelope glycoconjugates of Mycobacterium tuberculosis

Tuberculosis (TB) remains the second most common cause of death due to a single infectious agent. The cell envelope of Mycobacterium tuberculosis (Mtb), the causative agent of the disease in humans, is a source of unique glycoconjugates and the most distinctive feature of the biology of this organism. It is the basis of much of Mtb pathogenesis and one of the major causes of its intrinsic resistance to chemotherapeutic agents. At the same time, the unique structures of Mtb cell envelope glycoconjugates, their antigenicity and essentiality for mycobacterial growth provide opportunities for drug, vaccine, diagnostic and biomarker development, as clearly illustrated by recent advances in all of these translational aspects. This review focuses on our current understanding of the structure and biogenesis of Mtb glycoconjugates with particular emphasis on one of the most intriguing and least understood aspect of the physiology of mycobacteria: the translocation of these complex macromolecules across the different layers of the cell envelope. It further reviews the rather impressive progress made in the last 10 years in the discovery and development of novel inhibitors targeting their biogenesis.

Purification and characterization of the Staphylococcus aureus bacillithiol transferase BstA

BACKGROUND

Gram-positive bacteria in the phylum Firmicutes synthesize the low molecular weight thiol bacillithiol rather than glutathione or mycothiol. The bacillithiol transferase YfiT from Bacillus subtilis was identified as a new member of the recently discovered DinB/YfiT-like Superfamily. Based on structural similarity using the Superfamily program, we have determined 30 of 31 Staphylococcus aureus strains encode a single bacillithiol transferase from the DinB/YfiT-like Superfamily, while the remaining strain encodes two proteins.

METHODS

We have cloned, purified, and confirmed the activity of a recombinant bacillithiol transferase (henceforth called BstA) encoded by the S. aureus Newman ORF NWMN_2591. Moreover, we have studied the saturation kinetics and substrate specificity of this enzyme using in vitro biochemical assays.

RESULTS

BstA was found to be active with the co-substrate bacillithiol, but not with other low molecular weight thiols tested. BstA catalyzed bacillithiol conjugation to the model substrates monochlorobimane, 1-chloro-2,4-dinitrobenzene, and the antibiotic cerulenin. Several other molecules, including the antibiotic rifamycin S, were found to react directly with bacillithiol, but the addition of BstA did not enhance the rate of reaction. Furthermore, cells growing in nutrient rich medium exhibited low BstA activity.

CONCLUSIONS

BstA is a bacillithiol transferase from S. aureus that catalyzes the detoxification of cerulenin. Additionally, we have determined that bacillithiol itself might be capable of directly detoxifying electrophilic molecules.

GENERAL SIGNIFICANCE

BstA is an active bacillithiol transferase from S. aureus Newman and is the first DinB/YfiT-like Superfamily member identified from this organism. Interestingly, BstA is highly divergent from B. subtilis YfiT.

Rv2466c mediates the activation of TP053 to kill replicating and non-replicating Mycobacterium tuberculosis

The emergence of multidrug- and extensively drug-resistant strains of Mycobacterium tuberculosis highlights the need to discover new antitubercular agents. Here we describe the synthesis and characterization of a new series of thienopyrimidine (TP) compounds that kill both replicating and non-replicating M. tuberculosis. The strategy to determine the mechanism of action of these TP derivatives was to generate resistant mutants to the most effective compound TP053 and to isolate the genetic mutation responsible for this phenotype. The only non-synonymous mutation found was a g83c transition in the Rv2466c gene, resulting in the replacement of tryptophan 28 by a serine. The Rv2466c overexpression increased the sensitivity of M. tuberculosis wild-type and resistant mutant strains to TP053, indicating that TP053 is a prodrug activated by Rv2466c. Biochemical studies performed with purified Rv2466c demonstrated that only the reduced form of Rv2466c can activate TP053. The 1.7 Å resolution crystal structure of the reduced form of Rv2466c, a protein whose expression is transcriptionally regulated during the oxidative stress response, revealed a unique homodimer in which a β-strand is swapped between the thioredoxin domains of each subunit. A pronounced groove harboring the unusual active-site motif CPWC might account for the uncommon reactivity profile of the protein. The mutation of Trp28Ser clearly predicts structural defects in the thioredoxin fold, including the destabilization of the dimerization core and the CPWC motif, likely impairing the activity of Rv2466c against TP053. Altogether our experimental data provide insights into the molecular mechanism underlying the anti-mycobacterial activity of TP-based compounds, paving the way for future drug development programmes.

