Isolation of an Escherichia coli mutant deficient in glutathione synthesis

The Glutathione System: A Journey from Cyanobacteria to Higher Eukaryotes

From bacteria to plants and humans, the glutathione system plays a pleiotropic role in cell defense against metabolic, oxidative and metal stresses. Glutathione (GSH), the γ-L-glutamyl-L-cysteinyl-glycine nucleophile tri-peptide, is the central player of this system that acts in redox homeostasis, detoxification and iron metabolism in most living organisms. GSH directly scavenges diverse reactive oxygen species (ROS), such as singlet oxygen, superoxide anion, hydrogen peroxide, hydroxyl radical, nitric oxide and carbon radicals. It also serves as a cofactor for various enzymes, such as glutaredoxins (Grxs), glutathione peroxidases (Gpxs), glutathione reductase (GR) and glutathione-S-transferases (GSTs), which play crucial roles in cell detoxication. This review summarizes what is known concerning the GSH-system (GSH, GSH-derived metabolites and GSH-dependent enzymes) in selected model organisms (Escherichia coli, Saccharomyces cerevisiae, Arabidopsis thaliana and human), emphasizing cyanobacteria for the following reasons. Cyanobacteria are environmentally crucial and biotechnologically important organisms that are regarded as having evolved photosynthesis and the GSH system to protect themselves against the ROS produced by their active photoautotrophic metabolism. Furthermore, cyanobacteria synthesize the GSH-derived metabolites, ergothioneine and phytochelatin, that play crucial roles in cell detoxication in humans and plants, respectively. Cyanobacteria also synthesize the thiol-less GSH homologs ophthalmate and norophthalmate that serve as biomarkers of various diseases in humans. Hence, cyanobacteria are well-suited to thoroughly analyze the role/specificity/redundancy of the players of the GSH-system using a genetic approach (deletion/overproduction) that is hardly feasible with other model organisms (E. coli and S. cerevisiae do not synthesize ergothioneine, while plants and humans acquire it from their soil and their diet, respectively).

Genome-wide analysis of genes involved in efflux function and regulation within Escherichia coli and Salmonella enterica serovar Typhimurium

The incidence of multidrug-resistant bacteria is increasing globally, with efflux pumps being a fundamental platform limiting drug access and synergizing with other mechanisms of resistance. Increased expression of efflux pumps is a key feature of most cells that are resistant to multiple antibiotics. Whilst expression of efflux genes can confer benefits, production of complex efflux systems is energetically costly and the expression of efflux is highly regulated, with cells balancing benefits against costs. This study used TraDIS-Xpress, a genome-wide transposon mutagenesis technology, to identify genes in Escherichia coli and Salmonella Typhimurium involved in drug efflux and its regulation. We exposed mutant libraries to the canonical efflux substrate acriflavine in the presence and absence of the efflux inhibitor phenylalanine-arginine β-naphthylamide. Comparisons between conditions identified efflux-specific and drug-specific responses. Known efflux-associated genes were easily identified, including acrAB, tolC, marRA, ramRA and soxRS, confirming the specificity of the response. Further genes encoding cell envelope maintenance enzymes and products involved with stringent response activation, DNA housekeeping, respiration and glutathione biosynthesis were also identified as affecting efflux activity in both species. This demonstrates the deep relationship between efflux regulation and other cellular regulatory networks. We identified a conserved set of pathways crucial for efflux activity in these experimental conditions, which expands the list of genes known to impact on efflux efficacy. Responses in both species were similar and we propose that these common results represent a core set of genes likely to be relevant to efflux control across the Enterobacteriaceae.

Role of Pseudomonas aeruginosa Glutathione Biosynthesis in Lung and Soft Tissue Infection

The opportunistic pathogen Pseudomonas aeruginosa is a leading cause of morbidity and mortality worldwide. To survive in both the environment and the host, P. aeruginosa must cope with redox stress. In P. aeruginosa, a primary mechanism for protection from redox stress is the antioxidant glutathione (GSH). GSH is a low-molecular-weight thiol-containing tripeptide (l-γ-glutamyl-l-cysteinyl-glycine) that can function as a reversible reducing agent. GSH plays an important role in P. aeruginosa physiology and is known to modulate several cellular and social processes that are likely important during infection. However, the role of GSH biosynthesis during mammalian infection is not well understood. In this study, we created a P. aeruginosa mutant defective in GSH biosynthesis to examine how loss of GSH biosynthesis affects P. aeruginosa virulence. We found that GSH is critical for normal growth in vitro and provides protection against hydrogen peroxide, bleach, and ciprofloxacin. We also studied the role of P. aeruginosa GSH biosynthesis in four mouse infection models, including the surgical wound, abscess, burn wound, and acute pneumonia models. We discovered that the GSH biosynthesis mutant was slightly less virulent in the acute pneumonia infection model but was equally virulent in the three other models. This work provides new and complementary data regarding the role of GSH in P. aeruginosa during mammalian infection.

Glutathione Activates Type III Secretion System Through Vfr in Pseudomonas aeruginosa

Glutathione (GSH) is the most abundant antioxidant in all living organisms. Previously, we have shown that a deletion mutant in the glutathione synthetase gene (ΔgshB) decreases the expression of type III secretion system (T3SS) genes of Pseudomonas aeruginosa. However, the mechanism remains elusive. In this study, a comprehensive transcriptomic analysis of the GSH-deficient mutant ΔgshAΔgshB was used to elucidate the role of GSH in the pathogenesis of P. aeruginosa. The data show that the expression of genes in T3SS, type VI secretion system (T6SS) and some regulatory genes were impaired. ΔgshAΔgshB was attenuated in a mouse model of acute pneumonia, swimming and swarming motilities, and biofilm formation. Under T3SS inducing conditions, GSH enhanced the expression of T3SS in both wild-type PAO1 and ΔgshAΔgshB, but not in Δvfr. Genetic complementation of Δvfr restored the ability of GSH to induce the expression of T3SS genes. Site-directed mutagenesis based substitution of cysteine residues with alanine in Vfr protein abolished the induction of T3SS genes by GSH, confirming that GSH regulates T3SS genes through Vfr. Exposure to H₂O₂ decreased free thiol content on Vfr, indicating that the protein was sensitive to redox modification. Importantly, GSH restored the oxidized Vfr to reduced state. Collectively, these results suggest that GSH serves as an intracellular redox signal sensed by Vfr to upregulate T3SS expression in P. aeruginosa. Our work provides new insights into the role of GSH in P. aeruginosa pathogenesis.

