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

The oxidant scavenging capacity of the oral Mycoplasma salivarium

OBJECTIVE

Mycoplasma salivarium is a human oral potential pathogen that preferentially resides in dental plaques and gingival sulci. It has been suggested that this organism may play an etiological role in inflammatory processes in the oral cavity. The aim of this work was to determine whether M. salivarium possesses a potent oxidant scavenging capacity (OSC).

DESIGN

The OSC of M. salivarium was quantified by a highly sensitive luminal-dependent chemiluminescence assay in the presence of cocktails that induced a constant flux of luminescence resulting from the generation of peroxide, hydroxyl radical (cocktail A) and NO, superoxide and peroxynitrites (cocktail B).

RESULTS

M. salivarium markedly reduced oxidative stress by scavenging both free reactive oxygen and nitrogen species. The OSC of M. salivarium was much higher than that of other Mycoplasma species. Most of M. salivarium OSC was confined to the cytosolic fraction and was markedly increased in the presence of tannic acid, red blood cells or mucin. The cytosolic OSC of M. salivarium was heat stable and not affected by sodium azide or prolonged proteolysis. However, it was markedly decreased upon dialysis, suggesting that the major reducing activity is not enzymatic but rather, a low molecular weight compound(s).

CONCLUSIONS

The ability of M. salivarium to scavenge oxidants may play a role in the survival and pathogenicity of this microorganism. The enhanced OSC of M. salivarium in the presence of tannic acid, red blood cells or mucin might have a significant importance to assess complex interactions with polyphenols from nutrients, salivary proteins and red blood cells extravasated from injured capillaries during infection and inflammation in oral tissues.

Low-molecular-weight thiols in thiol-disulfide exchange

SIGNIFICANCE

Oxidative stress is widely invoked in inflammation, aging, and complex diseases. To avoid unwanted oxidations, the redox environment of cellular compartments needs to be tightly controlled. The complementary action of oxidoreductases and of high concentrations of low-molecular-weight (LMW) nonprotein thiols plays an essential role in maintaining the redox potential of the cell in balance.

RECENT ADVANCES

While LMW thiols are central players in an extensive range of redox regulation/metabolism processes, not all organisms use the same thiol cofactors to this effect, as evidenced by the recent discovery of mycothiol (MSH) and bacillithiol (BSH) among different gram-positive bacteria.

CRITICAL ISSUES

LMW thiol-disulfide exchange processes and their cellular implications are often oversimplified, as only the biology of the free thiols and their symmetrical disulfides is considered. In bacteria under oxidative stress, especially where concentrations of different LMW thiols are comparable [e.g., BSH, coenzyme A (CoA), and cysteine (Cys) in many low-G+C gram-positive bacteria (Firmicutes)], mixed disulfides (e.g., CoASSB and CySSCoA) must surely be major thiol-redox metabolites that need to be taken into consideration.

FUTURE DIRECTIONS

There are many microorganisms whose LMW thiol-redox buffers have not yet been identified (either bioinformatically or experimentally). Many elements of BSH and MSH redox biochemistry remain to be explored. The fundamental biophysical properties, thiol pK(a) and redox potential, have not yet been determined, and the protein interactome in which the biothiols MSH and BSH are involved needs further exploration.

Ergothioneine is a secreted antioxidant in Mycobacterium smegmatis

Ergothioneine (ERG) and mycothiol (MSH) are two low-molecular-weight thiols synthesized by mycobacteria. The role of MSH has been extensively investigated in mycobacteria; however, little is known about the role of ERG in mycobacterial physiology. In this study, quantification of ERG at various points in the growth cycle of Mycobacterium smegmatis revealed that a significant portion of ERG is found in the culture media, suggesting that it is actively secreted. A mutant of M. smegmatis lacking egtD (MSMEG_6247) was unable to synthesize ERG, confirming its role in ERG biosynthesis. Deletion of egtD from wild-type M. smegmatis and an MSH-deficient mutant did not affect their susceptibility to antibiotics tested in this study. The ERG- and MSH-deficient double mutant was significantly more sensitive to peroxide than either of the single mutants lacking either ERG or MSH, suggesting that both thiols play a role in protecting M. smegmatis against oxidative stress and that ERG is able to partly compensate for the loss of MSH.

The orphan protein bis-γ-glutamylcystine reductase joins the pyridine nucleotide disulfide reductase family

Facile DNA sequencing became possible decades after many enzymes had been purified and characterized. Consequently, there are still "orphan" enyzmes for which activities are known but for which encoding genes have not been identified. Identification of the genes encoding orphan enzymes is important because it allows correct annotation of genes of unknown function or with misassigned function. Bis-γ-glutamylcystine reductase (GCR) is an orphan protein that was purified in 1988. This enzyme catalyzes the reduction of bis-γ-glutamylcystine. γ-Glutamylcysteine is the major low-molecular weight thiol in halobacteria. We purified GCR from Halobacterium sp. NRC-1 and identified the sequence of 23 tryptic peptides by nano-liquid chromatography electrospray ionization tandem mass spectrometry. These peptides cover 62% of the protein predicted to be encoded by a gene in Halobacterium sp. NRC-1 that is annotated as mercuric reductase. GCR and mercuric reductase activities were assayed using enzyme that was expressed in Escherichia coli and refolded from inclusion bodies. The enzyme had robust GCR activity but no mercuric reductase activity. The genomes of most, but not all, halobacteria for which whole genome sequences are available have close homologues of GCR, suggesting that there is more to be learned about the low-molecular weight thiols used in halobacteria.

New targets and inhibitors of mycobacterial sulfur metabolism

The identification of new antibacterial targets is urgently needed to address multidrug resistant and latent tuberculosis infection. Sulfur metabolic pathways are essential for survival and the expression of virulence in many pathogenic bacteria, including Mycobacterium tuberculosis. In addition, microbial sulfur metabolic pathways are largely absent in humans and therefore, represent unique targets for therapeutic intervention. In this review, we summarize our current understanding of the enzymes associated with the production of sulfated and reduced sulfur-containing metabolites in Mycobacteria. Small molecule inhibitors of these catalysts represent valuable chemical tools that can be used to investigate the role of sulfur metabolism throughout the Mycobacterial lifecycle and may also represent new leads for drug development. In this light, we also summarize recent progress made in the development of inhibitors of sulfur metabolism enzymes.

Zn(II) stimulation of Fe(II)-activated repression in the iron-dependent repressor from Mycobacterium tuberculosis

Thermodynamic measurements of Fe(II) binding and activation of repressor function in the iron-dependent repressor from Mycobacterium tuberculosis (IdeR) are reported. IdeR, a member of the diphtheria toxin repressor family of proteins, regulates iron homeostasis and contributes to the virulence response in M. tuberculosis. Although iron is the physiological ligand, this is the first detailed analysis of iron binding and activation in this protein. The results showed that IdeR binds 2 equiv of Fe(II) with dissociation constants that differ by a factor of 25. The high- and low-affinity iron binding sites were assigned to physical binding sites I and II, respectively, using metal binding site mutants. IdeR was also found to contain a high-affinity Zn(II) binding site that was assigned to physical metal binding site II through the use of binding site mutants and metal competition assays. Fe(II) binding was modestly weaker in the presence of Zn(II), but the coupled metal binding-DNA binding affinity was significantly stronger, requiring 30-fold less Fe(II) to activate DNA binding compared to Fe(II) alone. Together, these results suggest that IdeR is a mixed-metal repressor, where Zn(II) acts as a structural metal and Fe(II) acts to trigger the physiologically relevant promoter binding. This new model for IdeR activation provides a better understanding of IdeR and the biology of iron homeostasis in M. tuberculosis.

