Utilization of glutathione as an exogenous sulfur source is independent of gamma-glutamyl transpeptidase in the yeast Saccharomyces cerevisiae: evidence for an alternative gluathione degradation pathway

CHAC1 exacerbates arsenite cytotoxicity by lowering intracellular glutathione levels

We here examined whether CHAC1 is implicated in arsenite (As(III))-induced cytotoxicity in HaCaT cells. We found that HaCaT cells in which the intracellular GSH levels were elevated by transfection with CHAC1 siRNA showed decreased sensitivity to As(III) compared to the control cells. Treatment with BSO (an inhibitor of GSH biosynthesis) abolished the decrease in sensitivity to As(III), suggesting that an increase in intracellular GSH levels was involved in the decrease in sensitivity to As(III) due to the decrease in the levels of CHAC1 expression. When we examined the expression of CHAC1 after exposure of HaCaT cells to As(III), the levels of CHAC1 were increased. Since CHAC1 is a proapoptotic factor, we examined appearance of apoptotic cells and cleavage of caspase-3 after exposure to As(III) to determine whether As(III)-induced CHAC1 up-regulation was involved in apoptosis induction. The results showed that induction of apoptosis by As(III) exposure was not detected in CHAC1 siRNA-transfected cells. Together, our findings indicate that CHAC1 is involved in the sensitivity of HaCaT cells to As(III) by regulating the intracellular GSH levels, and in particular, CHAC1 is involved in As(III)-induced apoptosis.

Defective in utilizing glutathione 3, DUG3, is required for conidiation and host infection in the rice blast fungus Magnaporthe oryzae

Glutathione, an important redox buffer of the cell, also functions as a source of sulphur and nitrogen under starvation conditions. The metabolism and maintenance of glutathione homeostasis are vital for the appropriate functioning of the cell. In addition to the γ-glutamyl transpeptidase, the fungus-specific alternative pathway involving DUG1, DUG2 and DUG3 genes also mediate glutathione degradation. Here, we studied the functional significance of DUG3 in the vegetative growth and infection cycle of the cereal blast fungus Magnaporthe oryzae . Cells lacking the DUG3 gene displayed reduced conidiation, delayed appressorium formation, and a decrease in the severity of host infection. Further, we show that the γ-glutamyl transpeptidase inhibitor severely compromises the vegetative growth of the M. oryzae cells lacking the DUG3 gene. Taken together, our results suggest a significant role of glutathione metabolism in the growth and virulence of M. oryzae .

The DUG Pathway Governs Degradation of Intracellular Glutathione in Aspergillus nidulans

Glutathione (GSH) is an abundant tripeptide that plays a crucial role in shielding cellular macromolecules from various reactive oxygen and nitrogen species in fungi. Understanding GSH metabolism is of vital importance for deciphering redox regulation in these microorganisms. In the present study, to better understand the GSH metabolism in filamentous fungi, we investigated functions of the dugB and dugC genes in the model fungus Aspergillus nidulans These genes are orthologues of dug2 and dug3, which are involved in cytosolic GSH degradation in Saccharomyces cerevisiae The deletion of dugB, dugC, or both resulted in a moderate increase in the GSH content in mycelia grown on glucose, reduced conidium production, and disturbed sexual development. In agreement with these observations, transcriptome data showed that genes encoding mitogen-activated protein (MAP) kinase pathway elements (e.g., steC, sskB, hogA, and mkkA) or regulatory proteins of conidiogenesis and sexual differentiation (e.g., flbA, flbC, flbE, nosA, rosA, nsdC, and nsdD) were downregulated in the ΔdugB ΔdugC mutant. Deletion of dugB and/or dugC slowed the depletion of GSH pools during carbon starvation. It also reduced accumulation of reactive oxygen species and decreased autolytic cell wall degradation and enzyme secretion but increased sterigmatocystin formation. Transcriptome data demonstrated that enzyme secretions-in contrast to mycotoxin production-were controlled at the posttranscriptional level. We suggest that GSH connects starvation and redox regulation to each other: cells utilize GSH as a stored carbon source during starvation. The reduction of GSH content alters the redox state, activating regulatory pathways responsible for carbon starvation stress responses.IMPORTANCE Glutathione (GSH) is a widely distributed tripeptide in both eukaryotes and prokaryotes. Owing to its very low redox potential, antioxidative character, and high intracellular concentration, GSH profoundly shapes the redox status of cells. Our observations suggest that GSH metabolism and/or the redox status of cells plays a determinative role in several important aspects of fungal life, including oxidative stress defense, protein secretion, and secondary metabolite production (including mycotoxin formation), as well as sexual and asexual differentiations. We demonstrated that even a slightly elevated GSH level can substantially disturb the homeostasis of fungi. This information could be important for development of new GSH-producing strains or for any biotechnologically relevant processes where the GSH content, antioxidant capacity, or oxidative stress tolerance of a fungal strain is manipulated.

