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

The role of metals in hypothiocyanite resistance in Escherichia coli

The innate immune system employs a variety of antimicrobial oxidants to control and kill host-associated bacteria. Hypothiocyanite/hypothiocyanous acid (-OSCN/HOSCN) is one such antimicrobial oxidant that is synthesized by lactoperoxidase, myeloperoxidase, and eosinophil peroxidase at sites throughout the human body. HOSCN has potent antibacterial activity while being largely non-toxic toward human cells. The molecular mechanisms by which bacteria sense and defend themselves against HOSCN have only recently begun to be elaborated, notably by the discovery of bacterial HOSCN reductase (RclA), an HOSCN-degrading enzyme widely conserved among bacteria that live on epithelial surfaces. In this paper, I show that Ni2+ sensitizes Escherichia coli to HOSCN by inhibiting glutathione reductase and that inorganic polyphosphate protects E. coli against this effect, probably by chelating Ni2+ ions. I also found that RclA is very sensitive to inhibition by Cu2+ and Zn2+, metals that are accumulated to high levels by innate immune cells, and that, surprisingly, thioredoxin and thioredoxin reductase are not involved in HOSCN stress resistance in E. coli. These results advance our understanding of the contribution of different oxidative stress responses and redox buffering pathways to HOSCN resistance in E. coli and illustrate important interactions between metal ions and the enzymes bacteria use to defend themselves against oxidative stress.

IMPORTANCE

Hypothiocyanite (HOSCN) is an antimicrobial oxidant produced by the innate immune system. The molecular mechanisms by which host-associated bacteria defend themselves against HOSCN have only recently begun to be understood. The results in this paper are significant because they show that the low molecular weight thiol glutathione and enzyme glutathione reductase are critical components of the Escherichia coli HOSCN response, working by a mechanism distinct from that of the HOSCN-specific defenses provided by the RclA, RclB, and RclC proteins and that metal ions (including nickel, copper, and zinc) may impact the ability of bacteria to resist HOSCN by inhibiting specific defensive enzymes (e.g., glutathione reductase or RclA).

Antioxidants are ineffective at quenching reactive oxygen species inside bacteria and should not be used to diagnose oxidative stress

A wide variety of stresses have been proposed to exert killing effects upon bacteria by stimulating the intracellular formation of reactive oxygen species (ROS). A key part of the supporting evidence has often been the ability of antioxidant compounds to protect the cells. In this study, some of the most-used antioxidants-thiourea, glutathione, N-acetylcysteine, and ascorbate-have been examined. Their ability to quench superoxide and hydrogen peroxide was verified in vitro, but the rate constants were orders of magnitude too slow for them to have an impact upon superoxide and peroxide concentrations in vivo, where these species are already scavenged by highly active enzymes. Indeed, the antioxidants were unable to protect the growth and ROS-sensitive enzymes of E. coli strains experiencing authentic oxidative stress. Similar logic posits that antioxidants cannot substantially quench hydroxyl radicals inside cells, which contain abundant biomolecules that react with them at diffusion-limited rates. Indeed, antioxidants were able to protect cells from DNA damage only if they were applied at concentrations that slow metabolism and growth. This protective effect was apparent even under anoxic conditions, when ROS could not possibly be involved, and it was replicated when growth was similarly slowed by other means. Experimenters should discard the use of antioxidants as a way of detecting intracellular oxidative stress and should revisit conclusions that have been based upon such experiments. The notable exception is that these compounds can effectively degrade hydrogen peroxide from environmental sources before it enters cells.

Genome-Scale Investigation of the Regulation of azoR Expression in Escherichia coli Using Computational Analysis and Transposon Mutagenesis

Bacterial azoreductases are enzymes that catalyze the reduction of ingested or industrial azo dyes. Although azoreductase genes have been well identified and characterized, the regulation of their expression has not been systematically investigated. To determine how different factors affect the expression of azoR, we extracted and analyzed transcriptional data from the Gene Expression Omnibus (GEO) resource, then confirmed computational predictions by quantitative reverse transcription polymerase chain reaction (qRT-PCR). Results showed that azoR expression was lower with higher glucose concentration, agitation speed, and incubation temperature, but higher at higher culture densities. Co-expression and clustering analysis indicated ten genes with similar expression patterns to azoR: melA, tpx, yhbW, yciK, fdnG, fpr, nfsA, nfsB, rutF, and chrR (yieF). In parallel, constructing a random transposon library in E. coli K-12 and screening 4320 of its colonies for altered methyl red (MR)-decolorizing activity identified another set of seven genes potentially involved in azoR regulation. Among these genes, arsC, relA, plsY, and trmM were confirmed as potential azoR regulators based on the phenotypic decolorization activity of their transposon mutants, and the expression of arsC and relA was confirmed, by qRT-PCR, to significantly increase in E. coli K-12 in response to different MR concentrations. Finally, the significant decrease in azoR transcription upon transposon insertion in arsC and relA (as compared to its expression in wild-type E. coli) suggests their probable involvement in azoR regulation. In conclusion, combining in silico analysis and random transposon mutagenesis suggested a set of potential regulators of azoR in E. coli.

The role of metals in hypothiocyanite resistance in Escherichia coli

The innate immune system employs a variety of antimicrobial oxidants to control and kill host-associated bacteria. Hypothiocyanite/hypothiocyanous acid (-OSCN/HOSCN) is one such antimicrobial oxidant that is synthesized by lactoperoxidase, myeloperoxidase, and eosinophil peroxidase at sites throughout the human body. HOSCN has potent antibacterial activity while being largely non-toxic towards human cells. The molecular mechanisms by which bacteria sense and defend themselves against HOSCN have only recently begun to be elaborated, notably by the discovery of bacterial HOSCN reductase (RclA), an HOSCN-degrading enzyme widely conserved among bacteria that live on epithelial surfaces. In this paper, I show that Ni2+ sensitizes Escherichia coli to HOSCN by inhibiting glutathione reductase, and that inorganic polyphosphate protects E. coli against this effect, probably by chelating Ni2+ ions. I also found that RclA is very sensitive to inhibition by Cu2+ and Zn2+, metals that are accumulated to high levels by innate immune cells, and that, surprisingly, thioredoxin and thioredoxin reductase are not involved in HOSCN stress resistance in E. coli. These results advance our understanding of the contribution of different oxidative stress response and redox buffering pathways to HOSCN resistance in E. coli and illustrate important interactions between metal ions and the enzymes bacteria use to defend themselves against oxidative stress.

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).