Distribution and infection-related functions of bacillithiol in Staphylococcus aureus

Bacillithiol (Cys-GlcN-malate, BSH) serves as a major low molecular weight thiol in low GC Gram-positive bacteria including Bacillus species and a variety of Staphylococcus aureus strains. These bacteria do not produce glutathione (GSH). In this study, HPLC analyses were used to determine BSH levels in different S. aureus strains. Furthermore, the role of BSH in the resistance against oxidants and antibiotics and its function in virulence was investigated. We and others (Newton, G.L., Fahey, R.C., Rawat, M., 2012. Microbiology 158, 1117-1126) found that BSH is not produced by members of the S. aureus NCTC8325 lineage, such as strains 8325-4 and SH1000. Using bioinformatics we show that the BSH-biosynthetic gene bshC is disrupted by an 8-bp duplication in S. aureus NCTC8325. The functional bshC-gene from BSH-producing S. aureus Newman (NWMN_1087) was expressed in S. aureus 8325-4 to reconstitute BSH-synthesis. Comparison of the BSH-producing and BSH-minus strains revealed higher resistance of the BSH-producing strain against the antibiotic fosfomycin and the oxidant hypochlorite but not against hydrogen peroxide or diamide. In addition, a higher bacterial load of the BSH-producing strain was detected in human upper-airway epithelial cells and murine macrophages. This indicates a potential role of BSH in protection of S. aureus during infection.

The mechanism of redox sensing in Mycobacterium tuberculosis

Tuberculosis epidemics have defied constraint despite the availability of effective treatment for the past half-century. Mycobacterium tuberculosis, the causative agent of TB, is continually exposed to a number of redox stressors during its pathogenic cycle. The mechanisms used by Mtb to sense redox stress and to maintain redox homeostasis are central to the success of Mtb as a pathogen. Careful analysis of the Mtb genome has revealed that Mtb lacks classical redox sensors such as FNR, FixL, and OxyR. Recent studies, however, have established that Mtb is equipped with various sophisticated redox sensors that can detect diverse types of redox stress, including hypoxia, nitric oxide, carbon monoxide, and the intracellular redox environment. Some of these sensors, such as heme-based DosS and DosT, are unique to mycobacteria, whereas others, such as the WhiB proteins and anti-σ factor RsrA, are unique to actinobacteria. This article provides a comprehensive review of the literature on these redox-sensory modules in the context of TB pathogenesis.

Glutathione analogs in prokaryotes

BACKGROUND

Oxygen is both essential and toxic to all forms of aerobic life and the chemical versatility and reactivity of thiols play a key role in both aspects. Cysteine thiol groups have key catalytic functions in enzymes but are readily damaged by reactive oxygen species (ROS). Low-molecular-weight thiols provide protective buffers against the hazards of ROS toxicity. Glutathione is the small protective thiol in nearly all eukaryotes but in prokaryotes the situation is far more complex.

SCOPE OF REVIEW

This review provides an introduction to the diversity of low-molecular-weight thiol protective systems in bacteria. The topics covered include the limitations of cysteine as a protector, the multiple origins and distribution of glutathione biosynthesis, mycothiol biosynthesis and function in Actinobacteria, recent discoveries involving bacillithiol found in Firmicutes, new insights on the biosynthesis and distribution of ergothioneine, and the potential protective roles played by coenzyme A and other thiols.

MAJOR CONCLUSIONS

Bacteria have evolved a diverse collection of low-molecular-weight protective thiols to deal with oxygen toxicity and environmental challenges. Our understanding of how many of these thiols are produced and utilized is still at an early stage.

GENERAL SIGNIFICANCE

Extensive diversity existed among prokaryotes prior to evolution of the cyanobacteria and the development of an oxidizing atmosphere. Bacteria that managed to adapt to life under oxygen evolved, or acquired, the ability to produce a variety of small thiols for protection against the hazards of aerobic metabolism. Many pathogenic prokaryotes depend upon novel thiol protection systems that may provide targets for new antibacterial agents. This article is part of a Special Issue entitled Cellular functions of glutathione.

NdgR, a common transcriptional activator for methionine and leucine biosynthesis in Streptomyces coelicolor

We show here that NdgR, a known transcriptional activator of isopropylmalate dehydratase in actinomycetes, may have other targets in the cell. An in-frame deletion mutant of ndgR showed unexpectedly poor growth in defined minimal medium even in the presence of leucine. To our surprise, it was supplementation of cysteine and methionine that corrected the growth. Based on this, we propose that NdgR induces cysteine-methionine biosynthesis. Direct involvement of NdgR in the very last steps of methionine synthesis with methionine synthase (metH) and 5,10-methylenetetrahydrofolate reductase (metF) was examined. From a pulldown assay, it was seen that NdgR was enriched from crude cell lysates with a strong affinity to metH and metF upstream sequences. Direct physical interaction of NdgR with these targets was further examined with a gel mobility shift assay. ndgR, leuC, metH, and metF were inducible in M145 cells upon nutrient downshift from rich to minimal medium but were not induced in the ndgR knockout mutant. Taking these observations together, NdgR-dependent metH-metF expression would account for the abnormal growth phenotype of the ndgR mutant although there may be additional NdgR-dependent genes in the Cys-Met metabolic pathways. As the first transcriptional factor reported for regulating Cys-Met metabolism in Streptomyces, NdgR links two disparate amino acid families, branched-chain amino acids (BCAAs) and sulfur amino acids, at the transcriptional level. Considering that Cys-Met metabolism is connected to mycothiol and one-carbon metabolism, NdgR may have broad physiological impacts.