Genome-Wide Analysis of Experimentally Evolved Candida auris Reveals Multiple Novel Mechanisms of Multidrug Resistance

Candida auris is globally recognized as an opportunistic fungal pathogen of high concern, due to its extensive multidrug resistance (MDR). Still, molecular mechanisms of MDR are largely unexplored. This is the first account of genome-wide evolution of MDR in C. auris obtained through serial in vitro exposure to azoles, polyenes, and echinocandins. We show the stepwise accumulation of copy number variations and novel mutations in genes both known and unknown in antifungal drug resistance. Echinocandin resistance was accompanied by a codon deletion in FKS1 hot spot 1 and a substitution in FKS1 "novel" hot spot 3. Mutations in ERG3 and CIS2 further increased the echinocandin MIC. Decreased azole susceptibility was linked to a mutation in transcription factor TAC1b and overexpression of the drug efflux pump Cdr1, a segmental duplication of chromosome 1 containing ERG11, and a whole chromosome 5 duplication, which contains TAC1b The latter was associated with increased expression of ERG11, TAC1b, and CDR2 but not CDR1 The simultaneous emergence of nonsense mutations in ERG3 and ERG11 was shown to decrease amphotericin B susceptibility, accompanied with fluconazole cross-resistance. A mutation in MEC3, a gene mainly known for its role in DNA damage homeostasis, further increased the polyene MIC. Overall, this study shows the alarming potential for and diversity of MDR development in C. auris, even in a clade until now not associated with MDR (clade II), stressing its clinical importance and the urge for future research.IMPORTANCECandida auris is a recently discovered human fungal pathogen and has shown an alarming potential for developing multi- and pan-resistance toward all classes of antifungals most commonly used in the clinic. Currently, C. auris has been globally recognized as a nosocomial pathogen of high concern due to this evolutionary potential. So far, this is the first study in which the stepwise progression of multidrug resistance (MDR) in C. auris is monitored in vitro Multiple novel mutations in known resistance genes and genes previously not or vaguely associated with drug resistance reveal rapid MDR evolution in a C. auris clade II isolate. Additionally, this study shows that in vitro experimental evolution can be a powerful tool to discover new drug resistance mechanisms, although it has its limitations.

Glutathione Degradation

SIGNIFICANCE

Glutathione degradation has for long been thought to occur only on noncytosolic pools. This is because there has been only one enzyme known to degrade glutathione (γ-glutamyl transpeptidase) and this localizes to either the plasma membrane (mammals, bacteria) or the vacuolar membrane (yeast, plants) and acts on extracellular or vacuolar pools. The last few years have seen the discovery of several new enzymes of glutathione degradation that function in the cytosol, throwing new light on glutathione degradation. Recent Advances: The new enzymes that have been identified in the last few years that can initiate glutathione degradation include the Dug enzyme found in yeast and fungi, the ChaC1 enzyme found among higher eukaryotes, the ChaC2 enzyme found from bacteria to man, and the RipAY enzyme found in some bacteria. These enzymes play roles ranging from housekeeping functions to stress responses and are involved in processes such as embryonic neural development and pathogenesis.

CRITICAL ISSUES

In addition to delineating the pathways of glutathione degradation in detail, a critical issue is to find how these new enzymes impact cellular physiology and homeostasis.

FUTURE DIRECTIONS

Glutathione degradation plays a far greater role in cellular physiology than previously envisaged. The differential regulation and differential specificities of various enzymes, each acting on distinct pools, can lead to different consequences to the cell. It is likely that the coming years will see these downstream effects being unraveled in greater detail and will lead to a better understanding and appreciation of glutathione degradation. Antioxid. Redox Signal. 27, 1200-1216.

