Differential regulation of glutaredoxin gene expression in response to stress conditions in the yeast Saccharomyces cerevisiae

Synergistic soil-less medium for enhanced yield of crops: a step towards incorporating genomic tools for attaining net zero hunger

Globally, industrial farming endangers crucial ecological mechanisms upon which food production relies, while 815 million people are undernourished and a significant number are malnourished. Zero Hunger aims to concurrently solve global ecological sustainability and food security concerns. Recent breakthroughs in molecular tools and approaches have allowed scientists to detect and comprehend the nature and structure of agro-biodiversity at the molecular and genetic levels, providing us an advantage over traditional methods of crop breeding. These bioinformatics techniques let us optimize our target plants for our soil-less medium and vice versa. Most of the soil-borne and seed-borne diseases are the outcomes of non-treated seed and growth media, which are important factors in low productivity. The farmers do not consider these issues, thereby facing problems growing healthy crops and suffering economic losses. This study is going to help the farmers increase their eco-friendly, chemical residue-free, quality yield of crops and their economic returns. The present invention discloses a synergistic soil-less medium that consists of only four ingredients mixed in optimal ratios by weight: vermicompost (70-80%), vermiculite (10-15%), coco peat (10-15%), and Rhizobium (0-1%). The medium exhibits better physical and chemical characteristics than existing conventional media. The vermiculite to coco peat ratio is reduced, while the vermicompost ratio is increased, with the goals of lowering toxicity, increasing plant and water holding capacity, avoiding drying of the media, and conserving water. The medium provides balanced nutrition and proper ventilation for seed germination and the growth of seedlings. Rhizobium is also used to treat the plastic bags and seeds. The results clearly show that the current synergistic soil-less environment is best for complete plant growth. Securing genetic advantages via sexual recombination, induced random mutations, and transgenic techniques have been essential for the development of improved agricultural varieties. The recent availability of targeted genome-editing technology provides a new path for integrating beneficial genetic modifications into the most significant agricultural species on the planet. Clustered regularly interspaced short palindromic repeats and associated protein 9 (CRISPR/Cas9) has evolved into a potent genome-editing tool for imparting genetic modifications to crop species. In addition, the integration of analytical methods like population genomics, phylogenomics, and metagenomics addresses conservation problems, while whole genome sequencing has opened up a new dimension for explaining the genome architecture and its interactions with other species. The in silico genomic and proteomic investigation was also conducted to forecast future investigations for the growth of French beans on a synergistic soil-less medium with the purpose of studying how a blend of vermicompost, vermiculite, cocopeat, and Rhizobium secrete metal ions, and other chemical compounds into the soil-less medium and affect the development of our target plant as well as several other plants. This interaction was studied using functional and conserved region analysis, phylogenetic analysis, and docking tools.

Insights into biodegradation mechanisms of triphenyl phosphate by a novel fungal isolate and its potential in bioremediation of contaminated river sediment

In this study, Aspergillus sydowii FJH-1 isolated from soil was verified to be a novel triphenyl phosphate (TPhP) degrader. Biodegradation efficiency of TPhP by Aspergillus sydowii FJH-1 exceeded 90% within 6 days under the optimal conditions (pH 4-9, 30 ℃, initial concentration less than 20 mg/L). Proteomics analysis uncovered the proteins perhaps involved in hydrolysis, hydroxylation, methylation and sulfonation of TPhP and the primary intracellular adaptive responses of Aspergillus sydowii FJH-1 to TPhP stress. The expression of carboxylic ester hydrolase along with several thioredoxin- and glutathione-dependent oxidoreductases were induced to withstand the toxicity of TPhP. The presence of TPhP also caused obvious upregulation of proteins concerned with glycolysis, pentose phosphate pathway and tricarboxylic acid cycle. Data from toxicological tests confirmed that the cytotoxicity and phytotoxicity of TPhP was effectively decreased after treatment with Aspergillus sydowii FJH-1. Additionally, bioaugmentation with Aspergillus sydowii FJH-1 was available for promoting TPhP removal in real water and water-sediment system. Collectively, the present study offered a deeper insight into the biodegradation mechanism and pathway of TPhP by a newly screened fungal strain Aspergillus sydowii FJH-1 and validated the feasibility of applying this novel degrader in the bioremediation of TPhP-polluted matrices.

Glutaredoxin Interacts with GR and AhpC to Enhance Low-Temperature Tolerance of Antarctic Psychrophile Psychrobacter sp. ANT206

Glutaredoxin (Grx) is an important oxidoreductase to maintain the redox homoeostasis of cells. In our previous study, cold-adapted Grx from Psychrobacter sp. ANT206 (PsGrx) has been characterized. Here, we constructed an in-frame deletion mutant of psgrx (Δpsgrx). Mutant Δpsgrx was more sensitive to low temperature, demonstrating that psgrx was conducive to the growth of ANT206. Mutant Δpsgrx also had more malondialdehyde (MDA) and protein carbonylation content, suggesting that PsGrx could play a part in the regulation of tolerance against low temperature. A yeast two-hybrid system was adopted to screen interacting proteins of 26 components. Furthermore, two target proteins, glutathione reductase (GR) and alkyl hydroperoxide reductase subunit C (AhpC), were regulated by PsGrx under low temperature, and the interactions were confirmed via bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (Co-IP). Moreover, PsGrx could enhance GR activity. trxR expression in Δpsgrx, Δahpc, and ANT206 were illustrated 3.7, 2.4, and 10-fold more than mutant Δpsgrx Δahpc, indicating that PsGrx might increase the expression of trxR by interacting with AhpC. In conclusion, PsGrx may participate in glutathione metabolism and ROS-scavenging by regulating GR and AhpC to protect the growth of ANT206. These findings preliminarily suggest the role of PsGrx in the regulation of oxidative stress, which could improve the low-temperature tolerance of ANT206.