Glutathione is Involved in Detoxification of Peroxide and Root Nodule Symbiosis of Mesorhizobium huakuii

Legumes interact with symbiotic rhizobia to produce nitrogen-fixation root nodules under nitrogen-limiting conditions. The contribution of glutathione (GSH) to this symbiosis and anti-oxidative damage was investigated using the M. huakuii gshB (encoding GSH synthetase) mutant. The gshB mutant grew poorly with different monosaccharides, including glucose, sucrose, fructose, maltose, or mannitol, as sole sources of carbon. The antioxidative capacity of gshB mutant was significantly decreased by these treatments with H₂O₂ under the lower concentrations and cumene hydroperoxide (CUOOH) under the higher concentrations, indicating that GSH plays different roles in response to organic peroxide and inorganic peroxide. The gshB mutant strain displayed no difference in catalase activity, but significantly lower levels of the peroxidase activity and the glutathione reductase activity than the wild type. The same level of catalase activity could be associated with upregulation of the transcriptional activity of the catalase genes under H₂O₂-induced conditions. The nodules infected by the gshB mutant were severely impaired in abnormal nodules, and showed a nodulation phenotype coupled to a 60% reduction in the nitrogen fixation capacity. A 20-fold decrease in the expression of two nitrogenase genes, nifH and nifD, is observed in the nodules induced by gshB mutant strain. The symbiotic deficiencies were linked to bacteroid early senescence.

Cysteine homeostasis under inhibition of protein synthesis in Escherichia coli cells

Increased intracellular cysteine poses a potential danger to cells due to the high ability of cysteine to reduce free iron and promote the Fenton reaction. Here, we studied ways to maintain cysteine homeostasis in E. coli cells while inhibiting protein synthesis with valine or chloramphenicol. When growing wild-type bacteria on minimal medium with sulfate, an excess of cysteine resulting from the inhibition of protein synthesis is mainly incorporated into glutathione (up to 90%), which, therefore, can be considered as cysteine buffer. The share of hydrogen sulfide, which is the product of cysteine degradation by cysteine synthase B (CysM), does not exceed 1-3%, the rest falls on free cysteine, exported from cells. As a result, intracellular free cysteine is maintained at a low level (about 0.1 mM). The lack of glutathione in the gshA mutant increases H₂S production and excretion of cysteine and leads to a threefold increase in the level of intracellular cysteine in response to valine and chloramphenicol. The relA mutants, exposed to valine, produce more H₂S, dramatically accelerate the export of glutathione and accumulate more cysteine in the cytoplasm than their parent, which indicates that the regulatory nucleotide (p)ppGpp is involved in maintaining cysteine homeostasis. Disruption of cysteine homeostasis in gshA and relA mutants increases their sensitivity to peroxide stress.

Oxidative post-translational modifications in histones

Epigenetic regulation is attracting much attention because it explains many of the effects that the external environment induces in organisms. Changes in the cellular redox status and even more specifically in its nuclear redox compartment is one of these examples. Redox changes can induce modulation of the epigenetic regulation in cells. Here we present a few cases where reactive oxygen or nitrogen species induces epigenetic marks in histones. Posttranslational modification of these proteins like histone nitrosylation, carbonylation, or glutathionylation together with other mechanisms not reviewed here are the cornerstones of redox-related epigenetic regulation. We currently face a new field of research with potential important consequences for the treatment of many pathologies.

Modes of action and adverse effects of gamma radiation in an aquatic macrophyte Lemna minor

High dose rates of ionizing radiation have been reported to cause adverse effects such as reduction in reproduction and growth, and damage to protein and lipids in primary producers. However, the relevant effects of ionizing radiation are still poorly understood in aquatic plants. This study was intended to characterize the biological effects and modes of action (MoAs) of ionizing radiation using gamma radiation as the prototypical stressor and duckweed Lemna minor as a model organism. Lemna minor was exposed to 1, 14, 24, 46, 70 mGy/h gamma radiation dose rates from a cobalt-60 source for 7 days following the testing principles of the OECD test guideline 221. A suite of bioassays was applied to assess the biological effects of gamma radiation at multiple levels of biological organization, including detection of reactive oxygen species (ROS), oxidative stress responses (total glutathione, tGSH; lipid peroxidation, LPO), DNA damage, mitochondrial dysfunctions (mitochondrial membrane potential, MMP), photosynthetic parameters (chlorophyll a, chl a; chlorophyll b, chl b; carotenoids; Photosystem II (PSII) performance; CO₂ uptake), intercellular signaling (Ca²⁺ release) and growth. Gamma radiation increased DNA damage, tGSH level and Ca²⁺ content together with reduction in chlorophyll content, maximal PSII efficiency and CO₂ uptake at dose rates between 1 and 14 mGy/h, whereas increases in cellular ROS and LPO, inhibition of MMP and growth were observed at higher dose rates (≥24 mGy/h). A network of toxicity pathways was proposed to portray the causal relationships between gamma radiation-induced physiological responses and adverse outcomes to support the development of Adverse Outcome Pathways (AOPs) for ionizing radiation-mediated effects in primary producers.

Sinorhizobium meliloti Glutathione Reductase Is Required for both Redox Homeostasis and Symbiosis

Glutathione (l-γ-glutamyl-l-cysteinylglycine) (GSH), one of the key antioxidants in Sinorhizobium meliloti, is required for the development of alfalfa (Medicago sativa) nitrogen-fixing nodules. Glutathione exists as either reduced glutathione (GSH) or oxidized glutathione (GSSG), and its content is regulated by two pathways in S. meliloti The first pathway is the de novo synthesis of glutathione from its constituent amino acids, namely, Glu, Cys, and Gly, catalyzed by γ-glutamylcysteine synthetase (GshA) and glutathione synthetase (GshB). The second pathway is the recycling of GSSG via glutathione reductase (GR). However, whether the S. meliloti GR functions similarly to GshA and GshB1 during symbiotic interactions with alfalfa remains unknown. In this study, a plasmid insertion mutation of the S. melilotigor gene, which encodes GR, was constructed, and the mutant exhibited delayed alfalfa nodulation, with 75% reduction in nitrogen-fixing capacity. The gor mutant demonstrated increased accumulation of GSSG and a decreased GSH/GSSG ratio in cells. The mutant also showed defective growth in rich broth and minimal broth and was more sensitive to the oxidants H₂O₂ and sodium nitroprusside. Interestingly, the expression of gshA, gshB1, katA, and katB was induced in the mutant. These findings reveal that the recycling of glutathione is important for S. meliloti to maintain redox homeostasis and to interact symbiotically with alfalfa.IMPORTANCE The antioxidant glutathione is regulated by its synthetase and reductase in cells. In the symbiotic bacterium S. meliloti, the de novo synthesis of glutathione is essential for alfalfa nodulation and nitrogen fixation. In this study, we observed that the recycling of glutathione from GSSG not only was required for redox homeostasis and oxidative stress protection in S. meliloti cells but also contributed to alfalfa nodule development and competition capacity. Our findings demonstrate that the recycling of glutathione plays a key role in nitrogen fixation symbiosis.