Bacillus anthracis thioredoxin systems, characterization and role as electron donors for ribonucleotide reductase

Bacillus anthracis is the causative agent of anthrax, which is associated with a high mortality rate. Like several medically important bacteria, B. anthracis lacks glutathione but encodes many genes annotated as thioredoxins, thioredoxin reductases, and glutaredoxin-like proteins. We have cloned, expressed, and characterized three potential thioredoxins, two potential thioredoxin reductases, and three glutaredoxin-like proteins. Of these, thioredoxin 1 (Trx1) and NrdH reduced insulin, 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), and the manganese-containing type Ib ribonucleotide reductase (RNR) from B. anthracis in the presence of NADPH and thioredoxin reductase 1 (TR1), whereas thioredoxin 2 (Trx2) could only reduce DTNB. Potential TR2 was verified as an FAD-containing protein reducible by dithiothreitol but not by NAD(P)H. The recently discovered monothiol bacillithiol did not work as a reductant for RNR, either directly or via any of the redoxins. The catalytic efficiency of Trx1 was 3 and 20 times higher than that of Trx2 and NrdH, respectively, as substrates for TR1. Additionally, the catalytic efficiency of Trx1 as an electron donor for RNR was 7-fold higher than that of NrdH. In extracts of B. anthracis, Trx1 was responsible for almost all of the disulfide reductase activity, whereas Western blots showed that the level of Trx1 was 15 and 60 times higher than that of Trx2 and NrdH, respectively. Our findings demonstrate that the most important general disulfide reductase system in B. anthracis is TR1/Trx1 and that Trx1 is the physiologically relevant electron donor for RNR. This information may provide a basis for the development of novel antimicrobial therapies targeting this severe pathogen.

The antibiotic CJ-15,801 is an antimetabolite that hijacks and then inhibits CoA biosynthesis

The natural product CJ-15,801 is an inhibitor of Staphylococcus aureus, but not other bacteria. Its close structural resemblance to pantothenic acid, the vitamin precursor of coenzyme A (CoA), and its Michael acceptor moiety suggest that it irreversibly inhibits an enzyme involved in CoA biosynthesis or utilization. However, its mode of action and the basis for its specificity have not been elucidated to date. We demonstrate that CJ-15,801 is transformed by the uniquely selective S. aureus pantothenate kinase, the first CoA biosynthetic enzyme, into a substrate for the next enzyme, phosphopantothenoylcysteine synthetase, which is inhibited through formation of a tight-binding structural mimic of its native reaction intermediate. These findings reveal CJ-15,801 as a vitamin biosynthetic pathway antimetabolite with a mechanism similar to that of the sulfonamide antibiotics and highlight CoA biosynthesis as a viable antimicrobial drug target.

Bacterial mechanisms of reversible protein S-thiolation: structural and mechanistic insights into mycoredoxins

Mycobacteria produce millimolar concentrations of mycothiol (MSH) as their major low molecular weight thiol redox buffer. MSH-deficient mutants display increased sensitivity towards reactive oxygen, nitrogen and electrophilic species as well as alkylating agents and antibiotics. MSH is maintained in its reduced thiol state by the NADPH-dependent mycothiol disulphide reductase (Mtr). However, the redoxin that uses the MSH/Mtr/NADPH pathway for reduction of MSH-mixed protein disulphides, formed during oxidative stress, has long remained unknown. In this issue, Van Laer et al. report that MSH provides the reducing power for mycoredoxin-1 (Mrx1) in reduction of synthetic MSH-mixed disulphides. The reduced (dithiol) and oxidized (disulphide) solution structures of Mrx1 have been solved by nuclear magnetic resonance (NMR) spectroscopy. NMR time course experiments have also demonstrated the transient S-mycothiolation of the active site Cys14 of oxidized Mrx1 during reduction by the MSH/Mtr/NADPH electron pathway. The paper opens a new era of research to identify S-mycothiolated Mrx1 substrates and the function of MSH in redox regulation and virulence in Mycobacterium tuberculosis.

Nickel-catalyzed α-glycosylation of C(1)-hydroxyl D-myo-inositol: a formal synthesis of mycothiol

Formal synthesis of mycothiol has been developed via nickel-catalyzed α-glycosylation of the C(1)-hydroxyl group of D-myo-inositols with C(2)-N-substituted benzylideneamino N-phenyl trifluoroacetimidate donors. The pseudo-oligosaccharides were obtained in good yield and with excellent α-selectivity. Removal of the C(2)-N-2-trifluoromethylphenyl-benzylidene group under mild conditions provides a pseudo-disaccharide, completing the formal synthesis of mycothiol.

Nonsteroidal anti-inflammatory drug sensitizes Mycobacterium tuberculosis to endogenous and exogenous antimicrobials

Existing drugs are slow to eradicate Mycobacterium tuberculosis (Mtb) in patients and have failed to control tuberculosis globally. One reason may be that host conditions impair Mtb's replication, reducing its sensitivity to most antiinfectives. We devised a high-throughput screen for compounds that kill Mtb when its replication has been halted by reactive nitrogen intermediates (RNIs), acid, hypoxia, and a fatty acid carbon source. At concentrations routinely achieved in human blood, oxyphenbutazone (OPB), an inexpensive anti-inflammatory drug, was selectively mycobactericidal to nonreplicating (NR) Mtb. Its cidal activity depended on mild acid and was augmented by RNIs and fatty acid. Acid and RNIs fostered OPB's 4-hydroxylation. The resultant 4-butyl-4-hydroxy-1-(4-hydroxyphenyl)-2-phenylpyrazolidine-3,5-dione (4-OH-OPB) killed both replicating and NR Mtb, including Mtb resistant to standard drugs. 4-OH-OPB depleted flavins and formed covalent adducts with N-acetyl-cysteine and mycothiol. 4-OH-OPB killed Mtb synergistically with oxidants and several antituberculosis drugs. Thus, conditions that block Mtb's replication modify OPB and enhance its cidal action. Modified OPB kills both replicating and NR Mtb and sensitizes both to host-derived and medicinal antimycobacterial agents.

Thiol/Disulfide system plays a crucial role in redox protection in the acidophilic iron-oxidizing bacterium Leptospirillum ferriphilum

Thiol/disulfide systems are involved in the maintenance of the redox status of proteins and other molecules that contain thiol/disulfide groups. Leptospirillum ferriphilum DSM14647, an acidophilic bacterium that uses Fe²⁺ as electron donor, and withstands very high concentrations of iron and other redox active metals, is a good model to study how acidophiles preserve the thiol/disulfide balance. We studied the composition of thiol/disulfide systems and their role in the oxidative stress response in this extremophile bacterium. Bioinformatic analysis using genomic data and enzymatic assays using protein extracts from cells grown under oxidative stress revealed that the major thiol/disulfide system from L. ferriphilum are a cytoplasmic thioredoxin system (composed by thioredoxins Trx and thioredoxin reductase TR), periplasmic thiol oxidation system (DsbA/DsbB) and a c-type cytochrome maturation system (DsbD/DsbE). Upon exposure of L. ferriphilum to reactive oxygen species (ROS)-generating compounds, transcriptional activation of the genes encoding Trxs and the TR enzyme, which results in an increase of the corresponding activity, was observed. Altogether these data suggest that the thioredoxin-based thiol/disulfide system plays an important role in redox protection of L. ferriphilum favoring the survival of this microorganism under extreme environmental oxidative conditions.

Transcriptional regulation of the operon encoding stress-responsive ECF sigma factor SigH and its anti-sigma factor RshA, and control of its regulatory network in Corynebacterium glutamicum

Background

The expression of genes in Corynebacterium glutamicum, a Gram-positive non-pathogenic bacterium used mainly for the industrial production of amino acids, is regulated by seven different sigma factors of RNA polymerase, including the stress-responsive ECF-sigma factor SigH. The sigH gene is located in a gene cluster together with the rshA gene, putatively encoding an anti-sigma factor. The aim of this study was to analyze the transcriptional regulation of the sigH and rshA gene cluster and the effects of RshA on the SigH regulon, in order to refine the model describing the role of SigH and RshA during stress response.