Pseudomonas aeruginosa gshA Mutant Is Defective in Biofilm Formation, Swarming, and Pyocyanin Production

Pseudomonas aeruginosa is a ubiquitous bacterium that can cause severe opportunistic infections, including many hospital-acquired infections. It is also a major cause of infections in patients with cystic fibrosis. P. aeruginosa is intrinsically resistant to a number of drugs and is capable of forming biofilms that are difficult to eradicate with antibiotics. The number of drug-resistant strains is also increasing, making treatment of P. aeruginosa infections very difficult. Thus, there is an urgent need to understand how P. aeruginosa causes disease in order to find novel ways to treat infections. We show that the principal redox buffer, glutathione (GSH), is involved in intrinsic resistance to the fosfomycin and rifampin antibiotics. We further demonstrate that GSH plays a role in P. aeruginosa disease and infection, since a mutant lacking GSH has less biofilm formation, is less able to swarm, and produces less pyocyanin, a pigment associated with infection.

Pseudomonas aeruginosa is a ubiquitous Gram-negative bacterium that can cause severe opportunistic infections. The principal redox buffer employed by this organism is glutathione (GSH). To assess the role of GSH in the virulence of P. aeruginosa, a number of analyses were performed using a mutant strain deficient in gshA, which does not produce GSH. The mutant strain exhibited a growth delay in minimal medium compared to the wild-type strain. Furthermore, the gshA mutant was defective in biofilm and persister cell formation and in swimming and swarming motility and produced reduced levels of pyocyanin, a key virulence factor. Finally, the gshA mutant strain demonstrated increased sensitivity to methyl viologen (a redox cycling agent) as well as the thiol-reactive antibiotics fosfomycin and rifampin. Taken together, these data suggest a key role for GSH in the virulence of P. aeruginosa.

IMPORTANCEPseudomonas aeruginosa is a ubiquitous bacterium that can cause severe opportunistic infections, including many hospital-acquired infections. It is also a major cause of infections in patients with cystic fibrosis. P. aeruginosa is intrinsically resistant to a number of drugs and is capable of forming biofilms that are difficult to eradicate with antibiotics. The number of drug-resistant strains is also increasing, making treatment of P. aeruginosa infections very difficult. Thus, there is an urgent need to understand how P. aeruginosa causes disease in order to find novel ways to treat infections. We show that the principal redox buffer, glutathione (GSH), is involved in intrinsic resistance to the fosfomycin and rifampin antibiotics. We further demonstrate that GSH plays a role in P. aeruginosa disease and infection, since a mutant lacking GSH has less biofilm formation, is less able to swarm, and produces less pyocyanin, a pigment associated with infection.

Isolation and characterization of Xenorhabdus nematophila transposon insertion mutants defective in lipase activity against Tween

We identified Xenorhabdus nematophila transposon mutants with defects in lipase activity. One of the mutations, in yigL, a conserved gene of unknown function, resulted in attenuated virulence against Manduca sexta insects. We discuss possible connections between lipase production, YigL, and specific metabolic pathways.

Glutathione and transition-metal homeostasis in Escherichia coli

Glutathione (GSH) and its derivative phytochelatin are important binding factors in transition-metal homeostasis in many eukaryotes. Here, we demonstrate that GSH is also involved in chromate, Zn(II), Cd(II), and Cu(II) homeostasis and resistance in Escherichia coli. While the loss of the ability to synthesize GSH influenced metal tolerance in wild-type cells only slightly, GSH was important for residual metal resistance in cells without metal efflux systems. In mutant cells without the P-type ATPase ZntA, the additional deletion of the GSH biosynthesis system led to a strong decrease in resistance to Cd(II) and Zn(II). Likewise, in mutant cells without the P-type ATPase CopA, the removal of GSH led to a strong decrease of Cu(II) resistance. The precursor of GSH, gamma-glutamylcysteine (gammaEC), was not able to compensate for a lack of GSH. On the contrary, gammaEC-containing cells were less copper and cadmium tolerant than cells that contained neither gammaEC nor GSH. Thus, GSH may play an important role in trace-element metabolism not only in higher organisms but also in bacteria.

Synthesis of gamma-glutamylcysteine as a major low-molecular-weight thiol in lactic acid bacteria Leuconostoc spp

Some members of lactic acid bacteria are known to synthesize glutathione (GSH) or to import it from growth medium, whereas others are not. Analysis of the genome sequences of several Leuconostoc spp. indicate the presence of the gene gshA that encodes gamma-glutamylcysteine synthetase, but not the gene gshB encoding glutathione synthetase. We report here that, in cells of Leuconostoc kimchii and Leuconostoc mesenteroides, gamma-glutamylcysteine (gamma-GC) is present in large amount, whereas GSH is not detectable. The level of gamma-GC was higher at the stationary phase than at the exponential phase. Expression of the gshA gene in Leuconostoc spp. analyzed by S1 mapping showed the increased mRNA level upon hydrogen peroxide treatment. From high-resolution S1 mapping, the transcriptional start site was mapped and the putative promoter elements were suggested. This work suggests that gamma-GC has a significant role in Leuconostoc spp. as the major low-molecular-weight thiol.