Cadmium induces GAPDH- and- MDH mediated delayed cell aging and dysfunction in Candida tropicalis 3Aer

Eukaryotes employ various mechanisms to survive environmental stress conditions. Multicellular organisms eliminate permanently damaged cells by apoptosis, while unicellular eukaryotes like yeast react by decelerating cell aging. In the present study, transcriptomic and proteomic approaches were employed to elucidate the underlying mechanism of delayed apoptosis. Our findings suggest that Candida tropicalis 3Aer has a set of tightly controlled genes that are activated under Cd⁺² exposition. Acute exposure to Cd⁺² halts the cell cycle at the G₂/M phase checkpoint and activates multiple cytoplasmic proteins that overcome effects of Cd⁺²-induced reactive oxygen species. Prolonged Cd⁺² stress damages DNA and initiates GAPDH amyloid formation. This is the first report that Cd⁺² challenge initiates dynamic redistribution of GAPDH and MDH and alters various metabolic pathways including the pentose phosphate pathway. In conclusion, the intracellular redistribution of GAPDH and MDH induced by prolonged cadmium stress modulates various cellular reactions, which facilitate delayed aging in the yeast cell.

Protein S-glutathionylation: The linchpin for the transmission of regulatory information on redox buffering capacity in mitochondria

Protein S-glutathionylation reactions are a ubiquitous oxidative modification required to control protein function in response to changes in redox buffering capacity. These reactions are rapid and reversible and are, for the most part, enzymatically mediated by glutaredoxins (GRX) and glutathione S-transferases (GST). Protein S-glutathionylation has been found to control a range of cell functions in response to different physiological cues. Although these reactions occur throughout the cell, mitochondrial proteins seem to be highly susceptible to reversible S-glutathionylation, a feature attributed to the unique physical properties of this organelle. Indeed, mitochondria contain a number of S-glutathionylation targets which includes proteins involved in energy metabolism, solute transport, reactive oxygen species (ROS) production, proton leaks, apoptosis, antioxidant defense, and mitochondrial fission and fusion. Moreover, it has been found that conjugation and removal of glutathione from proteins in mitochondria fulfills a number of important physiological roles and defects in these reactions can have some dire pathological consequences. Here, we provide an updated overview on mitochondrial protein S-glutathionylation reactions and their importance in cell functions and physiology.

Glutaredoxin-deficiency confers bloodstream Trypanosoma brucei with improved thermotolerance

As constituents of their unusual trypanothione-based thiol metabolism, African trypanosomes express two dithiol glutaredoxins (Grxs), a cytosolic Grx1 and a mitochondrial Grx2, with so far unknown biological functions. As revealed by gel shift assays, in the mammalian bloodstream form of Trypanosoma brucei, Grx1 is in the fully reduced state. Upon diamide treatment of the cells, Grx1 forms an active site disulfide bridge that is rapidly re-reduced after stress removal; Cys76, a conserved non-active site Cys remains in the thiol state. Deletion of both grx1 alleles does not result in any proliferation defect of neither the procyclic insect form nor the bloodstream form, even not under various stress conditions. In addition, the Grx1-deficient parasites are fully infectious in the mouse model. A functional compensation by Grx2 is unlikely as identical levels of Grx2 were found in wildtype and Grx1-deficient cells. In the classical hydroxyethyl disulfide assay, Grx1-deficient bloodstream cells display 50-60% of the activity of wildtype cells indicating that the cytosolic oxidoreductase accounts for a major part of the total deglutathionylation capacity of the parasite. Intriguingly, at elevated temperature, proliferation of the Grx1-deficient bloodstream parasites is significantly less affected compared to wildtype cells. When cultured for three days at 39°C, only 51% of the cells in the wildtype population retained normal morphology with single mitochondrial and nuclear DNA (1K1N), whereas 27% of the cells displayed ≥2K2N. In comparison, 64% of the Grx1-deficient cells kept the 1K1N phenotype and only 18% had ≥2K2N. The data suggest that Grx1 plays a role in the regulation of the thermotolerance of the parasites by (in)directly interfering with the progression of the cell cycle, a process that may comprise protein (de)glutathionylation step(s).

A genome-wide screen in yeast identifies specific oxidative stress genes required for the maintenance of sub-cellular redox homeostasis

Maintenance of an optimal redox environment is critical for appropriate functioning of cellular processes and cell survival. Despite the importance of maintaining redox homeostasis, it is not clear how the optimal redox potential is sensed and set, and the processes that impact redox on a cellular/organellar level are poorly understood. The genetic bases of cellular redox homeostasis were investigated using a green fluorescent protein (GFP) based redox probe, roGFP2 and a pH sensitive GFP-based probe, pHluorin. The use of roGFP2, in conjunction with pHluorin, enabled determination of pH-adjusted sub-cellular redox potential in a non-invasive and real-time manner. A genome-wide screen using both the non-essential and essential gene collections was carried out in Saccharomyces cerevisiae using cytosolic-roGFP2 to identify factors essential for maintenance of cytosolic redox state under steady-state conditions. 102 genes of diverse function were identified that are required for maintenance of cytosolic redox state. Mutations in these genes led to shifts in the half-cell glutathione redox potential by 75-10 mV. Interestingly, some specific oxidative stress-response processes were identified as over-represented in the data set. Further investigation of the role of oxidative stress-responsive systems in sub-cellular redox homeostasis was conducted using roGFP2 constructs targeted to the mitochondrial matrix and peroxisome and E(GSH) was measured in cells in exponential and stationary phase. Analyses allowed for the identification of key redox systems on a sub-cellular level and the identification of novel genes involved in the regulation of cellular redox homeostasis.

Transcriptomic analysis of acclimation to temperature and light stress in Saccharina latissima (Phaeophyceae)

Kelps, brown algae of the order Laminariales, dominate rocky shores and form huge kelp beds which provide habitat and nurseries for various marine organisms. Whereas the basic physiological and ecophysiological characteristics of kelps are well studied, the molecular processes underlying acclimation to different light and temperature conditions are still poorly understood. Therefore we investigated the molecular mechanisms underlying the physiological acclimation to light and temperature stress. Sporophytes of S. latissima were exposed to combinations of light intensities and temperatures, and microarray hybridizations were performed to determine changes in gene expression patterns. This first large-scale transcriptomic study of a kelp species shows that S. latissima responds to temperature and light stress with a multitude of transcriptional changes: up to 32% of genes showed an altered expression after the exposure experiments. High temperature had stronger effects on gene expression in S. latissima than low temperature, reflected by the higher number of temperature-responsive genes. We gained insights into underlying molecular processes of acclimation, which includes adjustment of the primary metabolism as well as induction of several ROS scavengers and a sophisticated regulation of Hsps. We show that S. latissima, as a cold adapted species, must make stronger efforts for acclimating to high than to low temperatures. The strongest response was caused by the combination of high temperatures with high light intensities, which proved most harmful for the alga.