γ-Glutamyl transpeptidase (GgtA) of Aspergillus nidulans is not necessary for bulk degradation of glutathione

Aspergillus nidulans exhibited high γ-glutamyl transpeptidase (γGT) activity in both carbon-starved and carbon-limited cultures. Glucose repressed, but casein peptone increased γGT production. Null mutation of creA did not influence γGT formation, but the functional meaB was necessary for the γGT induction. Deletion of the AN10444 gene (ggtA) completely eliminated the γGT activity, and the mRNA levels of ggtA showed strong correlation with the observed γGT activities. While ggtA does not contain a canonical signal sequence, the γGT activity was detectable both in the fermentation broth and in the hyphae. Deletion of the ggtA gene did not prevent the depletion of glutathione observed in carbon-starved and carbon-limited cultures. Addition of casein peptone to carbon-starved cultures lowered the formation of reactive species (RS). Deletion of ggtA could hinder this decrease and resulted in elevated RS formation. This effect of γGT on redox homeostasis may explain the reduced cleistothecia formation of ΔggtA strains in surface cultures.

Role of glutathione in the oxidative stress response in the fungal pathogen Candida glabrata

Candida glabrata, an opportunistic fungal pathogen, accounts for 18-26 % of all Candida systemic infections in the US. C. glabrata has a robust oxidative stress response (OSR) and in this work we characterized the role of glutathione (GSH), an essential tripeptide-like thiol-containing molecule required to keep the redox homeostasis and in the detoxification of metal ions. GSH is synthesized from glutamate, cysteine, and glycine by the sequential action of Gsh1 (γ-glutamyl-cysteine synthetase) and Gsh2 (glutathione synthetase) enzymes. We first screened for suppressor mutations that would allow growth in the absence of GSH1 (gsh1∆ background) and found a single point mutation in PRO2 (pro2-4), a gene that encodes a γ-glutamyl phosphate reductase and catalyzes the second step in the biosynthesis of proline. We demonstrate that GSH is important in the OSR since the gsh1∆ pro2-4 and gsh2∆ mutant strains are more sensitive to oxidative stress generated by H2O2 and menadione. GSH is also required for Cadmium tolerance. In the absence of Gsh1 and Gsh2, cells show decreased viability in stationary phase. Furthermore, C. glabrata does not contain Saccharomyces cerevisiae high affinity GSH transporter ortholog, ScOpt1/Hgt1, however, our genetic and biochemical experiments show that the gsh1∆ pro2-4 and gsh2∆ mutant strains are able to incorporate GSH from the medium. Finally, GSH and thioredoxin, which is a second redox system in the cell, are not essential for the catalase-independent adaptation response to H2O2.

γ-Glutamyltransferases (GGT) in Colletotrichum graminicola: mRNA and enzyme activity, and evidence that CgGGT1 allows glutathione utilization during nitrogen deficiency

Gamma-glutamyltransferase (GGT, EC 2.3.2.2) cleaves the γ-glutamyl linkage in glutathione (GSH). Three GGTs in the hemibiotrophic plant pathogen Colletotrichum graminicola were identified in silico. GGT mRNA expression was monitored by quantitative reverse-transcriptase PCR. Expression of all three genes was detected in planta during the biotrophic and necrotrophic stages of infection. Of the three GGTs, CgGGT1 mRNA (from gene GLRG_09590) was the most highly expressed. All three GGT mRNAs were up-regulated in wild type nitrogen-starved germlings in comparison to non-starved germlings. CgGGT1 was insertionally mutagenized in C. graminicola, complemented with the wild type form of the gene, and over-expressed. Enzyme assays of two independent CgGGT1 knockouts and the wild type indicated that CgGGT1 is the major GGT and accounts for 86% and 68% of total GGT activity in conidia and mycelia, respectively. The over-expressing strain had 8-fold and 3-fold more enzyme activity in conidia and mycelia, respectively, than the wild type. In an analysis of the GGT knockout, complemented and over-expressing strains, GGT1 transcript levels are highly correlated (r=0.95) with levels of total GGT enzyme activity. CgGGT1 and CgGGT2 genes in strains that had ectopic copies of CgGGT1 were not up-regulated by nitrogen-starvation, in contrast to the wild type. Deletion or over-expression of CgGGT1 had no effect on mRNA expression of CgGGT2 and CgGGT3. In broth in which 3 and 6mM glutathione (GSH) was the nitrogen source, the CgGGT1 over-expressing strain produced significantly (P<0.0001) more biomass than the wild type and complemented strains, whereas the CgGGT1Δ strains produced significantly (P<0.0001) less biomass than the wild type strain. This suggests that CgGGT1 is involved in utilizing GSH as a nitrogen source. However, deletion and over-expression of CgGGT1 had no effect on either virulence in wounded corn leaf sheaths or GSH levels in conidia and mycelia. Thus, the regulation of GSH concentration is apparently independent of CgGGT1 activity.