Results

Transcription analyses revealed that the sigH gene and rshA gene are cotranscribed from four sigH housekeeping promoters in C. glutamicum. In addition, a SigH-controlled rshA promoter was found to only drive the transcription of the rshA gene. To test the role of the putative anti-sigma factor gene rshA under normal growth conditions, a C. glutamicum rshA deletion strain was constructed and used for genome-wide transcription profiling with DNA microarrays. In total, 83 genes organized in 61 putative transcriptional units, including those previously detected using sigH mutant strains, exhibited increased transcript levels in the rshA deletion mutant compared to its parental strain. The genes encoding proteins related to disulphide stress response, heat stress proteins, components of the SOS-response to DNA damage and proteasome components were the most markedly upregulated gene groups. Altogether six SigH-dependent promoters upstream of the identified genes were determined by primer extension and a refined consensus promoter consisting of 45 original promoter sequences was constructed.

Conclusions

The rshA gene codes for an anti-sigma factor controlling the function of the stress-responsive sigma factor SigH in C. glutamicum. Transcription of rshA from a SigH-dependent promoter may serve to quickly shutdown the SigH-dependent stress response after the cells have overcome the stress condition. Here we propose a model of the regulation of oxidative and heat stress response including redox homeostasis by SigH, RshA and the thioredoxin system.

Kinetic modelling of GlmU reactions - prioritization of reaction for therapeutic application

Mycobacterium tuberculosis(Mtu), a successful pathogen, has developed resistance against the existing anti-tubercular drugs necessitating discovery of drugs with novel action. Enzymes involved in peptidoglycan biosynthesis are attractive targets for antibacterial drug discovery. The bifunctional enzyme mycobacterial GlmU (Glucosamine 1-phosphate N-acetyltransferase/ N-acetylglucosamine-1-phosphate uridyltransferase) has been a target enzyme for drug discovery. Its C- and N- terminal domains catalyze acetyltransferase (rxn-1) and uridyltransferase (rxn-2) activities respectively and the final product is involved in peptidoglycan synthesis. However, the bifunctional nature of GlmU poses difficulty in deciding which function to be intervened for therapeutic advantage. Genetic analysis showed this as an essential gene but it is still unclear whether any one or both of the activities are critical for cell survival. Often enzymatic activity with suitable high-throughput assay is chosen for random screening, which may not be the appropriate biological function inhibited for maximal effect. Prediction of rate-limiting function by dynamic network analysis of reactions could be an option to identify the appropriate function. With a view to provide insights into biochemical assays with appropriate activity for inhibitor screening, kinetic modelling studies on GlmU were undertaken. Kinetic model of Mtu GlmU-catalyzed reactions was built based on the available kinetic data on Mtu and deduction from Escherichia coli data. Several model variants were constructed including coupled/decoupled, varying metabolite concentrations and presence/absence of product inhibitions. This study demonstrates that in coupled model at low metabolite concentrations, inhibition of either of the GlmU reactions cause significant decrement in the overall GlmU rate. However at higher metabolite concentrations, rxn-2 showed higher decrement. Moreover, with available intracellular concentration of the metabolites and in vivo variant of model, uncompetitive inhibition of rxn-2 caused highest decrement. Thus, at physiologically relevant metabolite concentrations, targeting uridyltranferase activity of Mtu GlmU would be a better choice for therapeutic intervention.

Current status and emerging role of glutathione in food grade lactic acid bacteria

Lactic acid bacteria (LAB) have taken centre stage in perspectives of modern fermented food industry and probiotic based therapeutics. These bacteria encounter various stress conditions during industrial processing or in the gastrointestinal environment. Such conditions are overcome by complex molecular assemblies capable of synthesizing and/or metabolizing molecules that play a specific role in stress adaptation. Thiols are important class of molecules which contribute towards stress management in cell. Glutathione, a low molecular weight thiol antioxidant distributed widely in eukaryotes and Gram negative organisms, is present sporadically in Gram positive bacteria. However, new insights on its occurrence and role in the latter group are coming to light. Some LAB and closely related Gram positive organisms are proposed to possess glutathione synthesis and/or utilization machinery. Also, supplementation of glutathione in food grade LAB is gaining attention for its role in stress protection and as a nutrient and sulfur source. Owing to the immense benefits of glutathione, its release by probiotic bacteria could also find important applications in health improvement. This review presents our current understanding about the status of glutathione and its role as an exogenously added molecule in food grade LAB and closely related organisms.

Changes in the Staphylococcus aureus transcriptome during early adaptation to the lung

Staphylococcus aureus is a common inhabitant of the human nasopharynx. It is also a cause of life-threatening illness, producing a potent array of virulence factors that enable survival in normally sterile sites. The transformation of S. aureus from commensal to pathogen is poorly understood. We analyzed S. aureus gene expression during adaptation to the lung using a mouse model of S. aureus pneumonia. Bacteria were isolated by bronchoalveolar lavage after residence in vivo for up to 6 hours. S. aureus in vivo RNA transcription was compared by microarray to that of shake flask grown stationary phase and early exponential phase cells. Compared to in vitro conditions, the in vivo transcriptome was dramatically altered within 30 minutes. Expression of central metabolic pathways changed significantly in response to the lung environment. Gluconeogenesis (fbs, pckA) was down regulated, as was TCA cycle and fermentation pathway gene expression. Genes associated with amino acid synthesis, RNA translation and nitrate respiration were upregulated, indicative of a highly active metabolic state during the first 6 hours in the lung. Virulence factors regulated by agr were down regulated in vivo and in early exponential phase compared to stationary phase cells. Over time in vivo, expression of ahpCF, involved in H₂O₂ scavenging, and uspA, which encodes a universal stress regulator, increased. Transcription of leukotoxic α and β-type phenol-soluble modulins psmα1-4 and psmβ1-2 increased 13 and 8-fold respectively; hld mRNA, encoding δ-hemolysin, was increased 9-fold. These were the only toxins to be significantly upregulated in vivo. These data provide the first complete survey of the S. aureus transcriptome response to the mammalian airway. The results present intriguing contrasts with previous work in other in vitro and in vivo models and provide novel insights into the adaptive and temporal response of S. aureus early in the pathogenesis of pneumonia.