The glutathione-glutaredoxin system in Rhodobacter capsulatus: part of a complex regulatory network controlling defense against oxidative stress

Mutants with defects in components of the glutathione-glutaredoxin (GSH/Grx) system of Rhodobacter capsulatus were constructed to study its role in defense against oxidative stress and the redox-dependent formation of photosynthetic complexes. The lack of the glutaredoxin 3 gene (grxC) or the glutathione synthetase B gene (gshB) resulted in lower growth rates under aerobic conditions and higher sensitivity to oxidative stress, confirming the role of the GSH/Grx system in oxidative stress defense. Both mutants are highly sensitive to disulfide stress, indicating a major contribution of the GSH/Grx system to the thiol-disulfide redox buffer in the cytoplasm. Like mutations in the thioredoxin system, mutations in the GSH/Grx system affected the formation of photosynthetic complexes, which is redox dependent in R. capsulatus. Expression of the genes grxC, gshB, grxA for glutaredoxin 1, and gorA for glutathione reductase, all encoding components of the GSH/Grx system, was not induced by oxidative stress. Other genes, for which a role in oxidative stress was established in Escherichia coli, acnA, fpr, fur, and katG, were strongly induced by oxidative stress in R. capsulatus. Mutations in the grxC, and/or gshB, and/or trxC (thioredoxin 2) genes affected expression of these genes, indicating an interplay of the different defense systems against oxidative stress. The OxyR and the SoxRS regulons control the expression of many genes involved in oxidative stress defense in E. coli in response to H2O2 and superoxide, respectively. Our data and the available genome sequence of R. capsulatus suggest that a SoxRS system is lacking but an alternative superoxide specific regulator exists in R. capsulatus. While the expression of gorA and grxA is regulated by H2O2 in E. coli this is not the case in R. capsulatus, indicating that the OxyR regulons of these two species are significantly different.

Disruption of gamma-glutamylcysteine synthetase results in absolute glutathione auxotrophy and apoptosis in Candida albicans

Glutathione is the most abundant non-protein thiol and a major source of reducing equivalents in eukaryotes. We examined the role of glutathione in Candida albicans by the disruption of gamma-glutamylcysteine synthetase (GCS1), an essential enzyme in glutathione biosynthesis. The gcs1/gcs1 null mutants exhibited glutathione auxotrophy, which could be rescued by supplementing with reduced and oxidized glutathione and gamma-glutamylcysteine. When the mutants were depleted of glutathione, they showed typical markers of apoptosis. These results suggest that glutathione itself is an essential metabolite and C. albicans lacking GCS1 undergoes apoptosis.

Glutathione, Altruistic Metabolite in Fungi

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

Roles of thiol-redox pathways in bacteria

Disulfide bonds in proteins play various important roles. They are either formed as structural features to stabilize the protein or are found only transiently as part of a catalytic or regulatory cycle. In vivo, the formation and reduction of disulfide bonds is catalyzed by specialized thiol-disulfide exchanging enzymes that contain an active site with the sequence motif Cys-X-X-Cys. These proteins have structurally evolved to catalyze predominantly either oxidative reactions or reductive steps. There is mounting evidence that, in addition to the thiol redox potential, the spatial distribution within different cell compartments and the overall redox state of the cell are equally important. In the cytoplasm, multiple pathways play overlapping roles in the reduction of disulfide bonds and additionally, the expression of several components of thiol-redox pathways was shown to respond to the changes in the cellular thiol-redox equilibrium. In the periplasm, two systems coexist, one catalyzing thiol oxidation and the other disulfide reduction. Recent results suggest that two different mechanisms are used to translocate reducing power from the cytoplasm or to dissipate the electrons after oxidation.

The evolution of glutathione metabolism in phototrophic microorganisms

Of the many roles ascribed to glutathione (GSH) the one most clearly established is its role in the protection of higher eucaryotes against oxygen toxicity through destruction of thiol-reactive oxygen byproducts. If this is the primary function of GSH then GSH metabolism should have evolved during or after the evolution of oxygenic photosynthesis. That many bacteria do not produce GSH is consistent with this view. In the present study we have examined the low-molecular-weight thiol composition of a variety of phototrophic microorganisms to ascertain how evolution of GSH production is related to evolution of oxygenic photosynthesis. Cells were extracted in the presence of monobromobimane (mBBr) to convert thiols to fluorescent derivatives, which were analyzed by high-pressure liquid chromatography. Significant levels of GSH were not found in the green bacteria (Chlorobium thiosulfatophilum and Chloroflexus aurantiacus). Substantial levels of GSH were present in the purple bacteria (Chromatium vinosum, Rhodospirillum rubrum, Rhodobacter sphaeroides, and Rhodocyclus gelatinosa), the cyanobacteria [Anacystis nidulans, Microcoleus chthonoplastes S.G., Nostoc muscorum, Oscillatoria amphigranulata, Oscillatoria limnetica, Oscillatoria sp. (Stinky Spring, Utah), Oscillatoria terebriformis, Plectonema boryanum, and Synechococcus lividus], and eucaryotic algae (Chlorella pyrenoidsa, Chlorella vulgaris, Euglena gracilis, Scenedesmus obliquus, and Chlamydomonas reinhardtii). Other thiols measured included cysteine, gamma-glutamylcysteine, thiosulfate, coenzyme A, and sulfide; several unidentified thiols were also detected. Many of the organisms examined also exhibited a marked ability to reduce mBBr to syn-(methyl,methyl)bimane, an ability that was quenched by treatment with 2-pyridyl disulfide or 5,5'-bisdithio-(2-nitrobenzoic acid) prior to reaction with mBBr. These observations indicate the presence of a reducing system capable of electron transfer to mBBr and reduction of reactive disulfides. The distribution of GSH in phototrophic eubacteria indicates that GSH synthesis evolved at or around the time that oxygenic photosynthesis evolved.