Simultaneous quantification of coenzyme A and its salvage pathway intermediates in in vitro and whole cell-sourced samples

A method for the quantitative analysis of CoA and its thiolated precursors was developed, addressing the analytical shortcomings of previous methods. Its utility was showcased by analysis ofin vitroenzyme reactions and samples extracted from various bacterial strains.

Glutathione and plant response to the biotic environment

Glutathione (GSH) is a major antioxidant molecule in plants. It is involved in regulating plant development and responses to the abiotic and biotic environment. In recent years, numerous reports have clarified the molecular processes involving GSH in plant-microbe interactions. In this review, we summarize recent studies, highlighting the roles of GSH in interactions between plants and microbes, whether pathogenic or beneficial to plants.

Low-molecular-weight thiol-dependent antioxidant and antinitrosative defences in Salmonella pathogenesis

We found herein that the intracytoplasmic pool of the low-molecular-weight (LMW) thiol glutathione (GSH) is readily oxidized in Salmonella exposed to nitric oxide (NO). The hypersusceptibility of gshA and gshB mutants lacking γ-glutamylcysteine and glutathione synthetases to NO and S-nitrosoglutathione indicates that GSH antagonizes the bacteriostatic activity of reactive nitrogen species. Metabolites of the GSH biosynthetic pathway do not affect the enzymatic activity of classical NO targets such as quinol oxidases. In contrast, LMW thiols diminish the nitrosative stress experienced by enzymes, such as glutamine oxoglutarate amidotransferase, that contain redox active cysteines. LMW thiols also preserve the transcription of Salmonella pathogenicity island 2 gene targets from the inhibitory activity of nitrogen oxides. These findings are consistent with the idea that GSH scavenges reactive nitrogen species (RNS) other than NO. Compared with the adaptive response afforded by inducible systems such as the hmp-encoded flavohaemoprotein, gshA, encoding the first step of GSH biosynthesis, is constitutively expressed in Salmonella. An acute model of salmonellosis has revealed that the antioxidant and antinitrosative properties associated with the GSH biosynthetic pathway represent a first line of Salmonella resistance against reactive oxygen and nitrogen species engendered in the context of a functional NRAMP1(R) divalent metal transporter.

Escherichia coli glutathionylspermidine synthetase/amidase: phylogeny and effect on regulation of gene expression

Glutathionylspermidine synthetase/amidase (Gss) and the encoding gene (gss) have only been studied in Escherichia coli and several members of the Kinetoplastida phyla. In the present article, we have studied the phylogenetic distribution of Gss and have found that Gss sequences are largely limited to certain bacteria and Kinetoplastids and are absent in a variety of invertebrate and vertebrate species, Archea, plants, and some Eubacteria. It is striking that almost all of the 75 Enterobacteria species that have been sequenced contain sequences with very high degree of homology to the E. coli Gss protein. To find out the physiological significance of glutathionylspermidine in E. coli, we have performed global transcriptome analyses. The microarray studies comparing gss(+) and Δgss strains of E. coli show that a large number of genes are either up-regulated (76 genes more than threefold) or down-regulated (35 genes more than threefold) by the loss of the gss gene. Most significant categories of up-regulated genes include sulfur utilization, glutamine and succinate metabolism, polyamine and arginine metabolism, and purine and pyrimidine metabolism.

Nuclear glutathione

Glutathione (GSH) is a linchpin of cellular defences in plants and animals with physiologically-important roles in the protection of cells from biotic and abiotic stresses. Moreover, glutathione participates in numerous metabolic and cell signalling processes including protein synthesis and amino acid transport, DNA repair and the control of cell division and cell suicide programmes. While it is has long been appreciated that cellular glutathione homeostasis is regulated by factors such as synthesis, degradation, transport, and redox turnover, relatively little attention has been paid to the influence of the intracellular partitioning on glutathione and its implications for the regulation of cell functions and signalling. We focus here on the functions of glutathione in the nucleus, particularly in relation to physiological processes such as the cell cycle and cell death. The sequestration of GSH in the nucleus of proliferating animal and plant cells suggests that common redox mechanisms exist for DNA regulation in G1 and mitosis in all eukaryotes. We propose that glutathione acts as "redox sensor" at the onset of DNA synthesis with roles in maintaining the nuclear architecture by providing the appropriate redox environment for the DNA replication and safeguarding DNA integrity. In addition, nuclear GSH may be involved in epigenetic phenomena and in the control of nuclear protein degradation by nuclear proteasome. Moreover, by increasing the nuclear GSH pool and reducing disulfide bonds on nuclear proteins at the onset of cell proliferation, an appropriate redox environment is generated for the stimulation of chromatin decompaction. This article is part of a Special Issue entitled Cellular functions of glutathione.

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.

Haemophilus influenzae and oxidative stress

Haemophilus influenzae is a commensal of the human upper respiratory tract. H. influenzae can, however, move out of its commensal niche and cause multiple respiratory tract diseases. Such diseases include otitis media in young children, as well as exacerbations of chronic obstructive pulmonary disease (COPD), sinusitis, conjunctivitis, and bronchitis. During the course of colonization and infection, H. influenzae must withstand oxidative stress generated by multiple reactive oxygen species produced endogenously, by other co-pathogens and by host cells. H. influenzae has, therefore, evolved multiple mechanisms that protect the cell against oxygen-generated stresses. In this review, we will describe these systems relative to the well-described systems in Escherichia coli. Moreover, we will compare how H. influenzae combats the effect of oxidative stress as a necessary phenotype for its roles as both a successful commensal and pathogen.

Glutathione is required by Rhizobium etli for glutamine utilization and symbiotic effectiveness

Here, we provide genetic and biochemical evidence indicating that the ability of Rhizobium etli bacteria to efficiently catabolize glutamine depends on its ability to produce reduced glutathione (l-γ-glutamyl-l-cysteinylglycine [GSH]). We find that GSH-deficient strains, namely a gshB (GSH synthetase) and a gor (GSH reductase) mutant, can use different amino acids, including histidine, alanine, and asparagine but not glutamine, as sole source of carbon, energy, and nitrogen. Moreover, l-buthionine(S,R)-sulfoximine, a GSH synthesis inhibitor, or diamide that oxidizes GSH, induced the same phenotype in the wild-type strain. Among the steps required for its utilization, glutamine uptake, occurring through the two well-characterized carriers (Aap and Bra systems) but not glutamine degradation or respiration, was largely reduced in GSH-deficient strains. Furthermore, GSH-deficient mutants of R. etli showed a reduced symbiotic efficiency. Exogenous GSH was sufficient to rescue glutamine uptake or degradation ability, as well as the symbiotic effectiveness of GSH mutants. Our results suggest a previously unknown GSH-glutamine metabolic relationship in bacteria.