Yeast genes involved in sulfur and nitrogen metabolism affect the production of volatile thiols from Sauvignon Blanc musts

Two volatile thiols, 3-mercaptohexan-1-ol (3MH), and 3-mercaptohexyl-acetate (3MHA), reminiscent of grapefruit and passion fruit respectively, are critical varietal aroma compounds in Sauvignon Blanc (SB) wines. These aromatic thiols are not present in the grape juice but are synthesized and released by the yeast during alcoholic fermentation. Single deletion mutants of 67 candidate genes in a laboratory strain of Saccharomyces cerevisiae were screened using gas chromatography mass spectrometry for their thiol production after fermentation of SB grape juice. None of the deletions abolished production of the two volatile thiols. However, deletion of 17 genes caused increases or decreases in production by as much as twofold. These 17 genes, mostly related to sulfur and nitrogen metabolism in yeast, may act by altering the regulation of the pathway(s) of thiol production or altering substrate supply. Deleting subsets of these genes in a wine yeast strain gave similar results to the laboratory strain for sulfur pathway genes but showed strain differences for genes involved in nitrogen metabolism. The addition of two nitrogen sources, urea and di-ammonium phosphate, as well as two sulfur compounds, cysteine and S-ethyl-L-cysteine, increased 3MH and 3MHA concentrations in the final wines. Collectively these results suggest that sulfur and nitrogen metabolism are important in regulating the synthesis of 3MH and 3MHA during yeast fermentation of grape juice.

Glutathione degradation by the alternative pathway (DUG pathway) in Saccharomyces cerevisiae is initiated by (Dug2p-Dug3p)2 complex, a novel glutamine amidotransferase (GATase) enzyme acting on glutathione

The recently identified, fungi-specific alternative pathway of glutathione degradation requires the participation of three genes, DUG1, DUG2, and DUG3. Dug1p has earlier been shown to function as a Cys-Gly-specific dipeptidase. In the present study, we describe the characterization of Dug2p and Dug3p. Dug3p has a functional glutamine amidotransferase (GATase) II domain that is catalytically important for glutathione degradation as demonstrated through mutational analysis. Dug2p, which has an N-terminal WD40 and a C-terminal M20A peptidase domain, has no peptidase activity. The previously demonstrated Dug2p-Dug3p interaction was found to be mediated through the WD40 domain of Dug2p. Dug2p was also shown to be able to homodimerize, and this was mediated by its M20A peptidase domain. In vitro reconstitution assays revealed that Dug2p and Dug3p were required together for the cleavage of glutathione into glutamate and Cys-Gly. Purification through gel filtration chromatography confirmed the formation of a Dug2p-Dug3p complex. The functional complex had a molecular weight that corresponded to (Dug2p-Dug3p)(2) in addition to higher molecular weight oligomers and displayed Michaelis-Menten kinetics. (Dug2p-Dug3p)(2) had a K(m) for glutathione of 1.2 mm, suggesting a novel GATase enzyme that acted on glutathione. Dug1p activity in glutathione degradation was found to be restricted to its Cys-Gly peptidase activity, which functioned downstream of the (Dug2p-Dug3p)(2) GATase. The DUG2 and DUG3 genes, but not DUG1, were derepressed by sulfur limitation. Based on these studies and the functioning of GATases, a mechanism is proposed for the functioning of the Dug proteins in the degradation of glutathione.