Roles of the glutathione- and thioredoxin-dependent reduction systems in the Escherichia coli and saccharomyces cerevisiae responses to oxidative stress

The glutathione- and thioredoxin-dependent reduction systems are responsible for maintaining the reduced environment of the Escherichia coli and Saccharomyces cerevisiae cytosol. Here we examine the roles of these two cellular reduction systems in the bacterial and yeast defenses against oxidative stress. The transcription of a subset of the genes encoding glutathione biosynthetic enzymes, glutathione reductases, glutaredoxins, thioredoxins, and thioredoxin reductases, as well as glutathione- and thioredoxin-dependent peroxidases is clearly induced by oxidative stress in both organisms. However, only some strains carrying mutations in single genes are hypersensitive to oxidants. This is due, in part, to the redundant effects of the gene products and the overlap between the two reduction systems. The construction of strains carrying mutations in multiple genes is helping to elucidate the different roles of glutathione and thioredoxin, and studies with such strains have recently revealed that these two reduction systems modulate the activities of the E. coli OxyR and SoxR and the S. cerevisiae Yap1p transcriptional regulators of the adaptive responses to oxidative stress.

Lesions in gshA (Encoding gamma-L-glutamyl-L-cysteine synthetase) prevent aerobic synthesis of thiamine in Salmonella enterica serovar typhimurium LT2

Thiamine pyrophosphate is an essential cofactor that is synthesized de novo in Salmonella enterica serovar Typhimurium and other bacteria. In addition to genes encoding enzymes in the biosynthetic pathway, mutations in other metabolic loci have been shown to prevent thiamine synthesis. The latter loci identify the integration of the thiamine biosynthetic pathway with other metabolic processes and can be uncovered when thiamine biosynthesis is challenged. Mutations in gshA, encoding gamma-L-glutamyl-L-cysteine synthetase, prevent the synthesis of glutathione, the major free thiol in the cell, and are shown here to result in a thiamine auxotrophy in some of the strains tested, including S. enterica LT2. Phenotypic characterization of the gshA mutants indicated they were similar enough to apbC and apbE mutants to warrant the definition of a class of mutants unified by (i) a requirement for both the hydroxymethyl pyrimidine (HMP) and thiazole (THZ) moiety of thiamine, (ii) the ability of L-tryosine to satisfy the THZ requirement, (iii) suppression of the thiamine requirement by anaerobic growth, and (iv) suppression by a second-site mutation at a single locus. Genetic data indicated that a defective ThiH generates the THZ requirement in these strains, and we suggest this defect is due to a reduced ability to repair a critical [Fe-S] cluster.

Inactivation of dehydratase [4Fe-4S] clusters and disruption of iron homeostasis upon cell exposure to peroxynitrite

Phagocytes produce both nitric oxide and superoxide as components of the oxidative defense against pathogens. Neither molecule is likely at physiological concentrations to kill cells. However, two of their reaction products, hydrogen peroxide and peroxynitrite, are strong oxidants, cell-permeant, and toxic. Hydrogen peroxide generates oxidative DNA damage, while the primary mechanism of toxicity of peroxynitrite has not yet been determined. Recent in vitro studies indicated that peroxynitrite is capable of oxidizing the [4Fe-4S] clusters of a family of dehydratases (Hausladen, A., and Fridovich, I. (1994) J. Biol. Chem. 269, 29405-29408; Castro, L., Rodriguez, M., and Radi, R. (1994) J. Biol. Chem. 269, 29409-29415). We demonstrate here that peroxynitrite at 1% of its lethal dose almost fully inactivated the labile dehydratases in Escherichia coli. The rate at which peroxynitrite inactivated the clusters substantially exceeded the rate at which it oxidized thiols or spontaneously decomposed. These results suggest that these dehydratases may be primary targets of peroxynitrite in vivo. Another consequence of the cluster damage was the release of 100 microM iron into the cytosol. During phagocytosis, this intracellular free iron could increase lethal DNA damage by hydrogen peroxide or protein modification by additional peroxynitrite. In response to peroxynitrite challenges, E. coli rapidly sequestered the intracellular free iron using an undefined scavenging system. The iron-sulfur clusters were more gradually repaired by a process that drew iron from its iron-storage proteins. These are likely to be critical events in the struggle between phagocyte and pathogen.

The gshB gene in the cyanobacterium Synechococcus sp. PCC 7942 encodes a functional glutathione synthetase

The gene homologous to glutathione synthetase of Escherichia coli was inactivated in the cyanobacterium Synechococcus sp. PCC 7942. The region of genomic DNA including the mutation site was isolated from the mutant by plasmid rescue and the native gene of the wild-type was cloned from a genomic DNA library of the wild-type using the flanking DNA as a probe. The wild-type gene, designated gshB, encodes a polypeptide of 323 amino acids with a molecular mass of 35 kDa. The deduced amino acid sequence resembles glutathione synthetases of bacteria, but not those of higher organisms. When gshB was overexpressed in E. coli, glutathione synthetase activity was increased markedly in the E. coli extract. In addition, the Synechococcus sp. PCC 7942 gshB mutants had lost their ability to synthesize glutathione. These findings demonstrate that the gshB gene of Synechococcus sp. PCC 7942 is a structural gene for glutathione synthetase and is involved in the biosynthesis of glutathione.

A glutathione reductase mutant of yeast accumulates high levels of oxidized glutathione and requires thioredoxin for growth

A glutathione reductase null mutant of Saccharomyces cerevisiae was isolated in a synthetic lethal genetic screen for mutations which confer a requirement for thioredoxin. Yeast mutants that lack glutathione reductase (glr1 delta) accumulate high levels of oxidized glutathione and have a twofold increase in total glutathione. The disulfide form of glutathione increases 200-fold and represents 63% of the total glutathione in a glr1 delta mutant compared with only 6% in wild type. High levels of oxidized glutathione are also observed in a trx1 delta, trx2 delta double mutant (22% of total), in a glr1 delta, trx1 delta double mutant (71% of total), and in a glr1 delta, trx2 delta double mutant (69% of total). Despite the exceptionally high ratio of oxidized/reduced glutathione, the glr1 delta mutant grows with a normal cell cycle. However, either one of the two thioredoxins is essential for growth. Cells lacking both thioredoxins and glutathione reductase are not viable under aerobic conditions and grow poorly anaerobically. In addition, the glr1 delta mutant shows increased sensitivity to the thiol oxidant diamide. The sensitivity to diamide was suppressed by deletion of the TRX2 gene. The genetic analysis of thioredoxin and glutathione reductase in yeast runs counter to previous studies in Escherichia coli and for the first time links thioredoxin with the redox state of glutathione in vivo.