Unraveling the hydroxypropionaldehyde (HPA) system: an active antimicrobial agent against human pathogens

The hydroxypropionaldehyde (HPA) system is a natural defense system synthesized by the probiotic bacterium Lactobacillus reuteri. To elucidate which of the molecules composing the HPA system (3-hydroxypropionaldehyde (3-HPA), reuterin (HPA dimer), and HPA hydrate) is responsible for the potent antimicrobial activity in biological systems, a combination of biochemical, genetic, and proteomic assays was used. The HPA system reacts with sulfhydryl-containing compounds such as cysteine and reduced glutathione (GSH) in solution. In situ, GSH knock-out Escherichia coli is significantly more susceptible to HPA-mediated cell death than E. coli wild type; GSH supplementation protects either bacteria from HPA attack. Proteomic analysis of HPA-treated bacteria ( Haemophilus influenzae ) revealed induction of redox- and heat shock-related proteins. A new antimicrobial mechanism of HPA is proposed, whereby the activity of HPA leads to depletion of free SH- groups in GSH and proteins through the action of 3-hydroxypropionaldehyde, causing an imbalance of the cellular redox status, ultimately resulting in cell death.

Oxygen is required for the L-cysteine-mediated decomposition of protein-bound dinitrosyl-iron complexes

Increasing evidence suggests that iron-sulfur proteins are the primary targets of nitric oxide (NO). Exposure of Escherichia coli cells to NO readily converts iron-sulfur proteins to protein-bound dinitrosyl-iron complexes (DNICs). Although the protein-bound DNICs are stable in vitro under aerobic or anaerobic conditions, they are efficiently repaired in aerobically growing E. coli cells even without new protein synthesis. The cellular repair mechanism for the NO-modified iron-sulfur proteins remains largely elusive. Here we report that, unlike aerobically growing E. coli cells, starved E. coli cells fail to reactivate the NO-modified iron-sulfur proteins. Significantly, the addition of L-cysteine, but not other related biological thiols, results in decomposition of the protein-bound DNICs in starved E. coli cells and in cell extracts under aerobic conditions. However, under anaerobic conditions, L-cysteine has little or no effect on the protein-bound DNICs in starved E. coli cells or in vitro, suggesting that oxygen is required for the L-cysteine-mediated decomposition of the protein-bound DNICs. Additional studies reveal that L-cysteine is able to release the DNIC from the protein and bind to it, and the L-cysteine-bound DNICs are rapidly disrupted by oxygen, resulting in the eventual decomposition of the protein-bound DNICs under aerobic conditions.

Protein S-thiolation by Glutathionylspermidine (Gsp): the role of Escherichia coli Gsp synthetASE/amidase in redox regulation

Certain bacteria synthesize glutathionylspermidine (Gsp), from GSH and spermidine. Escherichia coli Gsp synthetase/amidase (GspSA) catalyzes both the synthesis and hydrolysis of Gsp. Prior to the work reported herein, the physiological role(s) of Gsp or how the two opposing GspSA activities are regulated had not been elucidated. We report that Gsp-modified proteins from E. coli contain mixed disulfides of Gsp and protein thiols, representing a new type of post-translational modification formerly undocumented. The level of these proteins is increased by oxidative stress. We attribute the accumulation of such proteins to the selective inactivation of GspSA amidase activity. X-ray crystallography and a chemical modification study indicated that the catalytic cysteine thiol of the GspSA amidase domain is transiently inactivated by H(2)O(2) oxidation to sulfenic acid, which is stabilized by a very short hydrogen bond with a water molecule. We propose a set of reactions that explains how the levels of Gsp and Gsp S-thiolated proteins are modulated in response to oxidative stress. The hypersensitivities of GspSA and GspSA/glutaredoxin null mutants to H(2)O(2) support the idea that GspSA and glutaredoxin act synergistically to regulate the redox environment of E. coli.

The L-cysteine/L-cystine shuttle system provides reducing equivalents to the periplasm in Escherichia coli

Intracellular thiols like L-cysteine and glutathione play a critical role in the regulation of cellular processes. Escherichia coli has multiple L-cysteine transporters, which export L-cysteine from the cytoplasm into the periplasm. However, the role of L-cysteine in the periplasm remains unknown. Here we show that an L-cysteine transporter, YdeD, is required for the tolerance of E. coli cells to hydrogen peroxide. We also present evidence that L-cystine, a product from the oxidation of L-cysteine by hydrogen peroxide, is imported back into the cytoplasm in a manner dependent on FliY, the periplasmic L-cystine-binding protein. Remarkably, this protein, which is involved in the recycling of the oxidized L-cysteine, is also found to be important for the hydrogen peroxide resistance of this organism. Furthermore, our analysis of the transcription of relevant genes revealed that the transcription of genes encoding FliY and YdeD is highly induced by hydrogen peroxide rather than by L-cysteine. These findings led us to propose that the inducible L-cysteine/L-cystine shuttle system plays an important role in oxidative stress tolerance through providing a reducing equivalent to the periplasm in E. coli.

Role of nuclear glutathione as a key regulator of cell proliferation

Glutathione (GSH) is essential for survival of eukaryotic but not in prokaryotic cells. Its functions in nucleated cells are far from being known. In fact GSH plays an important role in cell proliferation. The purpose of the present review is to summarize the relationship between glutathione and the important events that take place in the nucleus during the cell cycle. Most GSH co-localizes with nuclear DNA when cells are proliferating. However, when cells were confluent no differences between nucleus and cytoplasm could be seen. A number of relevant nuclear proteins are strictly dependent on nuclear redox status. For instance, we found that telomerase is regulated by shifts in glutathione redox potential within values similar to those found in vivo, and alterations in telomerase activity are coordinated with changes in critical cell cycle proteins, particularly Id2 and E2F4. More studies are required to establish the role of nuclear glutathione in the epigenetic control of histone function. The information provided in the present review suggests an important role of nuclear glutathione as a key regulator of epigenetic events that may be critical in the regulation of cell proliferation.

The role and regulation of Trxl, a cytosolic thioredoxin in Schizosaccharomyces pombe

The genome of fission yeast Schizosaccharomyces pombe harbors two genes for thioredoxins, trx1(+) and trx2(+), which encode cytosolic and mitochondrial thioredoxins, respectively. The Deltatrx1 mutant was found sensitive to diverse external stressors such as various oxidants, heat, and salt, whereas Deltatrx2 mutant was not sensitive except to paraquat, a superoxide generator. Both Deltatrx1 and Deltatrx2 mutants were more resistant to diamide, a thiol-specific oxidant, than the wild type. The trx1(+) gene expression was induced by H(2)O(2) and menadione, being mediated through a stress-responsive transcription factor Papl. In Deltatrx1 cells, the basal expression of Pap1-regulated genes were elevated, suggesting a role for Trxl as a reducer for oxidized (activated) Papl. The Deltatrx1 mutant exhibited cysteine auxotrophy, which can be overcome by adding sulfite. This suggests that Trxl serves as a primary electron donor for 3'-phosphoadenosine-5'-phosphosulfate (PAPS) reductase and thus is an essential protein for sulfur assimilation in S. pombe. These results suggest that, in contrast to Trx2 whose role is more confined to mitochondrial functions, Trxl plays a major role in protecting S. pombe against various stressful conditions and enables proper sulfur metabolism.