Characterization of a cold-adapted glutathione synthetase from the psychrophile Pseudoalteromonas haloplanktis

Glutathione (GSH) biosynthesis occurs through two ATP-dependent reactions, usually involving distinct enzymes; in the second step of this process, catalysed by glutathione synthetase (GshB), GSH is formed from γ-glutamylcysteine and glycine. A recombinant form of GshB from the cold-adapted source Pseudoalteromonas haloplanktis (rPhGshB) was purified and characterised. The enzyme formed a disulfide adduct with β-mercaptoethanol, when purified in the presence of this reducing agent. The homotetrameric form of rPhGshB observed at high protein concentration disassembled into two homodimers at low concentration. A new method for directly determining the rPhGshB activity was developed, based on [γ-(32)P]ATP hydrolysis coupled to the GSH synthesis. The ATPase activity required the presence of both γ-glutamylcysteine and glycine and its optimum was reached in the 7.4-8.6 pH range; a divalent cation was absolutely required for the activity, whereas monovalent cations were dispensable. rPhGshB was active at low temperatures and had a similar affinity for ATP (K(m) 0.26 mM) and γ-glutamylcysteine (K(m) 0.25 mM); a lower affinity was measured for glycine (K(m) 0.75 mM). The oxidised form of glutathione (GSSG) acted as an irreversible inhibitor of rPhGshB (K(i) 10.7 mM) and formed disulfide adducts with the enzyme. rPhGshB displayed a great temperature-dependent increase in its activity with an unusually high value of energy of activation (75 kJ mol(-1)) for a psychrophilic enzyme. The enzyme was moderately thermostable, its half inactivation temperature being 50.5 °C after 10 min exposure. The energy of activation of the heat inactivation process was 208 kJ mol(-1). To our knowledge, this is the first contribution to the characterization of a GshB from cold-adapted sources.

An enzyme-initiated Smiles rearrangement enables the development of an assay of MshB, the GlcNAc-Ins deacetylase of mycothiol biosynthesis

MshB is the N-acetyl-1-D-myo-inosityl-2-amino-2-deoxy-D-glucopyranoside (GlcNAc-Ins) deacetylase active as one of the enzymes involved in the biosynthesis of mycothiol (MSH), a protective low molecular weight thiol present only in Mycobacterium tuberculosis and other actinomycetes. In this study, structural analogues of GlcNAc-Ins in which the inosityl moiety is replaced by a chromophore were synthesized and evaluated as alternate substrates of MshB, with the goal of identifying a compound that would be useful in high-throughput assays of the enzyme. In an unexpected and surprising finding one of the GlcNAc-Ins analogues is shown to undergo a Smiles rearrangement upon MshB-mediated deacetylation, uncovering a free thiol group. We demonstrate that this chemistry can be exploited for the development of the first continuous assay of MshB activity based on the detection of thiol formation by DTNB (Ellman's reagent); such an assay should be ideally suited for the identification of MshB inhibitors by means of high-throughput screens in microplates.

Novel L-cysteine-dependent maleylpyruvate isomerase in the gentisate pathway of Paenibacillus sp. strain NyZ101

Glutathione- and mycothiol-dependent maleylpyruvate isomerases are known to be involved, respectively, in gentisate catabolism in Gram-negative and high G+C Gram-positive strains. In the present study, a low-G+C Gram-positive Paenibacillus sp. strain, NyZ101, was isolated and shown to degrade 3-hydroxybenzoate via gentisate. A 6.5-kb fragment containing a conserved region of gentisate 1,2-dioxygenase genes was cloned and sequenced, and four genes (bagKLIX) were shown to encode the enzymes involved in the catabolism to central metabolites of 3-hydroxybenzoate via gentisate. The Bag proteins share moderate identities with the reported enzymes in the 3-hydroxybenzoate catabolism, except BagL that had no obvious homology with any functionally characterized proteins. Recombinant BagL was purified to homogeneity as a His-tagged protein and likely a dimer by gel filtration. BagL was demonstrated to be a novel thiol-dependent maleylpyruvate isomerase catalyzing the isomerization of maleylpyruvate to fumarylpyruvate with L-cysteine, cysteinylglycine, or glutathione, as its cofactor. The K(m) values of these three thiols for BagL were 15.5, 8.4, and 552 μM, respectively. Since cysteine and coenzyme A were reported to be abundant in low-G+C Gram-positive strains, BagL should utilize L-cysteine as its physiological cofactor in vivo. The addition of Ni(2+) increased BagL activity, and site-directed mutagenesis experiments indicated that three conserved histidines in BagL were associated with binding to Ni(2+) ion and were necessary for its enzyme activity. BagL is the first characterized L-cysteine-dependent catabolic enzyme in microbial metabolism and is likely a new and distinct member of DinB family, with a four-helix-bundle topology, as deduced by sequence analysis and homology modeling.

Characterization of BshA, bacillithiol glycosyltransferase from Staphylococcus aureus and Bacillus subtilis

The first step during bacillithiol (BSH) biosynthesis involves the formation of N-acetylglucosaminylmalate from UDP-N-acetylglucosamine and l-malate and is catalyzed by a GT4 class glycosyltransferase enzyme (BshA). Recombinant Staphylococcus aureus and Bacillus subtilis BshA were highly specific and active with l-malate but the former showed low activity with d-glyceric acid and the latter with d-malate. We show that BshA is inhibited by BSH and similarly that MshA (first enzyme of mycothiol biosynthesis) is inhibited by the final product MSH.

Importance and regulation of inositol biosynthesis during growth and differentiation of Streptomyces

Unusually among bacteria, actinobacteria possess myo-inositol 1-phosphate synthase (mIPS). In the developmentally complex Streptomyces coelicolor, the mIPS-encoding gene (inoA) is cotranscribed with a putative regulatory gene (inoR). The inoRA transcript was more abundant in an inoR in-frame deletion mutant, and InoR formed different complexes in vitro with an extensive region around the inoRA promoter. Binding was relieved by adding glucose 6-phosphate. Thus, InoR is a metabolite-sensitive autorepressor that influences inoA expression, and hence the level of inositol, by controlling transcription from P(inoRA) . Disruption of inoA resulted in inositol-dependent growth and development, with full phenotypic correction at 0.1 mM inositol: at lower inositol concentrations differentiation was arrested at intermediate stages. This pattern may partly reflect increased demand for membrane phospholipids during sporulation septation. A corresponding sharp upregulation of inoRA transcription coincident with sporulation was dependent on a developmental regulator, WhiI. A truncated form of WhiI could bind two sites downstream of P(inoRA) , and one of the WhiI-binding sites overlapped the InoR-binding site. The combined action of a metabolic regulator and a developmental regulator at the simple P(inoRA) promoter is a previously undescribed strategy for the differential provision of developmentally appropriate levels of a substance required during the formation of spore chains.

Breaking a paradigm: cosmopolitan and abundant freshwater actinobacteria are low GC

Free-living Actinobacteria are universally recognized as high-GC organisms. Freshwater Actinobacteria have been identified as abundant and prevalent members of freshwater microbial communities, but the two most common lineages (acI and acIV) have remained impossible to culture to date. We have analysed metagenomic data from lakes and estuaries, and show that members of acI and acIV are indeed abundant. We then show that the majority of actinobacterial reads from metagenomic datasets (both lakes and estuaries) are consistently low GC. Analysis of assembled scaffolds from these datasets also confirms that actinobacterial scaffolds are primarily low GC, although high-GC scaffolds were also observed, indicating both types of Actinobacteria coinhabit. Phylogenetic analysis of 16S rRNA gene sequences, both from PCR-based clone libraries and metagenomic reads, and the discovery of a low-GC scaffold containing a partial 16S rRNA gene, points to the abundance of the well-known acI and acIV lineages of freshwater in these habitats, both of which appear to be low GC.