Glutathione degradation is a key determinant of glutathione homeostasis

Glutathione (GSH) has several important functions in eukaryotic cells, and its intracellular concentration is tightly controlled. Combining mathematical models and (35)S labeling, we analyzed Saccharomyces cerevisiae sulfur metabolism. This led us to the observation that GSH recycling is markedly faster than previously estimated. We set up additional in vivo assays and concluded that under standard conditions, GSH half-life is around 90 min. Sulfur starvation and growth with GSH as the sole sulfur source strongly increase GSH degradation, whereas cadmium (Cd(2+)) treatment inhibits GSH degradation. Whatever the condition tested, GSH is degraded by the cytosolic Dug complex (composed of the three subunits Dug1, Dug2, and Dug3) but not by the γ-glutamyl-transpeptidase, raising the question of the role of this enzyme. In vivo, both DUG2/3 mRNA levels and Dug activity are quickly induced by sulfur deprivation in a Met4-dependent manner. This suggests that Dug activity is mainly regulated at the transcriptional level. Finally, analysis of dug2Δ and dug3Δ mutant cells shows that GSH degradation activity strongly impacts on GSH intracellular concentration and that GSH intracellular concentration does not affect GSH synthesis rate. Altogether, our data led us to reconsider important aspects of GSH metabolism, challenging notions on GSH synthesis and GSH degradation that were considered as established.

Dug1p Is a Cys-Gly peptidase of the gamma-glutamyl cycle of Saccharomyces cerevisiae and represents a novel family of Cys-Gly peptidases

GSH metabolism in yeast is carried out by the gamma-glutamyl cycle as well as by the DUG complex. One of the last steps in the gamma-glutamyl cycle is the cleavage of Cys-Gly by a peptidase to the constitutent amino acids. Saccharomyces cerevisiae extracts carry Cys-Gly dipeptidase activity, but the corresponding gene has not yet been identified. We describe the isolation and characterization of a novel Cys-Gly dipeptidase, encoded by the DUG1 gene. Dug1p had previously been identified as part of the Dug1p-Dug2p-Dug3p complex that operates as an alternate GSH degradation pathway and has also been suggested to function as a possible di- or tripeptidase based on genetic studies. We show here that Dug1p is a homodimer that can also function in a Dug2-Dug3-independent manner as a dipeptidase with high specificity for Cys-Gly and no activity toward tri- or tetrapeptides in vitro. This activity requires zinc or manganese ions. Yeast cells lacking Dug1p (dug1Delta) accumulate Cys-Gly. Unlike all other Cys-Gly peptidases, which are members of the metallopeptidase M17, M19, or M1 families, Dug1p is the first to belong to the M20A family. We also show that the Dug1p Schizosaccharomyces pombe orthologue functions as the exclusive Cys-Gly peptidase in this organism. The human orthologue CNDP2 also displays Cys-Gly peptidase activity, as seen by complementation of the dug1Delta mutant and by biochemical characterization, which revealed a high substrate specificity and affinity for Cys-Gly. The results indicate that the Dug1p family represents a novel class of Cys-Gly dipeptidases.

Glutathione and gamma-glutamyl transferases are involved in the formation of cadmium-glutathione complex

In a wild-type strain of Saccharomyces cerevisiae, cadmium induces the activities of both gamma-glutamyl transferase (gamma-GT) and glutathione transferase 2 (Gtt2). However, Gtt2 activity did not increase under gamma-GT or Ycf1 deficiencies, suggesting that the accumulation of glutathione-cadmium in the cytosol inhibits Gtt2. On the other hand, the balance between the cytoplasmic and vacuolar level of glutathione seems to regulate gamma-GT activity, since this enzyme was not activated in a gtt2 strain. Taken together, these results suggest that gamma-GT and Gtt2 work together to remove cadmium from the cytoplasm, a crucial mechanism for metal detoxification that is dependent on glutathione.