The multiple functions of the thiol-based electron flow pathways of Escherichia coli: Eternal concepts revisited

Electron flow via thiols is a theme with many variations in all kingdoms of life. The favourable physichochemical properties of the redox active couple of two cysteines placed in the optimised environment of the thioredoxin fold allow for two electron transfers in between top biological reductants and ultimate oxidants. The reduction of ribonucleotide reductases by thioredoxin and thioredoxin reductase of Escherichia coli (E. coli) was one of the first pathways to be elucidated. Diverse functions such as protein folding in the periplasm, maturation of respiratory enzymes, detoxification of hydrogen peroxide and prevention of oxidative damage may be based on two electron transfers via thiols. A growing field is the relation of thiol reducing pathways and the interaction of E. coli with different organisms. This concept combined with the sequencing of the genomes of different bacteria may allow for the identification of fine differences in the systems employing thiols for electron flow between pathogens and their corresponding mammalian hosts. The emerging possibility is the development of novel antibiotics.

Glutathione reductase and a mitochondrial thioredoxin play overlapping roles in maintaining iron-sulfur enzymes in fission yeast

In the fission yeast Schizosaccharomyces pombe, the pgr1+ gene encoding glutathione (GSH) reductase (GR) is essentially required for cell survival. Depletion of GR caused proliferation arrest at the G1 phase of the cell cycle under aerobic conditions. Multicopy suppressors that restore growth were screened, and one effective suppressor was found to be the trx2+ gene, encoding a mitochondrial thioredoxin. This suggests that GR is critically required for some mitochondrial function(s). We found that GR resides in both cytosolic and organellar fractions of the cell. Depletion of GR lowered the respiration rate and the activity of oxidation-labile Fe-S enzymes such as mitochondrial aconitase and cytosolic sulfite reductase. Trx2 did not reverse the high ratio of oxidized glutathione to GSH or the low respiration rate observed in GR-depleted cells. However, it brought the activity of oxidation-labile Fe-S enzymes to a normal level, suggesting that the maintenance of Fe-S enzymes is a critical factor in the survival of S. pombe. The activity of succinate dehydrogenase, an oxidation-insensitive Fe-S enzyme, however, was not affected by GR depletion, suggesting that GR is not required for the biogenesis of the Fe-S cluster. The total iron content was greatly increased by GR depletion and was brought to a nearly normal level by Trx2. These results indicate that the essentiality of GR in the aerobic growth of S. pombe is derived from its role in maintaining oxidation-labile Fe-S enzymes and iron homeostasis.

Glutathione in bacteria

Glutathione metabolism and its role in vital functions of bacterial cells are considered, as well as common features and differences between the functions of glutathione in prokaryotic and eukaryotic cells. Particular attention is given to the recent data for the role of glutathione in bacterial redox-regulation and adaptation to stresses.

The many faces of glutathione in bacteria

Glutathione is one of the most abundant thiols present in cyanobacteria and proteobacteria, and in all mitochondria or chloroplast-bearing eukaryotes. In bacteria, in addition to its key role in maintaining the proper oxidation state of protein thiols, glutathione also serves a key function in protecting the cell from the action of low pH, chlorine compounds, and oxidative and osmotic stresses. Moreover, glutathione has emerged as a posttranslational regulator of protein function under conditions of oxidative stress, by the direct modification of proteins via glutathionylation. This review summarizes the biosynthesis and function of glutathione in bacteria from physiological and biotechnological standpoints.

Involvement of oxidative stress in hydroquinone-induced cytotoxicity in catalase-deficient Escherichia coli mutants

Hydroquinone is a benzene-derived metabolite. To clarify whether the reactive oxygen species (ROS) are involved in hydroquinone-induced cytotoxicity, we constructed transformants of Escherichia coli (E. coli) strains that express mammalian catalase gene derived from catalase mutant mice (Cs(b), Cs(c)) and the wild-type (Cs(a)) using a catalase-deficient E. coli UM255 as a recipient. Specific catalase activities of these tester strains were in order of Cs(a) > Cs(c) > Cs(b) > UM255, and their susceptibility to hydrogen peroxide (H2O2) showed UM255 > Cs(b) > Cs(c) > Cs(a). We found that hydroquinone exposure reduced the survival of catalase-deficient E. coli mutants in a dose-dependent manner significantly, especially in the strains with lower catalase activities. Hydroquinone toxicity was also confirmed using zone of inhibition test, in which UM255 was the most susceptible, showing the largest zone of growth inhibition, followed by Cs(b), Cs(c) and Cs(a). Furthermore, we found that hydroquinone-induced cell damage was inhibited by the pretreatment of catalase, ascorbic acid, dimethyl sulfoxide (DMSO), and ethylenediaminetetraacetic acid (EDTA), and augmented by superoxide dismutase (both CuZnSOD and MnSOD). The present results suggest that H2O2 is probably involved in hydroquinone-induced cytotoxicity in catalase-deficient E. coli mutants and catalase plays an important role in protection of the cells against hydroquinone toxicity.

Mobilization of the iron centre in IscA for the iron-sulphur cluster assembly in IscU

The biogenesis of iron-sulphur clusters requires the co-ordinated delivery of both iron and sulphur. It is now clear that sulphur in iron-sulphur clusters is derived from L-cysteine by cysteine desulphurases. However, the iron donor for the iron-sulphur cluster assembly still remains elusive. Our previous studies indicated that Escherichia coli IscA, a member of the iron-sulphur cluster assembly machinery, is an iron-binding protein that can provide iron for the iron-sulphur cluster assembly in a proposed scaffold IscU. To determine how the iron centre in IscA is transferred for the iron-sulphur cluster assembly in IscU, we explore the mobility of the iron centre in IscA. The UV-visible and EPR measurements show that L-cysteine, but not IscU, is able to mobilize the iron centre in IscA and make the iron available for the iron-sulphur cluster assembly in IscU. Other related biological thiols such as N-acetyl-L-cysteine or reduced glutathione have no effect on the iron centre of IscA, suggesting that L-cysteine is unique in mobilizing the iron centre of IscA. Nevertheless, L-cysteine alone is not sufficient to transfer the iron from IscA to IscU. Both L-cysteine and cysteine desulphurase (IscS) are required for the IscA-mediated assembly of iron-sulphur clusters in IscU. The results suggest that L-cysteine may have two distinct functions in the biogenesis of iron-sulphur clusters: to mobilize the iron centre in IscA and to provide sulphur via cysteine desulphurase (IscS) for the iron-sulphur cluster assembly in IscU.