Detoxification of toxins by bacillithiol in Staphylococcus aureus

Bacillithiol (BSH), an α-anomeric glycoside of l-cysteinyl-d-glucosaminyl-l-malate, is a major low-molecular-mass thiol found in bacteria such as Bacillus sp., Staphylococcus aureus and Deinococcus radiodurans. Like other low-molecular-mass thiols such as glutathione and mycothiol, BSH is likely to be involved in protection against environmental toxins including thiol-reactive antibiotics. We report here a BSH-dependent detoxification mechanism in S. aureus. When S. aureus Newman strain was treated with monobromobimane and monochlorobimane, the cellular BSH was converted to the fluorescent S-conjugate BS-bimane. A bacillithiol conjugate amidase activity acted upon the BS-bimane to produce Cys-bimane, which was then acetylated by an N-acetyltransferase to generate N-acetyl-Cys-bimane, a mercapturic acid. An S. aureus mutant lacking BSH did not produce mercapturic acid when treated with monobromobimane and monochlorobimane, confirming the involvement of bacillithiol. Furthermore, treatment of S. aureus Newman with rifamycin, the parent compound of the first-line anti-tuberculosis drug, rifampicin, indicated that this thiol-reactive antibiotic is also detoxified in a BSH-dependent manner, since mercapturic acids of rifamycin were observed in the culture medium. These data indicate that toxins and thiol-reactive antibiotics are detoxified to less potent mercapturic acids in a BSH-dependent manner and then exported out of the cell in S. aureus.

Redox biology of tuberculosis pathogenesis

Mycobacterium tuberculosis (Mtb) is one of the most successful human pathogens. Mtb is persistently exposed to numerous oxidoreductive stresses during its pathogenic cycle of infection and transmission. The distinctive ability of Mtb, not only to survive the redox stress manifested by the host but also to use it for synchronizing the metabolic pathways and expression of virulence factors, is central to its success as a pathogen. This review describes the paradigmatic redox and hypoxia sensors employed by Mtb to continuously monitor variations in the intracellular redox state and the surrounding microenvironment. Two component proteins, namely, DosS and DosT, are employed by Mtb to sense changes in oxygen, nitric oxide, and carbon monoxide levels, while WhiB3 and anti-sigma factor RsrA are used to monitor changes in intracellular redox state. Using these and other unidentified redox sensors, Mtb orchestrates its metabolic pathways to survive in nutrient-deficient, acidic, oxidative, nitrosative, and hypoxic environments inside granulomas or infectious lesions. A number of these metabolic pathways are unique to mycobacteria and thus represent potential drug targets. In addition, Mtb employs versatile machinery of the mycothiol and thioredoxin systems to ensure a reductive intracellular environment for optimal functioning of its proteins even upon exposure to oxidative stress. Mtb also utilizes a battery of protective enzymes, such as superoxide dismutase (SOD), catalase (KatG), alkyl hydroperoxidase (AhpC), and peroxiredoxins, to neutralize the redox stress generated by the host immune system. This chapter reviews the current understanding of mechanisms employed by Mtb to sense and neutralize redox stress and their importance in TB pathogenesis and drug development.

Redox homeostasis in mycobacteria: the key to tuberculosis control?

Mycobacterium tuberculosis (Mtb) is a metabolically flexible pathogen that has the extraordinary ability to sense and adapt to the continuously changing host environment experienced during decades of persistent infection. Mtb is continually exposed to endogenous reactive oxygen species (ROS) as part of normal aerobic respiration, as well as exogenous ROS and reactive nitrogen species (RNS) generated by the host immune system in response to infection. The magnitude of tuberculosis (TB) disease is further amplified by exposure to xenobiotics from the environment such as cigarette smoke and air pollution, causing disruption of the intracellular prooxidant–antioxidant balance. Both oxidative and reductive stresses induce redox cascades that alter Mtb signal transduction, DNA and RNA synthesis, protein synthesis and antimycobacterial drug resistance. As reviewed in this article, Mtb has evolved specific mechanisms to protect itself against endogenously produced oxidants, as well as defend against host and environmental oxidants and reductants found specifically within the microenvironments of the lung. Maintaining an appropriate redox balance is critical to the clinical outcome because several antimycobacterial prodrugs are only effective upon bioreductive activation. Proper homeostasis of oxido-reductive systems is essential for Mtb survival, persistence and subsequent reactivation. The progress and remaining deficiencies in understanding Mtb redox homeostasis are also discussed.

Ni2+-activated glyoxalase I from Escherichia coli: substrate specificity, kinetic isotope effects and evolution within the βαβββ superfamily

The Escherichia coli glyoxalase system consists of the metalloenzymes glyoxalase I and glyoxalase II. Little is known regarding Ni(2+)-activated E. coli glyoxalase I substrate specificity, its thiol cofactor preference, the presence or absence of any substrate kinetic isotope effects on the enzyme mechanism, or whether glyoxalase I might catalyze additional reactions similar to those exhibited by related βαβββ structural superfamily members. The current investigation has shown that this two-enzyme system is capable of utilizing the thiol cofactors glutathionylspermidine and trypanothione, in addition to the known tripeptide glutathione, to convert substrate methylglyoxal to non-toxic D-lactate in the presence of Ni(2+) ion. E. coli glyoxalase I, reconstituted with either Ni(2+) or Cd(2+), was observed to efficiently process deuterated and non-deuterated phenylglyoxal utilizing glutathione as cofactor. Interestingly, a substrate kinetic isotope effect for the Ni(2+)-substituted enzyme was not detected; however, the proton transfer step was observed to be partially rate limiting for the Cd(2+)-substituted enzyme. This is the first non-Zn(2+)-activated GlxI where a metal ion-dependent kinetic isotope effect using deuterium-labelled substrate has been observed. Attempts to detect a glutathione conjugation reaction with the antibiotic fosfomycin, similar to the reaction catalyzed by the related superfamily member FosA, were unsuccessful when utilizing the E. coli glyoxalase I E56A mutein.