The response to heat shock and oxidative stress in Saccharomyces cerevisiae

A common need for microbial cells is the ability to respond to potentially toxic environmental insults. Here we review the progress in understanding the response of the yeast Saccharomyces cerevisiae to two important environmental stresses: heat shock and oxidative stress. Both of these stresses are fundamental challenges that microbes of all types will experience. The study of these environmental stress responses in S. cerevisiae has illuminated many of the features now viewed as central to our understanding of eukaryotic cell biology. Transcriptional activation plays an important role in driving the multifaceted reaction to elevated temperature and levels of reactive oxygen species. Advances provided by the development of whole genome analyses have led to an appreciation of the global reorganization of gene expression and its integration between different stress regimens. While the precise nature of the signal eliciting the heat shock response remains elusive, recent progress in the understanding of induction of the oxidative stress response is summarized here. Although these stress conditions represent ancient challenges to S. cerevisiae and other microbes, much remains to be learned about the mechanisms dedicated to dealing with these environmental parameters.

Purification, crystallization and preliminary crystallographic studies of the TLDc domain of oxidation resistance protein 2 from zebrafish

Cell metabolic processes are constantly producing reactive oxygen species (ROS), which have deleterious effects by triggering, for example, DNA damage. Numerous enzymes such as catalase, and small compounds such as vitamin C, provide protection against ROS. The TLDc domain of the human oxidation resistance protein has been shown to be able to protect DNA from oxidative stress; however, its mechanism of action is still not understood and no structural information is available on this domain. Structural information on the TLDc domain may therefore help in understanding exactly how it works. Here, the purification, crystallization and preliminary crystallographic studies of the TLDc domain from zebrafish are reported. Crystals belonging to the orthorhombic space group P2(1)2(1)2 were obtained and diffracted to 0.97 Å resolution. Selenomethionine-substituted protein could also be crystallized; these crystals diffracted to 1.1 Å resolution and the structure could be solved by SAD/MAD methods.

Protein expression regulation under oxidative stress

Oxidative stress is known to affect both translation and protein turnover, but very few large scale studies describe protein expression under stress. We measure protein concentrations in Saccharomyces cerevisiae over the course of 2 h in response to a mild oxidative stress induced by diamide, providing detailed time-resolved information for 815 proteins, with additional data for another ~1,100 proteins. For the majority of proteins, we discover major differences between the global transcript and protein response. Although mRNA levels often return to baseline 1 h after treatment, protein concentrations continue to change. Integrating our data with features of translation and protein degradation, we are able to predict expression patterns for 41% of the proteins in the core data set. Predictive features include, among others, targeting by RNA-binding proteins (Lhp1 and Khd1), RNA secondary structures, RNA half-life, and translation efficiency under unperturbed conditions and in response to oxidative reagents, but not chaperone binding. We are able to both describe general dynamics of protein concentration changes and suggest possible regulatory mechanisms for individual proteins.

Transgenic expression of fern Pteris vittata glutaredoxin PvGrx5 in Arabidopsis thaliana increases plant tolerance to high temperature stress and reduces oxidative damage to proteins

A glutaredoxin of the fern Pteris vittata PvGRX5 was previously implicated in arsenic tolerance. Because of possible involvements of glutaredoxins in metabolic adaptations to high temperature stress, transgenic Arabidopsis lines constitutively expressing PvGRX5 were evaluated for thermotolerance. Homozygous lines expressing PvGRX5 exhibited significantly greater tolerance to high temperature stress than the vector control and wild-type, based upon growth during stress and during recovery from stress, and this was related to leaf glutaredoxin specific activities. Measurements of tissue ion leakage, thiobarbituric acid reactive substances and protein carbonyl content showed that PvGRX5-expressors were significantly (P < 0.05) less affected by the high temperature treatment compared to wild-type and vector control lines for damage to membranes and proteins. Immunoblots indicated that specific protein bands, carbonylated during the stress treatment in the control lines, were protected in PvGRX5-expressors, thus implicating PvGRX5 in heat tolerance, likely mediated through cellular protection against oxidative stress.

The thioredoxin-thioredoxin reductase system can function in vivo as an alternative system to reduce oxidized glutathione in Saccharomyces cerevisiae

Cellular mechanisms that maintain redox homeostasis are crucial, providing buffering against oxidative stress. Glutathione, the most abundant low molecular weight thiol, is considered the major cellular redox buffer in most cells. To better understand how cells maintain glutathione redox homeostasis, cells of Saccharomyces cerevisiae were treated with extracellular oxidized glutathione (GSSG), and the effect on intracellular reduced glutathione (GSH) and GSSG were monitored over time. Intriguingly cells lacking GLR1 encoding the GSSG reductase in S. cerevisiae accumulated increased levels of GSH via a mechanism independent of the GSH biosynthetic pathway. Furthermore, residual NADPH-dependent GSSG reductase activity was found in lysate derived from glr1 cell. The cytosolic thioredoxin-thioredoxin reductase system and not the glutaredoxins (Grx1p, Grx2p, Grx6p, and Grx7p) contributes to the reduction of GSSG. Overexpression of the thioredoxins TRX1 or TRX2 in glr1 cells reduced GSSG accumulation, increased GSH levels, and reduced cellular glutathione E(h)'. Conversely, deletion of TRX1 or TRX2 in the glr1 strain led to increased accumulation of GSSG, reduced GSH levels, and increased cellular E(h)'. Furthermore, it was found that purified thioredoxins can reduce GSSG to GSH in the presence of thioredoxin reductase and NADPH in a reconstituted in vitro system. Collectively, these data indicate that the thioredoxin-thioredoxin reductase system can function as an alternative system to reduce GSSG in S. cerevisiae in vivo.

Sugar metabolism, redox balance and oxidative stress response in the respiratory yeast Kluyveromyces lactis

A lot of studies have been carried out on Saccharomyces cerevisiae, an yeast with a predominant fermentative metabolism under aerobic conditions, which allows exploring the complex response induced by oxidative stress. S. cerevisiae is considered a eukaryote model for these studies. We propose Kluyveromyces lactis as a good alternative model to analyse variants in the oxidative stress response, since the respiratory metabolism in this yeast is predominant under aerobic conditions and it shows other important differences with S. cerevisiae in catabolic repression and carbohydrate utilization. The knowledge of oxidative stress response in K. lactis is still a developing field. In this article, we summarize the state of the art derived from experimental approaches and we provide a global vision on the characteristics of the putative K. lactis components of the oxidative stress response pathway, inferred from their sequence homology with the S. cerevisiae counterparts. Since K. lactis is also a well-established alternative host for industrial production of native enzymes and heterologous proteins, relevant differences in the oxidative stress response pathway and their potential in biotechnological uses of this yeast are also reviewed.