Structure and function of the LmbE-like superfamily

The LmbE-like superfamily is comprised of a series of enzymes that use a single catalytic metal ion to catalyze the hydrolysis of various substrates. These substrates are often key metabolites for eukaryotes and prokaryotes, which makes the LmbE-like enzymes important targets for drug development. Herein we review the structure and function of the LmbE-like proteins identified to date. While this is the newest superfamily of metallohydrolases, a growing number of functionally interesting proteins from this superfamily have been characterized. Available crystal structures of LmbE-like proteins reveal a Rossmann fold similar to lactate dehydrogenase, which represented a novel fold for (zinc) metallohydrolases at the time the initial structure was solved. The structural diversity of the N-acetylglucosamine containing substrates affords functional diversity for the LmbE-like enzyme superfamily. The majority of enzymes identified to date are metal-dependent deacetylases that catalyze the hydrolysis of a N-acetylglucosamine moiety on substrate using a combination of amino acid side chains and a single bound metal ion, predominantly zinc. The catalytic zinc is coordinated to proteins via His₂-Asp-solvent binding site. Additionally, studies indicate that protein dynamics play important roles in regulating access to the active site and facilitating catalysis for at least two members of this protein superfamily.

The Physiology and Genetics of Oxidative Stress in Mycobacteria

During infection, Mycobacterium tuberculosis is exposed to a diverse array of microenvironments in the human host, each with its own unique set of redox conditions. Imbalances in the redox environment of the bacillus or the host environment serve as stimuli, which could regulate virulence. The ability of M. tuberculosis to evade the host immune response and cause disease is largely owing to the capacity of the mycobacterium to sense changes in its environment, such as host-generated gases, carbon sources, and pathological conditions, and alter its metabolism and redox balance accordingly for survival. In this article we discuss the redox sensors that are, to date, known to be present in M. tuberculosis , such as the Dos dormancy regulon, WhiB family, anti-σ factors, and MosR, in addition to the strategies present in the bacillus to neutralize free radicals, such as superoxide dismutases, catalase-peroxidase, thioredoxins, and methionine sulfoxide reductases, among others. M. tuberculosis is peculiar in that it appears to have a hierarchy of redox buffers, namely, mycothiol and ergothioneine. We discuss the current knowledge of their biosynthesis, function, and regulation. Ergothioneine is still an enigma, although it appears to have distinct and overlapping functions with mycothiol, which enable it to protect against a wide range of toxic metabolites and free radicals generated by the host. Developing approaches to quantify the intracellular redox status of the mycobacterium will enable us to determine how the redox balance is altered in response to signals and environments that mimic those encountered in the host.

Protein S-mycothiolation functions as redox-switch and thiol protection mechanism in Corynebacterium glutamicum under hypochlorite stress

AIMS

Protein S-bacillithiolation was recently discovered as important thiol protection and redox-switch mechanism in response to hypochlorite stress in Firmicutes bacteria. Here we used transcriptomics to analyze the NaOCl stress response in the mycothiol (MSH)-producing Corynebacterium glutamicum. We further applied thiol-redox proteomics and mass spectrometry (MS) to identify protein S-mycothiolation.

RESULTS

Transcriptomics revealed the strong upregulation of the disulfide stress σ(H) regulon by NaOCl stress in C. glutamicum, including genes for the anti sigma factor (rshA), the thioredoxin and MSH pathways (trxB1, trxC, cg1375, trxB, mshC, mca, mtr) that maintain the redox balance. We identified 25 S-mycothiolated proteins in NaOCl-treated cells by liquid chromatography-tandem mass spectrometry (LC-MS/MS), including 16 proteins that are reversibly oxidized by NaOCl in the thiol-redox proteome. The S-mycothiolome includes the methionine synthase (MetE), the maltodextrin phosphorylase (MalP), the myoinositol-1-phosphate synthase (Ino1), enzymes for the biosynthesis of nucleotides (GuaB1, GuaB2, PurL, NadC), and thiamine (ThiD), translation proteins (TufA, PheT, RpsF, RplM, RpsM, RpsC), and antioxidant enzymes (Tpx, Gpx, MsrA). We further show that S-mycothiolation of the thiol peroxidase (Tpx) affects its peroxiredoxin activity in vitro that can be restored by mycoredoxin1. LC-MS/MS analysis further identified 8 proteins with S-cysteinylations in the mshC mutant suggesting that cysteine can be used for S-thiolations in the absence of MSH.