The DinB superfamily includes novel mycothiol, bacillithiol, and glutathione S-transferases

The superfamily of glutathione S-transferases has been the subject of extensive study; however, Actinobacteria produce mycothiol (MSH) in place of glutathione, and no mycothiol S-transferase (MST) has been identified. Using mycothiol and monochlorobimane as substrates, an MST activity was detected in extracts of Mycobacterium smegmatis and purified sufficiently to allow identification of MSMEG_0887, a member the DUF664 family of the DinB superfamily, as the MST. The identity of the M. smegmatis and homologous Mycobacterium tuberculosis (Rv0443) enzymes was confirmed by cloning, and the expressed proteins were found to be active with MSH but not bacillithiol (BSH) or glutathione (GSH). Bacillus subtilis YfiT is another member of the DinB superfamily, but this bacterium produces BSH. The YfiT protein was shown to have S-transferase activity with monochlorobimane when assayed with BSH but not with MSH or GSH. Enterococcus faecalis EF_3021 shares some homology with MSMEG_0887, but En. faecalis produces GSH but not MSH or BSH. Cloned and expressed EF_0321 was active with monochlorobimane and GSH but not with MSH or BSH. MDMPI_2 is another member of the DinB superfamily and has been previously shown to have mycothiol-dependent maleylpyruvate isomerase activity. Three of the eight families of the DinB superfamily include proteins shown to catalyze thiol-dependent metabolic or detoxification activities. Because more than two-thirds of the sequences assigned to the DinB superfamily are members of these families, it seems likely that such activity is dominant in the DinB superfamily.

Structural variation in bacterial glyoxalase I enzymes: investigation of the metalloenzyme glyoxalase I from Clostridium acetobutylicum

The glyoxalase system catalyzes the conversion of toxic, metabolically produced α-ketoaldehydes, such as methylglyoxal, into their corresponding nontoxic 2-hydroxycarboxylic acids, leading to detoxification of these cellular metabolites. Previous studies on the first enzyme in the glyoxalase system, glyoxalase I (GlxI), from yeast, protozoa, animals, humans, plants, and Gram-negative bacteria, have suggested two metal activation classes, Zn(2+) and non-Zn(2+) activation. Here, we report a biochemical and structural investigation of the GlxI from Clostridium acetobutylicum, which is the first GlxI enzyme from Gram-positive bacteria that has been fully characterized as to its three-dimensional structure and its detailed metal specificity. It is a Ni(2+)/Co(2+)-activated enzyme, in which the active site geometry forms an octahedral coordination with one metal atom, two water molecules, and four metal-binding ligands, although its inactive Zn(2+)-bound form possesses a trigonal bipyramidal geometry with only one water molecule liganded to the metal center. This enzyme also possesses a unique dimeric molecular structure. Unlike other small homodimeric GlxI where two active sites are located at the dimeric interface, the C. acetobutylicum dimeric GlxI enzyme also forms two active sites but each within single subunits. Interestingly, even though this enzyme possesses a different dimeric structure from previously studied GlxI, its metal activation characteristics are consistent with properties of other GlxI. These findings indicate that metal activation profiles in this class of enzyme hold true across diverse quaternary structure arrangements.

The coenzyme A disulphide reductase of Borrelia burgdorferi is important for rapid growth throughout the enzootic cycle and essential for infection of the mammalian host

In a microarray analysis of the RpoS regulon in mammalian host-adapted Borrelia burgdorferi, bb0728 (cdr) was found to be dually transcribed by the sigma factors σ(70) and RpoS. The cdr gene encodes a coenzyme A disulphide reductase (CoADR) that reduces CoA-disulphides to CoA in an NADH-dependent manner. Based on the abundance of CoA in B. burgdorferi and the biochemistry of the enzyme, CoADR has been proposed to play a role in the spirochaete's response to reactive oxygen species. To better understand the physiologic function(s) of BbCoADR, we generated a B. burgdorferi mutant in which the cdr gene was disrupted. RT-PCR and 5'-RACE analysis revealed that cdr and bb0729 are co-transcribed from a single transcriptional start site upstream of the bb0729 coding sequence; a shuttle vector containing the bb0729-cdr operon and upstream promoter element was used to complement the cdr mutant. Although the mutant was no more sensitive to hydrogen peroxide than its parent, it did exhibit increased sensitivity to high concentrations of t-butyl-hydroperoxide, an oxidizing compound that damages spirochetal membranes. Characterization of the mutant during standard (15% oxygen, 6% CO(2)) and anaerobic (< 1% O(2) , 9-13% CO(2)) cultivation at 37°C revealed a growth defect under both conditions that was particularly striking during anaerobiosis. The mutant was avirulent by needle inoculation and showed decreased survival in feeding nymphs, but displayed no survival defect in unfed flat nymphs. Based on these results, we propose that BbCoADR is necessary to maintain optimal redox ratios for CoA/CoA-disulphide and NAD(+) /NADH during periods of rapid replication throughout the enzootic cycle, to support thiol-disulphide homeostasis, and to indirectly protect the spirochaete against peroxide-mediated membrane damage; one or more of these functions are essential for infection of the mammalian host by B. burgdorferi.

Distribution of glutathione transferases in Gram-positive bacteria and Archaea

Glutathione transferases (GSTs) have been widely studied in Gram-negative bacteria and the structure and function of several representatives have been elucidated. Conversely, limited information is available about the occurrence, classification and functional features of GSTs both in Gram-positive bacteria and in Archaea. An analysis of 305 fully-sequenced Gram-positive genomes highlights the presence of 49 putative GST genes in the genera of both Firmicutes and Actinobacteria phyla. We also performed an analysis on 81 complete genomes of the Archaea domain. Eleven hits were found in the Halobacteriaceae family of the Euryarchaeota phylum and only one in the Crenarchaeota phylum. A comparison of the identified sequences with well-characterized GSTs belonging to both Gram-negative and eukaryotic GSTs sheds light on their putative function and the evolutionary relationships within the large GST superfamily. This analysis suggests that the identified sequences mainly cluster in the new Xi class, while Beta class GSTs, widely distributed in Gram-negative bacteria, are under-represented in Gram-positive bacteria and absent in Archaea.

The Mycobacterium tuberculosis CysQ phosphatase modulates the biosynthesis of sulfated glycolipids and bacterial growth

CysQ is a 3'-phosphoadenosine-5'-phosphatase that dephosphorylates intermediates from the sulfate assimilation pathway of Mycobacterium tuberculosis (Mtb). Here, we demonstrate that cysQ disruption attenuates Mtb growth in vitro and decreases the biosynthesis of sulfated glycolipids but not major thiols, suggesting that the encoded enzyme specifically regulates mycobacterial sulfation.

Mass Spectrometric Analysis of Mycothiol levels in Wild-Type and Mycothiol Disulfide Reductase Mutant Mycobacterium smegmatis

Mycothiol (MSH), the primary low-molecular weight thiol produced in mycobacteria, acts to protect the cell from oxidative stress and to maintain redox homeostasis, notably in the pathogenic Mycobacterium tuberculosis in the course of human infection. The mycothiol disulfide reductase (Mtr) enzyme reduces the oxidized form of mycothiol, mycothione (MSSM), back to MSH, however its role in bacterial viability is not clear. In this study, we sought to determine the MSH levels of wild-type (WT) and Mtr mutant mycobacteria during oxidative stress. We describe a rapid method for the relative quantification of MSH using high-sensitivity mass spectrometry (MS) with selected ion monitoring (SIM). This method uses only minimal sample cleanup, and does not require advanced chromatographic equipment or fluorescent compounds. MSH levels decreased in the Mtr mutant only upon treatment with peroxide, and the results were consistent between our method and previously-described thiol quantification methods. Our results indicate that our MS-based method is a useful, high-throughput alternative tool for the quantification of MSH from mycobacteria.