The glutathione response to salt stress in the thermophilic fungus, Thermomyces lanuginosus

In order to investigate the role of glutathione in response to salt stress in the thermophilic fungus, Thermomyces lanuginosus, the biomass and the intracellular pool of protein and the glutathione + glutathione disulphid (GSH + GSSG) was measured for four days in a medium with NaCl or KCl added and in the basal medium. Due to the osmotic and ionic stress imposed by the salts, the growth of T. lanuginosus was delayed and the inhibitory effect of KCl exceeded that of NaCl. Glutathione seemed to be involved in the response of T. lanuginosus towards high concentrations of salt, as the level of stress was negatively correlated with the amount of total glutathione. Salt stress did not result in an increased intracellular protein production. GSH accumulated while nutrients were abundant and were subsequently degraded later, suggesting that nutrients stored in GSH are used when the medium is depleted.

Constitutive expression and functional characterization of mitochondrial glutaredoxin (Grx2) in mouse and human brain

Oxidative stress and mitochondrial dysfunction caused by loss of complex I activity are presumed to be primary events leading to neurodegeneration in Parkinson's disease. Mitochondrial glutaredoxin (Grx2), a glutathione-dependent thiol disulfide oxidoreductase helps maintain redox homeostasis in the mitochondria. We therefore, examined the constitutive expression of Grx2 in brain and its role in MPTP-mediated mitochondrial dysfunction in the extrapyramidal system. Grx2 is constitutively expressed in both neuron and glia in mouse and human brain including the neurons in human substantia nigra. Grx2 mRNA and protein were transiently upregulated in midbrain and striatum 1 h but not 4 h after a single dose of MPTP. Downregulation of Grx2 using antisense oligonucleotides, in vivo, in mouse brain resulted in partial loss of complex I activity indicating that Grx2 may help maintain complex I function in the mitochondria. Further, overexpression of Grx2 abolished MPP(+)-mediated toxicity in vitro in neuroblastoma cells. Our results demonstrate the probable role of Grx2 in maintenance of the redox milieu in mitochondria and its potential neuroprotective role in preserving mitochondrial integrity in neurodegenerative diseases, such as Parkinson's disease.

Oxidative stress and its effects during dehydration

Water is usually thought to be required for the living state, but several organisms are capable of surviving complete dehydration (anhydrobiotes). Elucidation of the mechanisms of tolerance against dehydration may lead to development of new methods for preserving biological materials that do not normally support drying, which is of enormous practical importance in industry, in clinical medicine as well as in agriculture. One of the molecular mechanisms of damage leading to death in desiccation-sensitive cells upon drying is free-radical attack to phospholipids, DNA and proteins. This review aims to summarize the strategies used by anhydrobiotes to cope with the danger of oxygen toxicity and to present our recent results about the importance of some antioxidant defense systems in the dehydration tolerance of Saccharomyces cerevisiae, a usual model in the study of stress response.

Old yellow enzymes protect against acrolein toxicity in the yeast Saccharomyces cerevisiae

Acrolein is a ubiquitous reactive aldehyde which is formed as a product of lipid peroxidation in biological systems. In this present study, we screened the complete set of viable deletion strains in Saccharomyces cerevisiae for sensitivity to acrolein to identify cell functions involved in resistance to reactive aldehydes. We identified 128 mutants whose gene products are localized throughout the cell. Acrolein-sensitive mutants were distributed among most major biological processes but particularly affected gene expression, metabolism, and cellular signaling. Surprisingly, the screen did not identify any antioxidants or similar stress-protective molecules, indicating that acrolein toxicity may not be mediated via reactive oxygen species. Most strikingly, a mutant lacking an old yellow enzyme (OYE2) was identified as being acrolein sensitive. Old yellow enzymes are known to reduce alpha,beta-unsaturated carbonyl compounds in vitro, but their physiological roles have remained uncertain. We show that mutants lacking OYE2, but not OYE3, are sensitive to acrolein, and overexpression of both isoenzymes increases acrolein tolerance. Our data indicate that OYE2 is required for basal levels of tolerance, whereas OYE3 expression is particularly induced following acrolein stress. Despite the range of alpha,beta-unsaturated carbonyl compounds that have been identified as substrates of old yellow enzymes in vitro, we show that old yellow enzymes specifically mediate resistance to small alpha,beta-unsaturated carbonyl compounds, such as acrolein, in vivo.

Glutaredoxins in fungi

Glutaredoxins (GRXs) can be subdivided into two subfamilies: dithiol GRXs with the CPY/FC active site motif, and monothiol GRXs with the CGFS motif. Both subfamilies share a thioredoxin-fold structure. Some monothiol GRXs exist with a single-Grx domain while others have a thioredoxin-like domain (Trx) and one or more Grx domains in tandem. Most fungi have both dithiol and monothiol GRXs with different subcellular locations. GRX-like molecules also exist in fungi that differ by one residue from one of the canonical active site motifs. Additionally, Omega-class glutathione transferases (GSTs) are active as GRXs. Among fungi, the GRXs more extensively studied are those from Saccharomyces cerevisiae. This organism contains two dithiol GRXs (ScGrx1 and ScGrx2) with partially overlapping functions in defence against oxidative stress. In this function, they cooperate with GSTs Gtt1 and Gtt2. While ScGrx1 is cytosolic, two pools exist for ScGrx2, a major one at the cytosol and a minor one at mitochondria. On the other hand, S. cerevisiae cells have two monothiol GRXs with the Trx-Grx structure (ScGrx3 and ScGrx4) that locate at the nucleus and probably regulate the activity of transcription factors such as Aft1, and one monothiol GRX with the Grx structure (ScGrx5) that localizes at the mitochondrial matrix, where it participates in the synthesis of iron-sulphur clusters. The function of yeast Grx5 seems to be conserved along the evolutionary scale.