INNOVATION AND CONCLUSION

We identified widespread protein S-mycothiolations in the MSH-producing C. glutamicum and demonstrate that S-mycothiolation reversibly affects the peroxidase activity of Tpx. Interestingly, many targets are conserved S-thiolated across bacillithiol- and MSH-producing bacteria, which could become future drug targets in related pathogenic Gram-positives.

Mycobacterium tuberculosis is extraordinarily sensitive to killing by a vitamin C-induced Fenton reaction

Drugs that kill tuberculosis more quickly could shorten chemotherapy significantly. In Escherichia coli, a common mechanism of cell death by bactericidal antibiotics involves the generation of highly reactive hydroxyl radicals via the Fenton reaction. Here we show that vitamin C, a compound known to drive the Fenton reaction, sterilizes cultures of drug-susceptible and drug-resistant Mycobacterium tuberculosis, the causative agent of tuberculosis. While M. tuberculosis is highly susceptible to killing by vitamin C, other Gram-positive and Gram-negative pathogens are not. The bactericidal activity of vitamin C against M. tuberculosis is dependent on high ferrous ion levels and reactive oxygen species production, and causes a pleiotropic effect affecting several biological processes. This study enlightens the possible benefits of adding vitamin C to an anti-tuberculosis regimen and suggests that the development of drugs that generate high oxidative burst could be of great use in tuberculosis treatment.

NrdH Redoxin enhances resistance to multiple oxidative stresses by acting as a peroxidase cofactor in Corynebacterium glutamicum

NrdH redoxins are small protein disulfide oxidoreductases behaving like thioredoxins but sharing a high amino acid sequence similarity to glutaredoxins. Although NrdH redoxins are supposed to be another candidate in the antioxidant system, their physiological roles in oxidative stress remain unclear. In this study, we confirmed that the Corynebacterium glutamicum NrdH redoxin catalytically reduces the disulfides in the class Ib ribonucleotide reductases (RNR), insulin and 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), by exclusively receiving electrons from thioredoxin reductase. Overexpression of NrdH increased the resistance of C. glutamicum to multiple oxidative stresses by reducing ROS accumulation. Accordingly, elevated expression of the nrdH gene was observed when the C. glutamicum wild-type strain was exposed to oxidative stress conditions. It was discovered that the NrdH-mediated resistance to oxidative stresses was largely dependent on the presence of the thiol peroxidase Prx, as the increased resistance to oxidative stresses mediated by overexpression of NrdH was largely abrogated in the prx mutant. Furthermore, we showed that NrdH facilitated the hydroperoxide reduction activity of Prx by directly targeting and serving as its electron donor. Thus, we present evidence that the NrdH redoxin can protect against the damaging effects of reactive oxygen species (ROS) induced by various exogenous oxidative stresses by acting as a peroxidase cofactor.

MoeH5: a natural glycorandomizer from the moenomycin biosynthetic pathway

The biosynthesis of the phosphoglycolipid antibiotic moenomycin A attracts the attention of researchers hoping to develop new moenomycin-based antibiotics against multidrug resistant Gram-positive infections. There is detailed understanding of most steps of this biosynthetic pathway in Streptomyces ghanaensis (ATCC14672), except for the ultimate stage, where a single pentasaccharide intermediate is converted into a set of unusually modified final products. Here we report that only one gene, moeH5, encoding a homologue of the glutamine amidotransferase (GAT) enzyme superfamily, is responsible for the observed diversity of terminally decorated moenomycins. Genetic and biochemical evidence support the idea that MoeH5 is a novel member of the GAT superfamily, whose homologues are involved in the synthesis of various secondary metabolites as well as K and O antigens of bacterial lipopolysaccharide. Our results provide insights into the mechanism of MoeH5 and its counterparts, and give us a new tool for the diversification of phosphoglycolipid antibiotics.

Mycothiol: a promising antitubercular target

Tuberculosis (TB) is the world's second commonest cause of death next to HIV/AIDS. The increasing emergence of multi drug resistance and the recalcitrant nature of persistent infections pose an additional challenge for the treatment of TB. Due to the development of resistance to conventional antibiotics there is a need for new therapeutic strategies to combat M. tuberculosis. One such target is Mycothiol (MSH), a major low molecular-mass thiol in mycobacteria, an important cellular anti-oxidant. MSH is present only in actinomycetes and hence is a good target. This review explores mycothiol as a potential target against tuberculosis and various research ongoing worldwide.