A Bacterial Glutathione Transporter (Escherichia coli CydDC) Exports Reductant to the Periplasm

Glutathione (GSH), a major biological antioxidant, maintains redox balance in prokaryotes and eukaryotic cells and forms exportable conjugates with compounds of pharmacological and agronomic importance. However, no GSH transporter has been characterized in a prokaryote. We show here that a heterodimeric ATP-binding cassette-type transporter, CydDC, mediates GSH transport across the Escherichia coli cytoplasmic membrane. In everted membrane vesicles, GSH is imported via an ATP-driven, protonophore-insensitive, orthovanadate-sensitive mechanism, equating with export to the periplasm in intact cells. GSH transport and cytochrome bd quinol oxidase assembly are abolished in the cydD1 mutant. Glutathione disulfide (GSSG) was not transported in either Cyd(+) or Cyd(-) strains. Exogenous GSH restores defective swarming motility and benzylpenicillin sensitivity in a cydD mutant and also benzylpenicillin sensitivity in a gshA mutant defective in GSH synthesis. Overexpression of the cydDC operon in dsbD mutants defective in disulfide bond formation restores dithiothreitol tolerance and periplasmic cytochrome b assembly, revealing redundant pathways for reductant export to the periplasm. These results identify the first prokaryotic GSH transporter and indicate a key role for GSH in periplasmic redox homeostasis.

Roles of thioredoxin reductase during the aerobic life of Lactococcus lactis

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

Glutathione plays a fundamental role in growth and symbiotic capacity of Sinorhizobium meliloti

Rhizobia form a symbiotic relationship with plants of the legume family to produce nitrogen-fixing root nodules under nitrogen-limiting conditions. We have examined the importance of glutathione (GSH) during free-living growth and symbiosis of Sinorhizobium meliloti. An S. meliloti mutant strain (SmgshA) which is unable to synthesize GSH due to a gene disruption in gshA, encoding the enzyme for the first step in the biosynthesis of GSH, was unable to grow under nonstress conditions, precluding any nodulation. In contrast, an S. meliloti strain (SmgshB) with gshB, encoding the enzyme involved in the second step in GSH synthesis, deleted was able to grow, indicating that gamma-glutamylcysteine, the dipeptide intermediate, can partially substitute for GSH. However, the SmgshB strain showed a delayed-nodulation phenotype coupled to a 75% reduction in the nitrogen fixation capacity. This phenotype was linked to abnormal nodule development. Both the SmgshA and SmgshB mutant strains exhibited higher catalase activity than the wild-type S. meliloti strain, suggesting that both mutant strains are under oxidative stress. Taken together, these results show that GSH plays a critical role in the growth of S. meliloti and during its interaction with the plant partner.

Glutathione plays a fundamental role in growth and symbiotic capacity of Sinorhizobium meliloti

Rhizobia form a symbiotic relationship with plants of the legume family to produce nitrogen-fixing root nodules under nitrogen-limiting conditions. We have examined the importance of glutathione (GSH) during free-living growth and symbiosis of Sinorhizobium meliloti. An S. meliloti mutant strain (SmgshA) which is unable to synthesize GSH due to a gene disruption in gshA, encoding the enzyme for the first step in the biosynthesis of GSH, was unable to grow under nonstress conditions, precluding any nodulation. In contrast, an S. meliloti strain (SmgshB) with gshB, encoding the enzyme involved in the second step in GSH synthesis, deleted was able to grow, indicating that gamma-glutamylcysteine, the dipeptide intermediate, can partially substitute for GSH. However, the SmgshB strain showed a delayed-nodulation phenotype coupled to a 75% reduction in the nitrogen fixation capacity. This phenotype was linked to abnormal nodule development. Both the SmgshA and SmgshB mutant strains exhibited higher catalase activity than the wild-type S. meliloti strain, suggesting that both mutant strains are under oxidative stress. Taken together, these results show that GSH plays a critical role in the growth of S. meliloti and during its interaction with the plant partner.

Cloning and characterization of gluconolactone oxidase of Penicillium cyaneo-fulvum ATCC 10431 and evaluation of its use for production of D-erythorbic acid in recombinant Pichia pastoris

A D-erythorbic acid-forming soluble flavoprotein, gluconolactone oxidase (GLO), was purified from Penicillium cyaneo-fulvum strain ATCC 10431 and partially sequenced. Peptide sequences were used to isolate a cDNA clone encoding the enzyme. The cloned gene exhibits high levels of similarity with the genes encoding other known eukaryotic lactone oxidases and also with the genes encoding some putative prokaryotic lactone oxidases. Analysis of the coding sequence of the GLO gene indicated the presence of a typical secretion signal sequence at the N terminus of GLO. No other targeting or anchoring signals were found, suggesting that GLO is the first known lactone oxidase that is secreted rather than targeted to the membranes of the endoplasmic reticulum or mitochondria. Experimental evidence, including the N-terminal sequence of mature GLO and data on glycosylation and localization of the enzyme in native and recombinant hosts, supports this analysis. The GLO gene was expressed in Pichia pastoris, and recombinant GLO was produced by using the strong methanol-induced AOX1 promoter. In order to evaluate the suitability of purified GLO for production of D-erythorbic acid, we immobilized it on N-hydroxysuccinimide-activated Sepharose and found that the immobilized GLO retained full activity during immobilization but was rather unstable under reaction conditions. Our results show that both soluble and immobilized forms of GLO can, in principle, be used for production of D-erythorbic acid from D-glucono-delta-lactone or (in combination with glucose oxidase and catalase) from glucose. We also demonstrated the feasibility of glucose-D-erythorbic acid fermentation with recombinant strains coexpressing GLO and glucose oxidase genes, and we analyzed problems associated with construction of efficient D-erythorbic acid-producing hosts.

Repair of oxidized iron-sulfur clusters in Escherichia coli

The [4Fe-4S]2+ clusters of dehydratases are rapidly damaged by univalent oxidants, including hydrogen peroxide, superoxide, and peroxynitrite. The loss of an electron destabilizes the cluster, causing it to release its catalytic iron atom and converting the cluster initially to an inactive [3Fe-4S]1+ form. Continued exposure to oxidants in vitro leads to further iron release. Experiments have shown that these clusters are repaired in vivo. We sought to determine whether repair is mediated by either the Isc or Suf cluster-assembly systems that have been identified in Escherichia coli. We found that all the proteins encoded by the isc operon were critical for de novo assembly, but most of these were unnecessary for cluster repair. IscS, a cysteine desulfurase, appeared to be an exception: although iscS mutants repaired damaged clusters, they did so substantially more slowly than did wild-type cells. Because sulfur mobilization should be required only if clusters degrade beyond the [3Fe-4S]1+ state, we used whole cell EPR to visualize the fate of oxidized enzymes in vivo. Fumarase A was overproduced. Brief exposure of cells to hydrogen peroxide resulted in the appearance of the characteristic [3Fe-4S]1+ signal of the oxidized enzyme. When hydrogen peroxide was then scavenged, the enzyme activity reappeared within minutes, in concert with the disappearance of the EPR signal. Thus it is unclear why IscS is required for efficient repair. The iscS mutants grew poorly, allowing the possibility that metabolic defects indirectly slow the repair process. Our data did indicate that damaged clusters decompose beyond the [3Fe-4S]1+ state in vivo when stress is prolonged. Under the conditions of our experiments, mutants that lacked other repair candidates--Suf proteins, glutathione, and NADPH: ferredoxin reductase--all repaired clusters at normal rates. We conclude that the mechanism of cluster repair is distinct from that of de novo assembly and that this is true because mild oxidative stress does not degrade clusters in vivo to the point of presenting an apoenzyme to the de novo cluster-assembly systems.