Thioredoxin induced antioxidant gene expressions in human lens epithelial cells

Thioredoxin (Trx) is one of the major redox-regulating proteins. It catalyzes dithiol/disulfide exchange reactions and displays many unique intracellular and extracellular activities thereby controlling multiple mammalian cell functions. In the present study we examine the effect of exogenous Trx on the expression of several antioxidant genes in human lens epithelial (HLE B3) cells. mRNA levels for gene expression were monitored by RT-PCR and real-time PCR while protein levels were measured by western blot analysis. We have found that recombinant human Trx (hTrx)-treated HLE B3 cells have a simultaneous increase in mRNA expressions of mitochondrial manganese superoxide dismutase (MnSOD), thioltranferase 1 (TTase 1) or glutaredoxin 1 (Grx1), mitochondrial thioltransferase (TTase 2) or glutaredoxin 2 (Grx2), and thioredoxin peroxidase IV (Prx IV). The increased MnSOD and TTase 1 mRNA expressions were accompanied with their respective increases in protein levels. Other antioxidant genes, including Cu/ZnSOD, catalase, glutathione peroxidase 1 (GPx1), thioredoxin reductase 1 (TrxR1), thioredoxin peroxidase III (Prx III), and gamma-glutamyl cysteine synthetase were not affected. The ability of Trx to induce selectively these antioxidant genes in the absence of oxidative stress suggest a cytokine/growth factor-like new physiological role of hTrx in HLE B3 cells. Our data also provide evidence of a strong antioxidant defense system in HLE B3 cells that can be activated by extracellular hTrx, as well as of a possible link between the thioredoxin (Trx) and glutathione (GSH) redox regulating systems in these cells.

Stress-dependent regulation of a monothiol glutaredoxin gene from Schizosaccharomyces pombe

Glutaredoxin (Grx) is a small, heat-stable protein acting as a multi-functional glutathione-dependent disulfide oxidoreductase. In this work, a gene encoding the monothiol glutaredoxin Grx4 was cloned from the genomic DNA of the fission yeast Schizosaccharomyces pombe. The determined DNA sequence carries 1706 bp, which is able to encode the putative 244 amino acid sequence of Grx with 27 099 Da. It does not contain an intron, and the sequence CGFS is found in the active site. Grx activity was increased 1.46-fold in S. pombe cells harboring the cloned Grx4 gene, indicating that the Grx4 gene is in vivo functioning. Although aluminum, cadmium, and hydrogen peroxide marginally enhanced the synthesis of beta-galactosidase from the Grx4-lacZ fusion gene, NO-generating sodium nitroprusside (0.5 mmol/L and 1.0 mmol/L) and potassium chloride (0.2 mol/L and 0.5 mol/L) significantly enhanced it. The Grx4 mRNA level was also enhanced after the treatment with sodium nitroprusside and potassium chloride. The synthesis of beta-galactosidase from the Grx4-lacZ gene was increased by fermentable carbon sources, such as glucose (lower than 2%) and sucrose, but not by nonfermentable carbon sources such as acetate and ethanol. The basal expression of the S. pombe Grx4 gene did not depend on the presence of Pap1. These results imply that the S. pombe monothiol Grx4 gene is genuinely functional and regulated by a variety of stresses.

Stressed worms: Responding to the post-genomics era

Nematodes are among the most successful organisms in withstanding stress conditions associated with water loss, and viable individuals have been recovered from dry desert soils. Little is known about the biochemical and molecular events underpinning nematodes' physiological responses to dehydration. Post-genomics research in Caenorhabditis elegans may offer an opportunity to understand the stress response better. This review focuses on recent progress in understanding the molecular mechanisms of water-loss associated stress response in the model nematode C. elegans and in parasitic nematodes and discusses the scope for applying the knowledge and tools derived from a model organism for the study of wild, environmentally-adapted, parasitic nematodes, in the light of the emergence of genomics research of non-model organisms.

Involvement of oxidative stress response genes in redox homeostasis, the level of reactive oxygen species, and ageing in Saccharomyces cerevisiae

Saccharomyces cerevisiae mutants lacking oxidative stress response genes were used to investigate which genes are required under normal aerobic conditions to maintain cellular redox homeostasis, using intracellular glutathione redox potential (glutathione E(h)) to indicate the redox environment of the cells. Levels of reactive oxygen species (ROS) and mitochondrial membrane potentials (MMP) were also assessed by FACS using dihydroethidium and rhodamine 123 as fluorescent probes. Cells became more oxidised as strains shifted from exponential growth to stationary phase. During both phases the presence of reduced thioredoxin and the activity of glutathione reductase were important for redox homeostasis. Thioredoxin reductase contributed less during exponential phase when there was a strong requirement for active Yap1p transcription factor, but was critical during stationary phase. The absence of ROS detoxification systems, such as catalases or superoxide dismutases, had a lesser effect on glutathione E(h), but a more pronounced effect on ROS levels and MMP. These results reflect the major shift in ROS generation as cells switch from fermentative to respiratory metabolism and also showed that there was not a strong correlation between ROS production, MMP and cellular redox environment. Heterogeneity was detected in populations of strains with compromised anti-oxidant defences, and as cells aged they shifted from one cell type with low ROS content to another with much higher intracellular ROS.

Degradation-mediated protein quality control in the nucleus

Protein quality control degradation systems rid the cell of aberrant proteins, preventing detrimental effects on normal cellular function. Although such systems have been identified in most subcellular compartments, none have been found in the nucleus. Here, we report the discovery of such a system in Saccharomyces cerevisiae. It is defined by San1p, a ubiquitin-protein ligase that, in conjunction with the ubiquitin-conjugating enzymes Cdc34p and Ubc1p, targets four distinct mutant nuclear proteins for ubiquitination and destruction by the proteasome. San1p has exquisite specificity for aberrant proteins and does not target the wild-type versions of its mutant substrates. San1p is nuclear localized and requires nuclear localization for function. Loss of SAN1 results in a chronic stress response, underscoring its role of protein quality control in the cell. We propose that San1p-mediated degradation acts as the last line of proteolytic defense against the deleterious accumulation of aberrant proteins in the nucleus and that analogous systems exist in other eukaryotes.