Vacuolar compartmentation of the cadmium-glutathione complex protects Saccharomyces cerevisiae from mutagenesis

In the yeast Saccharomyces cerevisiae, gamma-glutamyl transferase (gamma-GT; EC 2.3.2.2) is a vacuolar-membrane bound enzyme. In this work we verified that S. cerevisiae cells deficient in gamma-GT absorbed almost 2.5-fold as much cadmium as the wild-type (wt) cells, suggesting that this enzyme might be responsible for the recycle of cadmium-glutathione complex stored in the vacuole. The mutant strain showed difficulty in keeping constant levels of glutathione (GSH) during the stress, although the GSH-reductase activity was practically the same in both wt and mutant strains, before and after metal stress. This difficulty to maintain the GSH levels in the gamma-GT mutant strain led to high levels of lipid peroxidation and carbonyl proteins in response to cadmium, higher than in the wt, but lower than in a mutant deficient in GSH synthesis. Although the increased levels of oxidative stress, gamma-GT mutant strain showed to be tolerant to cadmium and showed similar mutation rates to the wt, indicating that the compartmentation of the GSH-cadmium complex in vacuole protects cells against the mutagenic action of the metal. Confirming this hypothesis, a mutant strain deficient in Ycf1, which present high concentrations of GSH-cadmium in cytoplasm due to its deficiency in transport the complex to vacuole, showed increased mutation rates.

The alternative pathway of glutathione degradation is mediated by a novel protein complex involving three new genes in Saccharomyces cerevisiae

Glutathione (GSH), L-gamma-glutamyl-L-cysteinyl-glycine, is the major low-molecular-weight thiol compound present in almost all eukaryotic cells. GSH degradation proceeds through the gamma-glutamyl cycle that is initiated, in all organisms, by the action of gamma-glutamyl transpeptidase. A novel pathway for the degradation of GSH that requires the participation of three previously uncharacterized genes is described in the yeast Saccharomyces cerevisiae. These genes have been named DUG1 (YFR044c), DUG2 (YBR281c), and DUG3 (YNL191w) (defective in utilization of glutathione). Although dipeptides and tripeptides with a normal peptide bond such as cys-gly or glu-cys-gly required the presence of only a functional DUG1 gene that encoded a protein belonging to the M20A metallohydrolase family, the presence of an unusual peptide bond such as in the dipeptide, gamma-glu-cys, or in GSH, required the participation of the DUG2 and DUG3 gene products as well. The DUG2 gene encodes a protein with a peptidase domain and a large WD40 repeat region, while the DUG3 gene encoded a protein with a glutamine amidotransferase domain. The Dug1p, Dug2p, and Dug3p proteins were found to form a degradosomal complex through Dug1p-Dug2p and Dug2p-Dug3p interactions. A model is proposed for the functioning of the Dug1p/Dug2p/Dug3p proteins as a specific GSH degradosomal complex.

In vivo specificity of Ure2 protection from heavy metal ion and oxidative cellular damage in Saccharomyces cerevisiae

The S. cerevisiae Ure2 protein is a prion precursor able to form large homopolymers with the characteristics of amyloid particles, a function largely restricted to its 90 N-terminal amino acids. The remaining C-terminal domain of Ure2 plays two important roles in cellular metabolism. First, it regulates nitrogen catabolic gene expression by forming a complex with the GATA transcription factor Gln3. This complex formation correlates with Gln3 being sequestered in the cytoplasm under conditions of excess nitrogen, where Gln3/Gat1-mediated transcription is minimal. Second, Ure2, which possesses structural homology with glutathione S-transferases and binds to xenobiotics and glutathione, has been recently shown to be required for Cd(II) and hydrogen peroxide detoxification. Present experiments demonstrate that Ure2 possesses a far broader protection specificity, being required to avoid the toxic effects of As(III), As(V), Cr(III), Cr(VI), Se(IV), as well as Cd(II) and Ni(II), and to varying lesser degrees Co(II), Cu(II), Fe(II), Ag(I), Hg(II), cumene and t-butyl hydroperoxides. In contrast, deletion of URE2 greatly enhances a cell's ability to withstand toxic concentrations of Zn(II) and Mo(VI). In the case of Cd(II), Ure2 does not function to decrease intracellular Cd(II) levels or influence glutathione availability for glutathionation. In fact, ure2 hypersensitivity to Cd(II) remains the same, even when glutathione is used as sole source of nitrogen for cell growth. These data suggest that Ure2 possesses a central role in metal ion detoxification, a role not demonstrably shared by either of the two known S. cerevisiae glutathione S-transferases, Gtt1 and Gtt2, or the two glutaredoxins, Grx1 and Grx2, that also possess glutathione S-transferase activity.