Glutathione is required for maximal transcription of the cobalamin biosynthetic and 1,2-propanediol utilization (cob/pdu) regulon and for the catabolism of ethanolamine, 1,2-propanediol, and propionate in Salmonella typhimurium LT2

Transcription of the cob/pdu regulon of Salmonella typhimurium is activated by the PocR regulatory protein in response to 1,2-propanediol (1,2-PDL) in the environment. Nutritional analysis and DNA sequencing confirmed that a strain defective in expression of the cob/pdu regulon in response to 1,2-PDL lacked a functional gshA gene. gshA encodes gamma-glutamylcysteine synthetase (L-glutamate:L-cysteine gamma-ligase [ADP forming]; EC 6.3.2.2), the enzyme that catalyzes the first step in the synthesis of glutathione (GSH). The DNA sequence of gshA was partially determined, and the location of gshA in the chromosome was established by two-factor crosses. P22 cotransduction of gshA with nearby markers showed 21% linkage to srl and 1% linkage to hyd; srl was 9% cotransducible with hyd. In light of these data, the gene order gshA srl hyd is suggested. The level of reduced thiols in the gshA strain was 87% lower than the levels measured in the wild-type strain in both aerobically and anaerobically grown cells. 1,2-PDL-dependent transcription of cob/pdu was studied by using M. Casadaban's Mu-lacZ fusions. In aerobically grown cells, transcription of a cbi-lacZ fusion (the cbi genes are the subset of cob genes that encode functions needed for the synthesis of the corrin ring) was 4-fold lower and transcription of a pdu-lacZ fusion was 10-fold lower in a gshA mutant than in the wild-type strain. Expression of the cob/pdu regulon in response to 1,2-PDL was restored when GSH was included in the medium. In anaerobically grown cells, cbi-lacZ transcription was only 0.4-fold lower than in the gshA+ strain; pdu-lacZ transcription was reduced only by 0.34-fold, despite the lower thiol levels in the mutant. cobA-lacZ transcription was used as negative control of gene whose transcription is not controlled by the PocR/1,2-PDL system; under both conditions, cobA transcription remained unaffected. The gshA mutant strain was unable to utilize 1,2-PDL, ethanolamine, or propionate as a carbon and energy source. The defect in ethanolamine utilization appears to be at the level of ethanolamine ammonia-lyase activity, not at the transcriptional level. Possible roles for GSH in ethanolamine, 1,2-PDL, and propionate catabolism are proposed and discussed.

A novel function of Escherichia coli chaperone DnaJ. Protein-disulfide isomerase

Molecular chaperones, protein-disulfide isomerases, and peptidyl prolyl cis-trans isomerases assist protein folding in both prokaryotes and eukaryotes. The DnaJ protein of Escherichia coli and the DnaJ-like proteins of eukaryotes are known as molecular chaperones and specific regulators of DnaK-like proteins and are involved in protein folding and renaturation after stress. In this study we show that DnaJ, like thioredoxin, protein-disulfide isomerase, and DsbA, possesses an active dithiol/disulfide group and catalyzes protein disulfide formation (oxidative renaturation of reduced RNase), reduction (reduction of insulin disulfides), and isomerization (refolding of randomly oxidized RNase). These results suggest that, in addition to its known function as a chaperone, DnaJ might be involved in controlling the redox state of cytoplasmic, membrane, or exported proteins.

Glutathionylspermidine metabolism in Escherichia coli. Purification, cloning, overproduction, and characterization of a bifunctional glutathionylspermidine synthetase/amidase

Glutathionylspermidine (GSP) synthetases of Trypanosomatidae and Escherichia coli couple hydrolysis of ATP (to ADP and Pi) with formation of an amide bond between spermidine (N-(3-aminopropyl)-1,4-diaminobutane) and the glycine carboxylate of glutathione (gamma-Glu-Cys-Gly). In the pathogenic trypanosomatids, this reaction is the penultimate step in the biosynthesis of the antioxidant metabolite, trypanothione (N1,N8-bis-(glutathionyl)spermidine), and is a target for drug design. In this study, GSP synthetase was purified to near homogeneity from E. coli B, the gene encoding it was isolated and sequenced, the enzyme was overexpressed and purified in quantity, and the recombinant enzyme was characterized. The 70-kDa protein was found to have an unexpected second catalytic activity, glutathionylspermidine amide bond hydrolysis. Thus, the bifunctional GSP synthetase/amidase catalyzes opposing amide bond-forming and -cleaving reactions, with net hydrolysis of ATP. The synthetase activity is selectively abrogated by proteolytic cleavage 81 residues from the C terminus, suggesting that the two activities reside in distinct domains (N-terminal amidase and C-terminal synthetase). Proteolysis at this site is facile in the absence of substrates, but is inhibited in the presence of ATP, glutathione, and Mg2+. A series of analogs was used to probe the spermidine-binding site of the synthetase activity. The activity of diaminopropane as a substrate, inactivity of the C4-C8 diaminoalkanes, and greater loss of specificity for analogs modified in the 3-aminopropyl moiety than for those modified in the 4-aminobutyl moiety indicate that the enzyme recognizes predominantly the diaminopropane portion of spermidine and corroborate N-1 (the aminopropyl N) as the site of glutathione linkage (Tabor, H. and Tabor, C. W. (1975) J. Biol. Chem. 250, 2648-2654). Trends in Km and kcat for a set of difluorosubstituted spermidine derivatives suggest that the enzyme may bind the minor, deprotonated form of the amine nucleophile.