Carbon source-dependent regulation of a second gene encoding glutaredoxin from the fission yeast Schizosaccharomyces pombe

Glutaredoxin (Grx), also known as thioltransferase (TTase), is an enzyme that catalyzes the reduction of a variety of disulfide compounds, including protein disulfides, in the presence of reduced glutathione. A second gene encoding Grx (Grx2) was cloned from the chromosomal DNA of the fission yeast Schizosaccharomyces pombe. The determined DNA sequence contains 1645 bp which is able to encode a polypeptide of 110 amino acids with a molecular mass of 12.2 kDa. The genomic DNA consists of 4 exons and 3 introns. The isolated gene was found to produce functional glutaredoxin that could accelerate the growth of the fission yeast, and is highly expressed at the mid- and late exponential phases. Aluminum, cadmium and hydrogen peroxide marginally enhanced the synthesis of beta-galactosidase from the Grx2-lacZ fusion gene. Shifts to lower concentrations (0.2, 0.4 or 0.8%) of D-glucose significantly enhanced the synthesis of beta-galactosidase from the Grx2-lacZ fusion gene. And shifts to sucrose (0.2, 0.4, 0.8 or 1.6%) as a sole carbon source markedly enhanced the synthesis of beta-galactosidase from the Grx2-lacZ fusion gene, the degree of which was inversely dependent on concentration. However, nonfermentable carbon sources reduced the expression of the Grx2 gene due to their growth arrest. The transcription factor Pap1 is not involved in the basal expression and induction of the Grx2 gene. The Grx2 protein was subcellularly localized in the nucleus of the yeast cells. Our results indicate that the Grx2 protein, located in the nucleus, is linked with the yeast growth, and that the gene is regulated by carbon sources.

The herbicide 2,4-dichlorophenoxyacetic acid induces the generation of free-radicals and associated oxidative stress responses in yeast

The pro-oxidant action of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is demonstrated in this study using Saccharomyces cerevisiae as a eukaryotic experimental model. Evidence is presented for the generation of hydroxyl-radicals, in yeast cells suddenly exposed to 2,4-D, detected by in vivo electron paramagnetic resonance (EPR) spectroscopy using 5,5'-dimethyl-1-pyrroline N-oxide and 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide as spin-traps. The intensity of the EPR spectra was dependent on the concentration of herbicide tested and was consistently higher in a mutant (Deltasod1) devoid of the cytosolic CuZn-superoxide dismutase. A time-course-dependent variation of the level of free-radical adducts was registered upon sudden exposure of an yeast cell population to concentrations of 2,4-D that lead to an initial period of viability loss, before resumption of inhibited growth by the viable adapted population. The variation pattern of the level of hydroxyl-radical adducts correlated with the one determined for the activity of Sod1p, cytosolic catalase Ctt1p, and the dithiol glutaredoxins Grx1p and Grx2p.

Disruption of yeast forkhead-associated cell cycle transcription by oxidative stress

The effects of oxidative stress on yeast cell cycle depend on the stress-exerting agent. We studied the effects of two oxidative stress agents, hydrogen peroxide (HP) and the superoxide-generating agent menadione (MD). We found that two small coexpressed groups of genes regulated by the Mcm1-Fkh2-Ndd1 transcription regulatory complex are sufficient to account for the difference in the effects of HP and MD on the progress of the cell cycle, namely, G1 arrest with MD and an S phase delay followed by a G2/M arrest with HP. Support for this hypothesis is provided by fkh1fkh2 double mutants, which are affected by MD as we find HP affects wild-type cells. The apparent involvement of a forkhead protein in HP-induced cell cycle arrest, similar to that reported for Caenorhabditis elegans and human, describes a potentially novel stress response pathway in yeast.

Dehydration-specific induction of hydrophilic protein genes in the anhydrobiotic nematode Aphelenchus avenae

Some organisms can survive exposure to extreme desiccation by entering a state of suspended animation known as anhydrobiosis. The free-living nematode Aphelenchus avenae can be induced to enter the anhydrobiotic state by exposure to a moderate reduction in relative humidity. During this preconditioning period, the nematode accumulates large amounts of the disaccharide trehalose, which is thought to be necessary, but not sufficient, for successful anhydrobiosis. To identify other adaptations that are required for anhydrobiosis, we developed a novel SL1-based mRNA differential display technique to clone genes that are upregulated by dehydration in A. avenae. Three such genes, Aav-lea-1, Aav-ahn-1, and Aav-glx-1, encode, respectively, a late embryogenesis abundant (LEA) group 3 protein, a novel protein that we named anhydrin, and the antioxidant enzyme glutaredoxin. Strikingly, the predicted LEA and anhydrin proteins are highly hydrophilic and lack significant secondary structure in the hydrated state. The dehydration-induced upregulation of Aav-lea-1 and Aav-ahn-1 was confirmed by Northern hybridization and quantitative PCR experiments. Both genes were also upregulated by an osmotic upshift, but not by cold, heat, or oxidative stress. Experiments to investigate the relationship between mRNA levels and protein expression for these genes are in progress. LEA proteins occur commonly in plants, accumulating during seed maturation and desiccation stress; the presence of a gene encoding an LEA protein in an anhydrobiotic nematode suggests that some mechanisms of coping with water loss are conserved between plants and animals.

Stress induction and mitochondrial localization of Oxr1 proteins in yeast and humans

Reactive oxygen species (ROS) are critical molecules produced as a consequence of aerobic respiration. It is essential for cells to control the production and activity of such molecules in order to protect the genome and regulate cellular processes such as stress response and apoptosis. Mitochondria are the major source of ROS within the cell, and as a result, numerous proteins have evolved to prevent or repair oxidative damage in this organelle. The recently discovered OXR1 gene family represents a set of conserved eukaryotic genes. Previous studies of the yeast OXR1 gene indicate that it functions to protect cells from oxidative damage. In this report, we show that human and yeast OXR1 genes are induced by heat and oxidative stress and that their proteins localize to the mitochondria and function to protect against oxidative damage. We also demonstrate that mitochondrial localization is required for Oxr1 protein to prevent oxidative damage.

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.

Regulation of redox homeostasis in the yeast Saccharomyces cerevisiae

An increasingly important area of research is based on sulphydryl chemistry, since the oxidation of -SH groups is one of the earliest observable events during oxidant-mediated damage and -SH groups play a critical role in the function of many macromolecular structures including enzymes, transcription factors and membrane proteins. Glutaredoxins and thioredoxins are small heat-stable oxidoreductases, conserved throughout evolution, which play key roles in maintaining the cellular redox balance. Much progress has been made in analysing these systems in the yeast Saccharomyces cerevisiae which is a very useful model eukaryote due to its ease of genetic manipulation, its compact genome, the availability of the entire genome sequence, and the current rate of progress in gene function research. Yeast, like all eukaryotes, contains a number of glutaredoxin and thioredoxin isoenzymes located in both the cytoplasm and the mitochondria. This review describes recent findings made in yeast that are leading to a better understanding of the regulation and role of redox homeostasis in eukaryotic cell metabolism.