Crystallization and diffraction analysis of thioredoxin reductase from Streptomyces coelicolor

Thioredoxin reductases are homodimeric flavoenzymes that catalyze the transfer of electrons from NADPH to oxidized thioredoxin substrate. Bacterial thioredoxin reductases represent a promising target for the development of new antibiotics. Recombinant thioredoxin reductase TrxB from Streptomyces coelicolor was crystallized using the hanging-drop vapour-diffusion method. X-ray diffraction data were collected from cryocooled crystals to 2.4 Å resolution using a synchrotron-radiation source. The crystals belonged to the primitive monoclinic space group P2(1), with unit-cell parameters a = 82.9, b = 60.6, c = 135.4 Å, α = γ = 90.0, β = 96.5°.

The regulation of sulfur metabolism in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) has evolved into a highly successful human pathogen. It deftly subverts the bactericidal mechanisms of alveolar macrophages, ultimately inducing granuloma formation and establishing long-term residence in the host. These hallmarks of Mtb infection are facilitated by the metabolic adaptation of the pathogen to its surrounding environment and the biosynthesis of molecules that mediate its interactions with host immune cells. The sulfate assimilation pathway of Mtb produces a number of sulfur-containing metabolites with important contributions to pathogenesis and survival. This pathway is regulated by diverse environmental cues and regulatory proteins that mediate sulfur transactions in the cell. Here, we discuss the transcriptional and biochemical mechanisms of sulfur metabolism regulation in Mtb and potential small molecule regulators of the sulfate assimilation pathway that are collectively poised to aid this intracellular pathogen in its expert manipulation of the host. From this global analysis, we have identified a subset of sulfur-metabolizing enzymes that are sensitive to multiple regulatory cues and may be strong candidates for therapeutic intervention.

Bacillithiol, a new player in bacterial redox homeostasis

Bacillithiol (BSH), the α-anomeric glycoside of l-cysteinyl-d-glucosamine with l-malic acid, plays a dominant role in the cytosolic thiol redox chemistry of the low guanine and cytosine (GC) Gram-positive bacteria (phylum Firmicutes). BSH is functionally analogous to glutathione (GSH) but differs sufficiently in chemical structure that cells have evolved a distinct set of enzymes that use BSH as cofactor. BSH was discovered in Bacillus subtilis as a mixed disulfide with the redox-sensing repressor OhrR and in B. anthracis by biochemical analysis of pools of labeled thiols. The structure of BSH was determined after purification from Deinococcus radiodurans. Similarities in structure between BSH and mycothiol (MSH) facilitated the identification of biosynthetic genes for BSH in the model organism B. subtilis. Phylogenomic analyses have identified several candidate BSH-using or associated proteins, including a BSH reductase, glutaredoxin-like thiol-dependent oxidoreductases (bacilliredoxins), and a BSH-S-transferase (FosB) involved in resistance to the epoxide antibiotic fosfomycin. Preliminary results implicate BSH in cellular processes to maintain cytosolic redox balance and for adaptation to reactive oxygen, nitrogen, and electrophilic species. BSH also is predicted to chelate metals avidly, in part due to the appended malate moiety, although the implications of BSH for metal ion homeostasis have yet to be explored in detail.

tdd8: a TerD domain-encoding gene involved in Streptomyces coelicolor differentiation

The Streptomyces coelicolor genome contains 17 TerD domain-encoding genes (tdd genes) of unknown function. The proteins encoded by these genes have been presumed to be involved in tellurite resistance on the basis of their homology with the protein TerD of Serratia marcescens. To elucidate the role of a Tdd protein (Tdd8), both a deletion mutant for the corresponding gene tdd8 (SCO2368) and a recombinant strain over-expressing tdd8 were produced in S. coelicolor M145. The deletion mutant (Δtdd8), like the wild strain, was not resistant to potassium tellurite. The deletion was not lethal but had a marked effect on differentiation. The deletion strain showed more rapid growth in liquid medium and produced long chains of short spores with a dense and non-spherical spore wall on agar plates. The strain over-expressing tdd8 had a growth delay in liquid medium and produced very few spores of irregular shapes and sizes on solid medium. The results of this study demonstrated that Tdd proteins might have a function other than tellurite resistance and this function seems to be of crucial importance for the proper development of the actinomycete S. coelicolor.

A thermophilic bacterial origin and subsequent constraints by redox, light and salinity on the evolution of the microbial mercuric reductase

Mercuric reductase (MerA) is central to the mercury (Hg) resistance (mer) system, catalyzing the reduction of ionic Hg to volatile Hg(0). A total of 213 merA homologues were identified in sequence databases, the majority of which belonged to microbial lineages that occupy oxic environments. merA was absent among phototrophs and in lineages that inhabit anoxic environments. Phylogenetic reconstructions of MerA indicate that (i) merA originated in a thermophilic bacterium following the divergence of the Archaea and Bacteria with a subsequent acquisition in Archaea via horizontal gene transfer (HGT), (ii) HGT of merA was rare across phylum boundaries and (iii) MerA from marine bacteria formed distinct and strongly supported lineages. Collectively, these observations suggest that a combination of redox, light and salinity conditions constrain MerA to microbial lineages that occupy environments where the most oxidized and toxic form of Hg, Hg(II), predominates. Further, the taxon-specific distribution of MerA with and without a 70 amino acid N-terminal extension may reflect intracellular levels of thiols. In conclusion, MerA likely evolved following the widespread oxygenation of the biosphere in a thermal environment and its subsequent evolution has been modulated by the interactions of Hg with the intra- and extracellular environment of the organism.

Repurposing lipoic acid changes electron flow in two important metabolic pathways of Escherichia coli

In bacteria, cysteines of cytoplasmic proteins, including the essential enzyme ribonucleotide reductase (RNR), are maintained in the reduced state by the thioredoxin and glutathione/glutaredoxin pathways. An Escherichia coli mutant lacking both glutathione reductase and thioredoxin reductase cannot grow because RNR is disulfide bonded and nonfunctional. Here we report that suppressor mutations in the lpdA gene, which encodes the oxidative enzyme lipoamide dehydrogenase required for tricarboxylic acid (TCA) cycle functioning, restore growth to this redox-defective mutant. The suppressor mutations reduce LpdA activity, causing the accumulation of dihydrolipoamide, the reduced protein-bound form of lipoic acid. Dihydrolipoamide can then provide electrons for the reactivation of RNR through reduction of glutaredoxins. Dihydrolipoamide is oxidized in the process, restoring function to the TCA cycle. Thus, two electron transfer pathways are rewired to meet both oxidative and reductive needs of the cell: dihydrolipoamide functionally replaces glutathione, and the glutaredoxins replace LpdA. Both lipoic acid and glutaredoxins act in the reverse manner from their normal cellular functions. Bioinformatic analysis suggests that such activities may also function in other bacteria.