Methylchloroisothiazolone-induced growth inhibition and lethality in Escherichia coli

Exposure of log phase Escherichia coli cells to inhibitory levels of 5-chloro-2-methyl-isothiazolin-3-one (MCI) results in rapid bacteriostasis and a delayed onset of bactericidal activity. Inhibition of respiration occurs within the same time frame as bacteriostasis, and is followed by a decline in intracellular ATP levels. In vitro and in vivo experiments suggest that growth inhibition is the result of selective inhibition of particular targets, with succinate dehydrogenase being identified as a possible target. Such selectivity was not anticipated from this highly reactive molecule. MCI-induced lethality is positively correlated with a loss of reduced protein sulphydryls (r2 = 0.79). A greater than equimolar loss of reduced protein sulphydryls, compared with the number of MCI molecules added, and a reduction in killing by MCI after induction of the OxyR regulon suggest that free radical generation may have a role in the antibacterial activity of MCI. We present an examination of the in vivo effects of MCI exposure on bacterial cells, and evidence that the isothiazolones exhibit selectivity in their cellular targets and antimicrobial effects.

Effect of isoniazid on glutathione biosynthesis and degradation in Mycobacterium smegmatis

The mechanism of glutathione (GSH) depletion by isoniazid (INH) was studied in M. smegmatis. INH increased the activity of gamma-glutamyl transferase (GGT) whether added before medium inoculation or to actively growing cells. The activity of GGT in cells grown from the beginning in INH-containing medium increased significantly on growth days 2-6. Three-day old M. smegmatis cells treated with INH exhibited a 30-65% increase in the activity of GGT. The activities of gamma-glutamyl-cysteine synthase (GGCS) and GSH synthase (GS) were lowered by 50 and 56% respectively on the second day of growth when M. smegmatis was grown in a medium supplemented with 1.5 mg INH per L. In 3-day old M. smegmatis, INH significantly inhibited the activities of GSH biosynthetic enzymes. The results demonstrate that the increased activity of GGT and decreased activities of GSH biosynthetic enzymes are responsible for GSH depletion by INH in M. smegmatis.

Escherichia coli ribonucleotide reductase expression is cell cycle regulated

The expression of the genes encoding ribonucleotide reductase in Escherichia coli was investigated in cultures synchronized by obtaining the smallest cells in a population after sucrose gradient centrifugation. Specific activity of ribonucleotide reductase and DNA initiation were found to increase in parallel, periodically as a function of the cell cycle. The expression of nrd was also determined in cells synchronized by periodic repeated doubling in a phosphate limited medium. Antibodies directed against the B2 subunit of ribonucleotide reductase were raised in a rabbit and purified. Immunoprecipitation of the B2 subunit and RNA-DNA dot blot hybridization assays were developed and employed to determine the expression of ribonucleotide reductase translational and transcriptional products during the cell cycle. Both of nrd-mRNA and B2 subunit expression were found to increase each generation at approximately the same time DNA synthesis was initiated and then to decrease back to the basal level shortly after DNA initiation. These results provided evidence of cell cycle dependent regulation of ribonucleotide reductase in E. coli. When the upstream regulatory region of nrd was fused to a promoterless lacZ gene on a single copy plasmid, lac-mRNA and beta-galactosidase were found to be synthesized in parallel to nrd expression from the chromosomal operon. When nrd sequences surrounding the promoter were removed from this construct, lac-mRNA and beta-galactosidase synthesis were no longer cell cycle regulated.

The oxygen effect: variation of the K-value and lifetimes of O2-dependent damage in some glutathione-deficient mutants of Escherichia coli

Radiosensitization by various concentrations of O2 has been studied in an Escherichia coli K-12 wild-type strain and some derived glutathione (GSH)-deficient mutants using 60Co gamma-irradiation. The maximum oxygen enhancement ratio (OER) and the K-value, the O2 concentration that produced half the maximum O2 effect, were found to depend on the GSH biosynthetic capacity of the strains. For the GSH+ wild-type strain, AB1157, and the GSH- mutant, 830, which is deficient in glutathione synthetase, the final enzyme in the GSH biosynthetic pathway, the maximum OERs were both about 3.9 and the K-values were 0.53% and 0.24% O2, respectively. On the other hand, the maximum OERs for two GSH- mutants, 7 and 821, both deficient in gamma-glutamylcysteine synthetase, the penultimate enzyme in the GSH biosynthetic pathway, were about 2.7 and the K-values were about 0.06% O2 for both. The fast chemical repair of O2-dependent damage in these strains was measured using a fast mixing and irradiation method, the gas explosion technique. The chemical repair rates in the various E. coli strains varied approximately in proportion to the O2 K-values, and both the rates of chemical repair and the K-values correlated approximately with the levels of non-protein sulphydryls in the various strains.

Overproduction of glutathione and its derivatives by genetically engineered microbial cells

In order to improve the biotechnological potentials of Escherichia coli cells to produce glutathione, S-D-lactoylglutathione and other gamma-glutamyl compounds, the genes for enzymes [gamma-L-glutamyl-L-cysteine synthetase (GSH A) in E. coli B, glutathione synthetase (GSH B) in E. coli B, glyoxalase I (GLO I) in Pseudomonas putida] were cloned and amplified in E. coli. E. coli B cells transformed with both GSH A and GSH B genes exhibited a high activity in the synthesis of glutathione and other gamma-glutamyl compounds in bioreactor systems containing immobilized cells. E. coli C600 cells transformed with GLO I gene of P. putida showed a high GLO I activity and were used for the preparation of S-D-lactoylglutathione and other glutathione thiol esters.

In vitro construction of gshB::kan in Escherichia coli and use of gshB::kan in mapping the gshB locus

The Escherichia coli structural gene for glutathione synthetase, gshB, was cloned into pBR322. Plasmids containing gshB were able to complement the glutathione requirement of a trxA gshB double mutant, and cells containing the plasmids were found to have elevated levels of glutathione synthetase. A mutant gshB allele was constructed by inserting the kan gene from pUC4K into a unique HpaI site located within gshB. The resulting plasmid-encoded allele was used to replace a genomic gshB+ by homologous recombination. The resulting strain had no detectable glutathione synthetase activity. The gshB allele containing the kan insertion was used to map gshB on the E. coli chromosome by P1 transduction. The results indicated that gshB is located at 63.4 min, between metK and speC. The allele was further localized to a region of 3,100 to 3,120 kilobase pairs on the physical map (restriction map) of E. coli by DNA-DNA hybridization to a series of lambda bacteriophages (Y. Kohara, K. Akiyama, and K. Isono, Cell 50:495-508, 1987).