Role of yeast glutaredoxins as glutathione S-transferases

The yeast Saccharomyces cerevisiae contains two glutaredoxins, encoded by GRX1 and GRX2, that are required for resistance to reactive oxygen species. We recently reported that Grx1 is active as a glutathione peroxidase and can directly reduce hydroperoxides (Collinson, E. J., Wheeler, G. L., Garrido, E. O., Avery, A. M., Avery, S. V., and Grant, C. M. (2002) J. Biol. Chem. 277, 16712-16717). We now show that Grx2 is also a general hydroperoxidase, and kinetic data indicate that both enzymes have a similar pattern of activity, which is highest with hydrogen peroxide, followed by cumene hydroperoxide and tert-butyl hydroperoxide. Furthermore, both Grx1 and Grx2 are shown be active as glutathione S-transferases (GSTs), and their activity with model substrates such as 1-chloro-2,4-dinitrobenzene is similar to their activity with hydroperoxides. Analysis of the Grx1 active site residues shows that Cys-27, but not Cys-30, is required for both the peroxidase and transferase activities, indicating that these reactions proceed via a monothiol mechanism. Deletion analysis shows that Grx1 and Grx2 have an overlapping function with yeast GSTs, encoded by GTT1 and GTT2, and are responsible for the majority of cellular GST activity. In addition, multiple mutants lacking GRX1, GRX2, GTT1, and GTT2 show increased sensitivity to stress conditions, including exposure to xenobiotics, heat, and oxidants. In summary, glutaredoxins are multifunctional enzymes with oxidoreductase, peroxidase, and GST activity, and are therefore ideally suited to detoxify the wide range of xenobiotics and oxidants that can be generated during diverse stress conditions.

Induction of hsp70 in transgenic Drosophila: biomarker of exposure against phthalimide group of chemicals

The expression of stress genes is suggested to be a potentially sensitive indicator of any chemical or physical assault. This led us to explore the possibility of using expression of one of the major stress genes, hsp70, in Drosophila as a biomarker against phthalimide group of chemicals, which may accordingly provide an early indication of exposure to these hazardous chemicals. We exposed third instar larvae of transgenic Drosophila melanogaster (hsp70-lacZ) Bg(9) to different concentrations of the test chemicals (Captan, Captafol and Folpet) for various time intervals (2-48 h) to evaluate expression of hsp70 by X-gal staining, ONPG assay and whole organ in situ immunohistochemistry. The study was further extended to examine the effect of the said chemicals on development of the organism and tissue damage occurring in them, thus raising the possibility of evaluating comparative deleterious effect inducing potential of the test chemicals. Our results showed a strong hsp70 expression in the Captafol-exposed larvae followed by weaker expression in Captan- and Folpet-treated larvae. The effect was further reflected on development as revealed by a delay in emergence of the flies by 3 days in 200 ppm Captafol-exposed group. Hsp70 was found not to be induced at 0.0002 ppm Captafol and at 0.002 ppm Captan and Folpet. The present study suggests that (a). hsp70 induction is sensitive enough to be used as a biomarker against phthalimide group of chemicals, (b). amongst the three test chemicals, Captafol is the most deleterious compound followed by Captan and Folpet, (c). 0.0002 ppm for Captafol and 0.002 ppm for Captan and Folpet, respectively, can be regarded as no observed adverse effect level (NOAEL).

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.

Two isoforms of Saccharomyces cerevisiae glutaredoxin 2 are expressed in vivo and localize to different subcellular compartments

Glutaredoxin (Grx)2 from Saccharomyces cerevisiae is a member of the two-cysteine (dithiol) subfamily of Grxs involved in the defence against oxidative stress in yeast. Recombinant yeast Grx2p, expressed in Escherichia coli, behaves as a 'classical' Grx that efficiently catalyses the reduction of hydroxyethyl disulphide by GSH. Grx2p also catalyses the reduction of GSSG by dihydrolipoamide with even higher efficiency. Western blot analysis of S. cerevisiae crude extracts identifies two isoforms of Grx2p of 15.9 and 11.9 kDa respectively. The levels of these two isoforms reach a peak during the exponential phase of growth in normal yeast extract/peptone/dextrose ('YPD') medium, with the long form predominating over the short one. From immunochemical analysis of subcellular fractions, it is shown that both isoforms are present in mitochondria, but only the short one is detected in the cytosolic fraction. On the other hand, only the long form is prominent in microsomes. Mitochondrial isoforms should represent the processed and unprocessed products of an open reading frame (YDR513W), with a putative start codon 99 bp upstream of the GRX2 start codon described thus far. These results indicate that GRX2 contains two in-frame start codons, and that translation from the first AUG results in a product that is targeted to mitochondria. The cytosolic form would result either by initiation from the second AUG, or by differential processing of one single translation product.

The yeast glutaredoxins are active as glutathione peroxidases

The yeast Saccharomyces cerevisiae contains two glutaredoxins, encoded by GRX1 and GRX2, which are active as glutathione-dependent oxidoreductases. Our studies show that changes in the levels of glutaredoxins affect the resistance of yeast cells to oxidative stress induced by hydroperoxides. Elevating the gene dosage of GRX1 or GRX2 increases resistance to hydroperoxides including hydrogen peroxide, tert-butyl hydroperoxide and cumene hydroperoxide. The glutaredoxin-mediated resistance to hydroperoxides is dependent on the presence of an intact glutathione system, but does not require the activity of phospholipid hydroperoxide glutathione peroxidases (GPX1-3). Rather, the mechanism appears to be mediated via glutathione conjugation and removal from the cell because it is absent in strains lacking glutathione-S-transferases (GTT1, GTT2) or the GS-X pump (YCF1). We show that the yeast glutaredoxins can directly reduce hydroperoxides in a catalytic manner, using reducing power provided by NADPH, GSH, and glutathione reductase. With cumene hydroperoxide, high pressure liquid chromatography analysis confirmed the formation of the corresponding cumyl alcohol. We propose a model in which the glutathione peroxidase activity of glutaredoxins converts hydroperoxides to their corresponding alcohols; these can then be conjugated to GSH by glutathione-S-transferases and transported into the vacuole by Ycf1.