Biophysical features of bacillithiol, the glutathione surrogate of Bacillus subtilis and other firmicutes

Bacillithiol (BSH) is the major low-molecular-weight (LMW) thiol in many low-G+C Gram-positive bacteria (Firmicutes). Evidence now emerging suggests that BSH functions as an important LMW thiol in redox regulation and xenobiotic detoxification, analogous to what is already known for glutathione and mycothiol in other microorganisms. The biophysical properties and cellular concentrations of such LMW thiols are important determinants of their biochemical efficiency both as biochemical nucleophiles and as redox buffers. Here, BSH has been characterised and compared with other LMW thiols in terms of its thiol pKa , redox potential and thiol-disulfide exchange reactivity. Both the thiol pKa and the standard thiol redox potential of BSH are shown to be significantly lower than those of glutathione whereas the reactivities of the two compounds in thiol-disulfide reactions are comparable. The cellular concentration of BSH in Bacillus subtilis varied over different growth phases and reached up to 5 mM, which is significantly greater than previously observed from single measurements taken during mid-exponential growth. These results demonstrate that the biophysical characteristics of BSH are distinctively different from those of GSH and that its cellular concentrations can reach levels much higher than previously reported.

Cross-functionalities of Bacillus deacetylases involved in bacillithiol biosynthesis and bacillithiol-S-conjugate detoxification pathways

BshB, a key enzyme in bacillithiol biosynthesis, hydrolyses the acetyl group from N-acetylglucosamine malate to generate glucosamine malate. In Bacillus anthracis, BA1557 has been identified as the N-acetylglucosamine malate deacetylase (BshB); however, a high content of bacillithiol (~70%) was still observed in the B. anthracis ∆BA1557 strain. Genomic analysis led to the proposal that another deacetylase could exhibit cross-functionality in bacillithiol biosynthesis. In the present study, BA1557, its paralogue BA3888 and orthologous Bacillus cereus enzymes BC1534 and BC3461 have been characterized for their deacetylase activity towards N-acetylglucosamine malate, thus providing biochemical evidence for this proposal. In addition, the involvement of deacetylase enzymes is also expected in bacillithiol-detoxifying pathways through formation of S-mercapturic adducts. The kinetic analysis of bacillithiol-S-bimane conjugate favours the involvement of BA3888 as the B. anthracis bacillithiol-S-conjugate amidase (Bca). The high degree of specificity of this group of enzymes for its physiological substrate, along with their similar pH-activity profile and Zn²⁺-dependent catalytic acid-base reaction provides further evidence for their cross-functionalities.

C1 metabolism in Corynebacterium glutamicum: an endogenous pathway for oxidation of methanol to carbon dioxide

Methanol is considered an interesting carbon source in "bio-based" microbial production processes. Since Corynebacterium glutamicum is an important host in industrial biotechnology, in particular for amino acid production, we performed studies of the response of this organism to methanol. The C. glutamicum wild type was able to convert (13)C-labeled methanol to (13)CO2. Analysis of global gene expression in the presence of methanol revealed several genes of ethanol catabolism to be upregulated, indicating that some of the corresponding enzymes are involved in methanol oxidation. Indeed, a mutant lacking the alcohol dehydrogenase gene adhA showed a 62% reduced methanol consumption rate, indicating that AdhA is mainly responsible for methanol oxidation to formaldehyde. Further studies revealed that oxidation of formaldehyde to formate is catalyzed predominantly by two enzymes, the acetaldehyde dehydrogenase Ald and the mycothiol-dependent formaldehyde dehydrogenase AdhE. The Δald ΔadhE and Δald ΔmshC deletion mutants were severely impaired in their ability to oxidize formaldehyde, but residual methanol oxidation to CO2 was still possible. The oxidation of formate to CO2 is catalyzed by the formate dehydrogenase FdhF, recently identified by us. Similar to the case with ethanol, methanol catabolism is subject to carbon catabolite repression in the presence of glucose and is dependent on the transcriptional regulator RamA, which was previously shown to be essential for expression of adhA and ald. In conclusion, we were able to show that C. glutamicum possesses an endogenous pathway for methanol oxidation to CO2 and to identify the enzymes and a transcriptional regulator involved in this pathway.