Increased capacity for glutathione synthesis enhances resistance to radiation in Escherichia coli: a possible model for mammalian cell protection

A strain of Escherichia coli, enriched in its content of gamma-glutamylcysteine synthetase and glutathione synthetase activities by recombinant DNA techniques, is more resistant to the lethal effects of gamma-irradiation than is the corresponding wild strain. Although the gene-enriched strain has higher glutathione levels than the wild strain, the observed radioresistance appears to be associated with the increased capacity of the gene-enriched strain to synthesize glutathione when irradiated rather than to the cellular levels of glutathione per se. Thus, resistance was abolished in the presence of buthionine sulfoximine, a selective inactivator of gamma-glutamylcysteine synthetase that decreases glutathione synthesis but that does not act directly to lower cellular glutathione levels. Conclusions drawn from studies on this E. coli model system may have relevance to protection of mammalian cells by glutathione.

Biochemical characterization of glutathione-deficient mutants of Escherichia coli K12 and Salmonella strains TA1535 and TA100

Glutathione-deficient mutants of Escherichia coli K12/343/408 and Salmonella typhimurium TA1535 and TA100 were characterized biochemically by measuring the rate of formation of (14C)gamma-glutamylcysteine and (14C)glutathione in cell-free extracts of the strains. gamma-Glutamylcysteine synthetase activity was found to be absent in the NGR-2 mutant of E. coli and in the Salmonella mutants TA1535/NG-19, TA100/NG-57 and TA100/NG-11, while only low activities were found in the NGR-9 and NG-54 mutant of E. coli and Salmonella respectively. These results correspond with the decreased levels of glutathione found in these strains. Extracts of the parent strains have normal glutathione levels and show high gamma-glutamylcysteine synthetase activities. It is concluded that the present GSH-deficient strains of E. coli and Salmonella are gshA mutants, analogous to those previously described in E. coli. In addition, the present results show that the fluorometric method used for the determination of glutathione, employing o-phthalaldehyde as a reagent, is not specific for glutathione (at pH 8.0), but also sensitively reacts with gamma-glutamylcysteine.

Isolation and characterization of an Escherichia coli K-12 mutant deficient in glutaredoxin

Mutants of Escherichia coli K-12 deficient in glutaredoxin were isolated and partially characterized. The mutants have detectable but significantly reduced glutaredoxin activity in assays of whole cells made permeable with ether as well as in assays of crude extracts coupled to ribonucleotide reductase. In vivo, the mutants appear to be deficient in both sulfate and ribonucleotide reduction, suggesting that in vivo glutaredoxin is the preferred cofactor for ribonucleotide reductase and adenosine 3'-phosphate 5'-phosphosulfate reductase. Complementation of the mutant phenotype by transformants was used to clone the wild-type glutaredoxin allele. The transformants had a high level of glutaredoxin activity and contained a plasmid with an insert that had a restriction endonuclease pattern identical to that predicted by the DNA sequence for glutaredoxin determined by Hoog et al. (J.-O. Hoog, H. von Bahr-Lindstrom, H. Jornvall, and A. Holmgren, Gene 43:13-21, 1986).

Effect of cellular glutathione content on the induction of DNA double strand breaks by 25 MeV electrons

The effect of endogenous glutathione (GSH) on the induction of DNA double strand breaks (dsb) by 25 MeV electrons was investigated using stationary haploid yeast cells defective in gamma-glutamyl-cysteine-synthetase (gsh 1) containing less than 5 per cent of the normal GSH content. In gsh 1 cells the induction of dsb is increased by a factor of 1.5 under oxic and 1.8 under anoxic irradiation conditions: whereas the oxygen enhancement ratio was only slightly decreased (1.9) compared to wild-type cells (2.4).

The effect of thioredoxin on the radiosensitivity of bacteria

The ability of Escherichia coli thioredoxin to protect cells from lethal amounts of gamma radiation was tested using bacterial strains engineered to contain different amounts of thioredoxin per cell. Cells grown to late stationary phase demonstrated a decreasing sensitivity to gamma-radiation with increasing amounts of thioredoxin per cell. Exponentially growing cells were equally sensitive to the gamma-radiation regardless of the intracellular concentration of thioredoxin. Cells exhibiting the radiation-resistant phenotype in the stationary phase reverted to the radiation-sensitive phenotype when diluted into fresh growth medium. These results suggest that thioredoxin can protect cells from gamma-radiation under certain metabolic conditions.

Glutathione status and sensitivity to GSH-reacting compounds of Escherichia coli strains deficient in glutathione metabolism and/or catalase activity

The intracellular concentrations of total glutathione, GSSG and protein X S-SG, the total excreted glutathione concentration, and the susceptibility towards GSH-reacting compounds were assayed in strains of Escherichia coli deficient in biosynthesis and/or reduction of glutathione. A deficiency in glutathione reductase displaced the glutathione status towards the oxidized forms. This displacement was more clearly appreciated in strains additionally deficient in glutathione biosynthesis. A deficiency in catalase activity also produced an increase in the oxidation of glutathione. The most severe changes were observed in the concentrations of protein-glutathione mixed disulfides and in the amount of glutathione excreted to the medium. Increased sensitivities towards compounds known to interact with cellular GSH were observed in glutathione reductase deficient strains, although these effects were enhanced in strains additionally deficient in GSH biosynthesis.