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 protects Lactococcus lactis against oxidative stress

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

Effects of hydrogen peroxide upon nicotinamide nucleotide metabolism in Escherichia coli: changes in enzyme levels and nicotinamide nucleotide pools and studies of the oxidation of NAD(P)H by Fe(III)

DNA is damaged in vivo by the Fenton reaction mediated by Fe2+ and cellular reductants such as NADH, which reduce Fe3+ to Fe2+ and allow the recycling of iron. To study the response of Escherichia coli to such cycling, the activities of several enzymes involved in nicotinamide nucleotide metabolism were measured following an H2O2 challenge. NADPH-dependent peroxidase, NADH/NADP+ transhydrogenase, and glucose-6-phosphate dehydrogenase were most strongly induced, increasing 2.5-3-fold. In addition, the cellular ratios of NADPH to NADH increased 6- or 92-fold 15 min after exposure to 0.5 or 5 mm H2O2, respectively. In vitro, NADH was oxidized by Fe3+ up to 16-fold faster than NADPH, despite their identical reduction potentials. To understand this rate difference, the interactions of Fe3+ and Ga3+ with NAD(P)H were examined by 1H, 13C, and 31P NMR spectroscopy. Association with NADH occurred primarily with adenine at N7 and the amino group, but for NADPH, strong metal interactions also occurred at the 2'-phosphate group. Interaction of M3+ (Fe3+ or Ga3+) with the adenine ring would bring it into close proximity to the redox-active nicotinamide ring in the folded form of NAD(P)H, but interaction of M3+ with the 2'-phosphate group would avoid this close contact. In addition, as determined by absorbance spectroscopy, the energy of the charge-transfer species was significantly higher for the Fe3+.NADPH complex than for the Fe3+.NADH complex. We therefore suggest that upon exposure to H2O2 the NADH pool is depleted, and NADPH, which is less reactive with Fe3+, functions as the major nicotinamide nucleotide reductant.

Glutathione and catalase provide overlapping defenses for protection against respiration-generated hydrogen peroxide in Haemophilus influenzae

Glutathione is an abundant and ubiquitous low-molecular-weight thiol that may play a role in many cellular processes, including protection against the deleterious effects of reactive oxygen species. We address here the role of glutathione in protection against hydrogen peroxide (H2O2) in Haemophilus influenzae and show that glutathione and catalase provide overlapping defense systems. H. influenzae is naturally glutathione deficient and imports glutathione from the growth medium. Mutant H. influenzae lacking catalase and cultured in glutathione-deficient minimal medium is completely devoid of H2O2 scavenging activity and, accordingly, substantial amounts of H2O2 accumulate in the growth medium. H. influenzae generates H2O2 at rates similar to those reported for Escherichia coli, but the toxicity of this harmful metabolite is averted by glutathione-based H2O2 removal, which appears to be the primary system for protection against H2O2 endogenously generated during aerobic respiration. When H2O2 concentrations exceed low micromolar levels, the hktE gene-encoded catalase becomes the predominant scavenger. The requirement for glutathione in protection against oxidative stress is analogous to that in higher and lower eukaryotes but is unlike the situation in other bacteria in which glutathione is dispensable for aerobic growth during both normal and oxidative stress conditions.

Regulation of the nitric oxide reduction operon (norRVW) in Escherichia coli. Role of NorR and sigma54 in the nitric oxide stress response

Nitric oxide (NO) induces NO-detoxifying enzymes in Escherichia coli suggesting sensitive mechanisms for coordinate control of NO defense genes in response to NO stress. Exposure of E. coli to sub-micromolar NO levels under anaerobic conditions rapidly induced transcription of the NO reductase (NOR) structural genes, norV and norW, as monitored by lac gene fusions. Disruption of rpoN (sigma(54)) impaired the NO-mediated induction of norV and norW transcription and NOR expression, whereas disruption of the upstream regulatory gene, norR, completely ablated NOR induction. NOR inducibility was restored to NorR null mutants by expressing NorR in trans. Furthermore, an internal deletion of the N-terminal domain of NorR activated NOR expression independent of NO exposure. Neither NorR nor sigma(54) was essential for NO-mediated induction of the NO dioxygenase (flavohemoglobin) encoded by hmp. However, elevated NOR activity inhibited NO dioxygenase induction, and, in the presence of dioxygen, NO dioxygenase inhibited norV induction by NO. The results demonstrate the role of NorR as a sigma(54)-dependent regulator of norVW expression. A role for the NorR N-terminal domain as a transducer or sensor for NO is suggested.

Exogenous glutathione completes the defense against oxidative stress in Haemophilus influenzae

Since they are equipped with several strategies by which they evade the antimicrobial defense of host macrophages, it is surprising that members of the genus Haemophilus appear to be deficient in common antioxidant systems that are well established to protect prokaryotes against oxidative stress. Among others, no genetic evidence for glutathione (gamma-Glu-Cys-Gly) (GSH) biosynthesis or for alkyl hydroperoxide reduction (e.g., the Ahp system characteristic or enteric bacteria) is apparent from the Haemophilus influenzae Rd genome sequence, suggesting that the organism relies on alternative systems to maintain redox homeostasis or to reduce small alkyl hydroperoxides. In this report we address this apparent paradox for the nontypeable H. influenzae type strain NCTC 8143. Instead of biosynthesis, we could show that this strain acquires GSH by importing the thiol tripeptide from the growth medium. Although such GSH accumulation had no effect on growth rates, the presence of cellular GSH protected against methylglyoxal, tert-butyl hydroperoxide (t-BuOOH), and S-nitrosoglutathione toxicity and regulated the activity of certain antioxidant enzymes. H. influenzae NCTC 8143 extracts were shown to contain GSH-dependent peroxidase activity with t-BuOOH as the peroxide substrate. The GSH-mediated protection against t-BuOOH stress is most probably catalyzed by the product of open reading frame HI0572 (Prx/Grx), which we isolated from a genomic DNA fragment that confers wild-type resistance to t-BuOOH toxicity in the Ahp-negative Escherichia coli strain TA4315 and that introduces GSH-dependent alkyl hydroperoxide reductase activity into naturally GSH peroxidase-negative E. coli. Finally, we demonstrated that cysteine is an essential amino acid for growth and that cystine, GSH, glutathione amide, and cysteinylglycine can be catabolized in order to complement cysteine deficiency.