Isoniazid inhibits the heme-based reactivity of Mycobacterium tuberculosis truncated hemoglobin N

Isoniazid represents a first-line anti-tuberculosis medication in prevention and treatment. This prodrug is activated by a mycobacterial catalase-peroxidase enzyme called KatG in Mycobacterium tuberculosis), thereby inhibiting the synthesis of mycolic acid, required for the mycobacterial cell wall. Moreover, isoniazid activation by KatG produces some radical species (e.g., nitrogen monoxide), that display anti-mycobacterial activity. Remarkably, the ability of mycobacteria to persist in vivo in the presence of reactive nitrogen and oxygen species implies the presence in these bacteria of (pseudo-)enzymatic detoxification systems, including truncated hemoglobins (trHbs). Here, we report that isoniazid binds reversibly to ferric and ferrous M. tuberculosis trHb type N (or group I; Mt-trHbN(III) and Mt-trHbN(II), respectively) with a simple bimolecular process, which perturbs the heme-based spectroscopic properties. Values of thermodynamic and kinetic parameters for isoniazid binding to Mt-trHbN(III) and Mt-trHbN(II) are K = (1.1±0.1)×10⁻⁴ M, k_on = (5.3±0.6)×10³ M⁻¹ s⁻¹ and k_off = (4.6±0.5)×10⁻¹ s⁻¹; and D = (1.2±0.2)×10⁻³ M, d_on = (1.3±0.4)×10³ M⁻¹ s⁻¹, and d_off = 1.5±0.4 s⁻¹, respectively, at pH 7.0 and 20.0°C. Accordingly, isoniazid inhibits competitively azide binding to Mt-trHbN(III) and Mt-trHbN(III)-catalyzed peroxynitrite isomerization. Moreover, isoniazid inhibits Mt-trHbN(II) oxygenation and carbonylation. Although the structure of the Mt-trHbN-isoniazid complex is not available, here we show by docking simulation that isoniazid binding to the heme-Fe atom indeed may take place. These data suggest a direct role of isoniazid to impair fundamental functions of mycobacteria, e.g. scavenging of reactive nitrogen and oxygen species, and metabolism.

Regulation of Bacillus subtilis bacillithiol biosynthesis operons by Spx

Bacillithiol is the major low molecular mass thiol produced by many firmicutes bacteria, including the model organism Bacillus subtilis and pathogens such as Bacillus anthracis and Staphylococcus aureus. We have previously shown that four genes (bshA, bshB1, bshB2 and bshC) are involved in bacillithiol biosynthesis. Here, we report that these four genes are encoded within three, unlinked operons all expressed from canonical σ(A)-dependent promoters as determined by 5'RACE (rapid amplification of cDNA ends). The bshA and bshB1 genes are embedded within a seven-gene operon additionally including mgsA, encoding methylglyoxal synthase, and the essential genes cca and birA, encoding tRNA nucleotidyltransferase (CCA transferase) and biotin-protein ligase, respectively. The bshB2 gene is co-transcribed with unknown function genes, while bshC is expressed both as part of a two-gene operon (with the upstream putative pantothenate biosynthesis gene ylbQ) and from its own promoter. All three operons are expressed at a reduced level in an spx null mutant, consistent with a direct role of Spx as a transcriptional activator for these operons, and all three operons are induced by the thiol oxidant diamide. In contrast with other Spx-regulated genes characterized to date, the effects of Spx on basal expression and diamide-stimulated expression appear to be independent of Cys10 in the redox centre of Spx. Consistent with the role of Spx as an activator of bacillithiol biosynthetic genes, cellular levels of bacillithiol are reduced several-fold in an spx null mutant.

Comparative proteomic analysis of Streptomyces lividans Wild-Type and ppk mutant strains reveals the importance of storage lipids for antibiotic biosynthesis

Streptomyces lividans TK24 is a strain that naturally produces antibiotics at low levels, but dramatic overproduction of antibiotics occurs upon interruption of the ppk gene. However, the role of the Ppk enzyme in relation to the regulation of antibiotic biosynthesis remains poorly understood. In order to gain a better understanding of the phenotype of the ppk mutant, the proteomes of the wild-type (wt) and ppk mutant strains, grown for 96 h on R2YE medium limited in phosphate, were analyzed. Intracellular proteins were separated on two-dimensional (2D) gels, spots were quantified, and those showing a 3-fold variation or more were identified by mass spectrometry. The expression of 12 proteins increased and that of 29 decreased in the ppk mutant strain. Our results suggested that storage lipid degradation rather than hexose catabolism was taking place in the mutant. In order to validate this hypothesis, the triacylglycerol contents of the wt and ppk mutant strains of S. lividans as well as that of Streptomyces coelicolor M145, a strain that produces antibiotics at high levels and is closely related to S. lividans, were assessed using electron microscopy and thin-layer chromatography. These studies highlighted the large difference in triacylglycerol contents of the three strains and confirmed the hypothetical link between storage lipid metabolism and antibiotic biosynthesis in Streptomyces.