Identification and characterization of a new mammalian glutaredoxin (thioltransferase), Grx2

A thiol/disulfide oxidoreductase component of the GSH system, glutaredoxin (Grx), is involved in the reduction of GSH-based mixed disulfides and participates in a variety of cellular redox pathways. A single cytosolic Grx (Grx1) was previously described in mammals. We now report identification and characterization of a second mammalian Grx, designated Grx2. Grx2 exhibited 36% identity with Grx1 and had a disulfide active center containing the Cys-Ser-Tyr-Cys motif. Grx2 was encoded in the genomes of mammals and birds and expressed in a variety of cell types. The gene for human Grx2 consisted of four exons and three introns, spanned 10 kilobase pairs, and localized to chromosome 1q31.2-31.3. The coding sequence was present in all exons, with the first exon encoding a mitochondrial signal peptide. The mitochondrial leader sequence was also present in mouse and rat Grx2 sequences and was shown to direct either Grx2 or green fluorescent protein to mitochondria. Alternative splicing forms of mammalian Grx2 mRNAs were identified that differed in sequences upstream of exon 2. To functionally characterize the new protein, human and mouse Grx2 proteins were expressed in Escherichia coli, and the purified proteins were shown to reduce mixed disulfides formed between GSH and S-sulfocysteine, hydroxyethyldisulfide, or cystine. Grx1 and Grx2 were sensitive to inactivation by iodoacetamide and H(2)O(2) and exhibited similar pH dependence of catalytic activity. However, H(2)O(2)-inactivated Grx2 could only be reactivated with 5 mm GSH, whereas Grx1 could also be reactivated with dithiothreitol or thioredoxin/thioredoxin reductase. The Grx2 structural model suggested a common reaction mechanism for this class of proteins. The data provide the first example of a mitochondrial Grx and also indicate the occurrence of a second functional Grx in mammals.

Adaptive response of the yeast Saccharomyces cerevisiae to reactive oxygen species: defences, damage and death

The yeast Saccharomyces cerevisiae has been extensively utilised to address the mechanisms underlying the oxidative stress response. The antioxidant defences can be induced either by respiratory growth or in the presence of pro-oxidants. The cell response involves the transcriptional control of genes by protein regulators that have been recently identified and post-translational activation of pre-existing defences. The current state of the art regarding the induction of antioxidant defences during respiratory growth and by exposure to hydrogen peroxide is reviewed.

Role of the glutathione/glutaredoxin and thioredoxin systems in yeast growth and response to stress conditions

Sulphydryl groups (-SH) play a remarkably broad range of roles in the cell, and the redox status of cysteine residues can affect both the structure and the function of numerous enzymes, receptors and transcription factors. The intracellular milieu is usually a reducing environment as a result of high concentrations of the low-molecular-weight thiol glutathione (GSH). However, reactive oxygen species (ROS), which are the products of normal aerobic metabolism, as well as naturally occurring free radical-generating compounds, can alter this redox balance. A number of cellular factors have been implicated in the regulation of redox homeostasis, including the glutathione/glutaredoxin and thioredoxin systems. Glutaredoxins and thioredoxins are ubiquitous small heat-stable oxidoreductases that have proposed functions in many cellular processes, including deoxyribonucleotide synthesis, repair of oxidatively damaged proteins, protein folding and sulphur metabolism. This review describes recent findings in the lower eukaryote Saccharomyces cerevisiae that are leading to a better understanding of their role in redox homeostasis in eukaryotic cell metabolism.

Cloning, expression and regulation of Schizosaccharomyces pombe gene encoding thioltransferase

The genomic DNA encoding thioltransferase was isolated from Schizosaccharomyces pombe using the polymerase chain reaction. The amplified DNA fragment was confirmed by Southern hybridization, completely digested with HindIII and BamHI, and then ligated into the yeast-Escherichia coli shuttle vector pRS316, which resulted in plasmid pEH1. The insert of plasmid pEH1 was transferred into the multi-copy vector YEp357 to generate plasmid pYEH1. The determined nucleotide sequence harbors an open reading frame consisting of four exons and three introns, which encodes a polypeptide of 101 amino acids with a molecular mass of 11261 Da. Thioltransferase activity was increased 1.6-fold in Saccharomyces cerevisiae containing plasmid pYEH1, and 1.8- and 2.7-fold in S. pombe containing plasmid pEH1 and pYEH1, respectively. The upstream sequence and the region encoding the N-terminal six amino acids were fused into promoterless beta-galactosidase gene of the shuttle vector YEp357R to generate the fusion plasmid pYEHR1. Synthesis of beta-galactosidase from the fusion plasmid was found to be enhanced by zinc and NO-generating S-nitroso-N-acetylpenicillamine.

A single glutaredoxin or thioredoxin gene is essential for viability in the yeast Saccharomyces cerevisiae

Glutaredoxins and thioredoxins are small heat-stable oxidoreductases that have been conserved throughout evolution. The yeast Saccharomyces cerevisiae contains two gene pairs encoding cytoplasmic glutaredoxins (GRX1, GRX2) and thioredoxins (TRX1, TRX2). We report here that the quadruple trx1 trx2 grx1 grx2 mutant is inviable and that either a single glutaredoxin or a single thioredoxin (i.e. grx1 grx2 trx1, grx1 grx2 trx2, grx1 trx1 trx2, grx2 trx1 trx2) is essential for viability. Loss of both thioredoxins has been reported previously to lead to methionine auxotrophy consistent with thioredoxins being the sole reductants for 3'-phosphoadenosine 5'-phosphosulphate reductase (PAPS) in yeast. However, we present evidence for the existence of a novel yeast hydrogen donor for PAPS reductase, as strains lacking both thioredoxins assimilated sulphate under conditions that minimized the generation of reactive oxygen species (low aeration and absence of functional mitochondria). In addition, the assimilation of [35S]-sulphate was approximately 60-fold higher in the trx1 trx2 grx1 and trx1 trx2 grx2 mutants compared with the trx1 trx2 mutant. Furthermore, in contrast to the trx1 trx2 mutant, the trx1 trx2 grx2 mutant grew on minimal agar plates, and the trx1 trx2 grx1 mutant grew on minimal agar plates under anaerobic conditions. We propose a model in which the novel reductase activity normally functions in the repair of oxidant-mediated protein damage but, under conditions that minimize the generation of reactive oxygen species, it can serve as a hydrogen donor for PAPS reductase.