High levels of intracellular cysteine promote oxidative DNA damage by driving the fenton reaction

Escherichia coli is generally resistant to H(2)O(2), with >75% of cells surviving a 3-min challenge with 2.5 mM H(2)O(2). However, when cells were cultured with poor sulfur sources and then exposed to cystine, they transiently exhibited a greatly increased susceptibility to H(2)O(2), with <1% surviving the challenge. Cell death was due to an unusually rapid rate of DNA damage, as indicated by their filamentation, a high rate of mutation among the survivors, and DNA lesions by a direct assay. Cell-permeable iron chelators eliminated sensitivity, indicating that intracellular free iron mediated the conversion of H(2)O(2) into a hydroxyl radical, the direct effector of DNA damage. The cystine treatment caused a temporary loss of cysteine homeostasis, with intracellular pools increasing about eightfold. In vitro analysis demonstrated that cysteine reduces ferric iron with exceptional speed. This action permits free iron to redox cycle rapidly in the presence of H(2)O(2), thereby augmenting the rate at which hydroxyl radicals are formed. During routine growth, cells maintain small cysteine pools, and cysteine is not a major contributor to DNA damage. Thus, the homeostatic control of cysteine levels is important in conferring resistance to oxidants. More generally, this study provides a new example of a situation in which the vulnerability of cells to oxidative DNA damage is strongly affected by their physiological state.

Reduced flavins promote oxidative DNA damage in non-respiring Escherichia coli by delivering electrons to intracellular free iron

When cells are exposed to external H(2)O(2), the H(2)O(2) rapidly diffuses inside and oxidizes ferrous iron, thereby forming hydroxyl radicals that damage DNA. Thus the process of oxidative DNA damage requires only H(2)O(2), free iron, and an as-yet unidentified electron donor that reduces ferric iron to the ferrous state. Previous work showed that H(2)O(2) kills Escherichia coli especially rapidly when respiration is inhibited either by cyanide or by genetic defects in respiratory enzymes. In this study we established that these respiratory blocks accelerate the rate of DNA damage. The respiratory blocks did not substantially affect the amounts of intracellular free iron or H(2)O(2), indicating that that they accelerated damage because they increased the availability of the electron donor. The goal of this work was to identify that donor. As expected, the respiratory inhibitors caused a large increase in the amount of intracellular NADH. However, NADH itself was a poor reductant of free iron in vitro. This suggests that in non-respiring cells electrons are transferred from NADH to another carrier that directly reduces the iron. Genetic manipulations of the amounts of intracellular glutathione, NADPH, alpha-ketoacids, ferredoxin, and thioredoxin indicated that none of these was the direct electron donor. However, cells were protected from cyanide-stimulated DNA damage if they lacked flavin reductase, an enzyme that transfers electrons from NADH to free FAD. The K(m) value of this enzyme for NADH is much higher than the usual intracellular NADH concentration, which explains why its flux increased when NADH levels rose during respiratory inhibition. Flavins that were reduced by purified flavin reductase rapidly transferred electrons to free iron and drove a DNA-damaging Fenton system in vitro. Thus the rate of oxidative DNA damage can be limited by the rate at which electron donors reduce free iron, and reduced flavins become the predominant donors in E. coli when respiration is blocked. It remains unclear whether flavins or other reductants drive Fenton chemistry in respiring cells.

Repair of nitric oxide-modified ferredoxin [2Fe-2S] cluster by cysteine desulfurase (IscS)

Iron-sulfur proteins are among the sensitive targets of the nitric oxide cytotoxicity. When Escherichia coli cells are exposed to nitric oxide, iron-sulfur clusters are modified forming protein-bound dinitrosyl iron complexes. Such modified protein dinitrosyl iron complexes are stable in vitro but are efficiently repaired in aerobically growing E. coli cells even without any new protein synthesis. Here we show that cysteine desulfurase encoded by the gene iscS of E. coli can directly convert the ferredoxin dinitrosyl iron complex to the ferredoxin [2Fe-2S] cluster in the presence of L-cysteine in vitro. A reassembly of the [2Fe-2S] cluster in the ferredoxin dinitrosyl iron complex does not require any addition of iron or other protein components. Furthermore, a complete removal of the dinitrosyl iron complex from ferredoxin prevents reassembly of the [2Fe-2S] cluster in the protein. The results suggest that cysteine desulfurase (IscS) together with L-cysteine can efficiently repair the nitric oxide-modified ferredoxin [2Fe-2S] cluster and that the iron center in the dinitrosyl iron complex may be recycled for the reassembly of iron-sulfur clusters in proteins.

Escherichia coli gamma-glutamylcysteine synthetase. Two active site metal ions affect substrate and inhibitor binding

Gamma-glutamylcysteine synthetase (gamma-GCS, glutamate-cysteine ligase), which catalyzes the first and rate-limiting step in glutathione biosynthesis, is present in many prokaryotes and in virtually all eukaryotes. Although all eukaryotic gamma-GCS isoforms examined to date are rapidly inhibited by buthionine sulfoximine (BSO), most reports indicate that bacterial gamma-GCS is resistant to BSO. We have confirmed the latter finding with Escherichia coli gamma-GCS under standard assay conditions, showing both decreased initial binding affinity for BSO and a reduced rate of BSO-mediated inactivation compared with mammalian isoforms. We also find that substitution of Mn2+ for Mg2+ in assay mixtures increases both the initial binding affinity of BSO and the rate at which BSO causes mechanism-based inactivation. Similarly, the specificity of E. coli gamma-GCS for its amino acid substrates is broadened in the presence of Mn2+, and the rate of reaction for some very poor substrates is improved. These results suggest that divalent metal ions have a role in amino acid binding to E. coli gamma-GCS. Electron paramagnetic resonance (EPR) studies carried out with Mn2+ show that E. coli gamma-GCS binds two divalent metal ions; Kd values for Mn2+ are 1.1 microm and 82 microm, respectively. Binding of l-glutamate or l-BSO to the two Mn2+/gamma-GCS species produces additional upfield and downfield X-band EPR hyperfine lines at 45 G intervals, a result indicating that the two Mn2+ are spin-coupled and thus apparently separated by 5 A or less in the active site. Additional EPR studies in which Cu2+ replaced Mg2+ or Mn2+ suggest that Cu2+ is bound by one N and three O ligands in the gamma-GCS active site. The results are discussed in the context of the catalytic mechanism of gamma-GCS and its relationship to the more fully characterized glutamine synthetase reaction.