Glutathione in Escherichia coli is dispensable for resistance to H2O2 and gamma radiation

Escherichia coli devoid of glutathione (because of transposon insertions in the gshA gene) has normal resistance to H2O2, cumene hydroperoxide, heat, or ionizing radiation. Intracellular glutathione thus does not protect E. coli from such lethal oxidative damage. The use of gshA::Tn10 mutants also revealed a glutathione-independent, H2O2-inducible resistance to N-ethylmaleimide.

Glutathione reductase from Escherichia coli: cloning and sequence analysis of the gene and relationship to other flavoprotein disulfide oxidoreductases

A glutathione reductase negative strain of Escherichia coli K-12 was isolated as a thermoresistant survivor when a gor::MuctsAp lysogen was subjected to elevated temperature. It was found that in addition to being ampicillin sensitive this mutant was hypersensitive to arsenate, which may be connected with the fact that the gor gene maps between 77 and 78 min on the E. coli genome, close to the pit locus encoding the major arsenate transport system of E. coli. A derivative of this mutant was used as the recipient in a screen of the Clarke and Carbon hybrid plasmid bank of E. coli DNA. A plasmid, pGR, was isolated that encodes both an arsenate-resistance element and glutathione reductase. Restriction mapping of this plasmid showed that the insert DNA is approximately 10 kilobase pairs in length, and a fragment of the gor gene was identified that allowed the gor gene to be accurately mapped on pGR by a combination of restriction analysis and Southern blotting. The DNA sequence of the gor gene was determined and found to encode a protein of 450 amino acid residues. The glutathione reductase of E. coli is very homologous to the human enzyme and is also related (though less closely) to other flavoprotein disulfide oxidoreductases whose sequences are available. These enzymes have retained a common mechanism while evolving different specificities.

S-methyl glutathione synthesis is catalyzed by the cheR methyltransferase in Escherichia coli

The cheR methyltransferase, known to be necessary for the methyl esterification of receptors involved in chemotaxis, is shown to be essential to the synthesis of S-methyl glutathione from glutathione and S-adenosylmethionine in intact Escherichia coli. S-Methyl glutathione is not, however, found to be essential for chemotaxis. It is suggested that the synthesis of S-methyl glutathione may be due to a "parasitic" reaction of glutathione with S-adenosylmethionine bound to the methyltransferase.

Isolation of glutathione-deficient mutants of the yeast Saccharomyces cerevisiae

Glutathione-deficient (gsh-) mutants of the yeast Saccharomyces cerevisiae were isolated after UV treatment using MNNG as selective agent. For genetic and biochemical characterization 5 mutant strains were chosen which exhibited considerably decreased residual GSH contents varying from 2 to 6% of the wild-type levels. All 5 isolates showed a 2:2 segregation of the gsh-:GSH+ phenotypes alluding to a monogenic recessive mutation. Complementation analysis indicates that all gsh- mutants belong to one complementation group.

Glutathione-independent isomerization of maleylpyruvate by Bacillus megaterium and other gram-positive bacteria

Maleylpyruvate, the ring fission product of gentisic acid, was found to be isomerized to fumarylpyruvate without a requirement for glutathione by an enzyme activity found in cell extracts of m-hydroxybenzoate-grown Bacillus megaterium 410. The isomerization reaction was detected as a shift in the absorbance maximum from 330 nm, the maximum for maleylpyruvate, to 345 nm, the maximum for fumarylpyruvate, when assayed at pH 8.0. Ammonium sulfate precipitation and dialysis of B. megaterium cell extracts resolved the isomerase activity from low-molecular-weight compounds such as glutathione but did not eliminate the isomerase activity. Iodoacetate and p-chloromercuribenzoate were potent inhibitors of the isomerase from B. megaterium. However, N-ethylmaleimide and iodoacetamide did not significantly inhibit this activity. In addition, fumaric acid was demonstrated as a product of gentisate oxidation by dialyzed cell extracts of B. megaterium. Glutathione-independent maleylpyruvate isomerases with properties similar to the isomerase found in B. megaterium were also found in other genera of gram-positive organisms. Eleven different organisms representing the genera Bacillus, Arthrobacter, Corynebacterium, Nocardia, and Rhodococcus were all found to possess this novel type of glutathione-independent maleylpyruvate isomerase.

Mutagenic activity of various chemicals in Salmonella strain TA100 and glutathione-deficient derivatives. On the role of glutathione in the detoxification or activation of mutagens inside bacterial cells

Several mutants with decreased levels of reduced glutathione (GSH) were isolated from the sensitive mutagen tester strain Salmonella typhimurium TA100 after treatment with u.v. and selection for resistance to N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) and its methyl analogue MNNG. Estimation of the GSH concentration and GSH S-transferase activity in extracts of these strains and of TA100 indicates that the GSH- derivatives contain 10-30% of the GSH level found in TA100, and that they exhibit normal GSH S-transferase activity. The mutagenic activities of 7 chemicals, namely, MNNG, ENNG, 1,2-dibromoethane (DBE), 1-chloro-2,4-dinitrobenzene (CDNB), styrene-7,8-oxide (STOX), N-ethyl-N-nitrosourea (ENU) and methyl methane sulphonate (MMS) were compared in TA100 and in one representative GSH- strain, denominated NG-57. MNNG, ENNG, DBE and CDNB are potent to extremely potent mutagens in TA100, but induce very low levels of His+ mutants in NG-57. Pretreatment of NG-57 with 1 mM GSH (partially) restores the mutant yields to the levels usually found in TA100. The mutagenic activities of STOX, ENU and MMS are similar in both strains. These results support some previous findings, namely that ENNG, MNNG and DBE, but not ENU are activated to mutagens inside the test bacteria, and also suggest that CDNB is activated by bacterial GSH. The latter finding is in contrast with the current view that CDNB is detoxified by GSH, as is also presently evidenced by a strong reduction of the compound's mutagenicity in the presence of extracts of rat liver, which contains GSH and GSH S-transferase activity. The results with STOX indicate that GSH plays in bacteria a much less important role in the detoxification of xenobiotics than in mammalian tissue, presumably due to a much lower GSH S-transferase activity in the first organism.