Multiple cis-regulatory elements and the yeast sulphur regulatory network are required for the regulation of the yeast glutathione transporter, Hgt1p

HGT1 encodes a high-affinity glutathione transporter in the yeast Saccharomyces cerevisiae that is induced under sulphur limitation. The present work demonstrates that repression by organic sulphur sources is under the control of the classic sulphur regulatory network, as seen by the absence of expression in a met4delta background. Cysteine appeared to be the principal regulatory molecule, since elevated levels were seen in str4delta strains (deficient in cysteine biosynthesis) that could be repressed by elevated levels of cysteine, but not by methionine or glutathione. Investigations into cis-regulatory elements revealed that the previously described motif, a 9-bp cis element, CCGCCACAC, located at the -356 to -364 region of the promoter could in fact be refined to a 7-bp CGCCACA motif that is also repeated at -333 to -340. The second copy of this motif was essential for activity, since mutations in the core region of the second copy completely abolished activity and regulation by sulphur sources. Activity, but not regulation, could be restored by reintroducing an additional copy upstream of the first copy. A third region, GCCGTCTGCAAGGCA, conserved in the HGT1 promoters of the different Saccharomyces spp, was observed at -300 to -285 but, while mutations in this region did not lead to any loss in repression, the basal and induced levels were significantly increased. In contrast to a previous report, no evidence was found for regulation by the VDE endonuclease. The strong repression at the transport level by glutathione seen in strains overexpressing HGT1 was due to a glutathione-dependent toxicity in these cells.

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.

Characterization and regulation of the γ-glutamyl transpeptidase gene from the fission yeast Schizosaccharomyces pombe

The structural gene for the putative γ-glutamyl transpeptidase (GGT) was isolated from the chromosomal DNA of the fission yeast Schizosaccharomyces pombe. The determined sequence contained 3324 bp and encoded the predicted 630 amino acid sequence of GGT, which resembles counterparts in Homo sapiens, Rattus norvegicus, Saccharomyces cerevisiae, and Escherichia coli. The S. pombe cells harboring the cloned GGT gene showed about twofold higher GGT activity in the exponential phase than the cells harboring the vector only, indicating that the cloned GGT gene was functional. To monitor the expression of the S. pombe GGT gene, we fused the fragment 1085 bp upstream of the cloned GGT gene into the promoterless β-galactosidase gene of the shuttle vector YEp367R to generate the fusion plasmid pGT98. The synthesis of β-galactosidase from the fusion plasmid in S. pombe cells was enhanced by treatments with NO-generating sodium nitroprusside (SN), L-buthionine-(S,R)-sulfoximine (BSO), and glycerol. The GGT mRNA level in the S. pombe cells was increased by SN and BSO. Involvement of Pap1 in the induction of the GGT gene by SN and BSO was observed.Key words: fission yeast, genomic DNA, γ-glutamyl transpeptidase, Pap1, regulation, Schizosaccharomyces pombe.

Investigations into the polymorphisms at the ECM38 locus of two widely used Saccharomyces cerevisiae S288C strains, YPH499 and BY4742

The ECM38 gene encodes the gamma-glutamyl transpeptidase enzyme, an enzyme involved in glutathione turnover. The enzyme was found to be present in the S288C strain, BY4742, but absent in another widely used strain congenic to S288C, YPH499. Cloning and sequencing the genes from these yeasts indicated the presence of 11 single nucleotide polymorphisms in the coding region and eight single nucleotide polymorphisms in the promoter region of the ECM38 gene of YPH499 (but none in that of BY4742). One of the SNPs in the ECM38 ORF led to a G --> D conversion in a region conserved in all gamma-GT enzymes and was found to be responsible for the loss of activity in this strain. The presence of gamma-GT activity in other YPH strains led us to trace the origins of the polymorphisms in YPH499. Our results indicated that among the progenitor strains, YPH1 and YPH2, YPH1 carried the polymorphisms seen in YPH499 and also lacked the gamma-GT activity. The implications of these results for the use of these widely used S288C strains and the origin of these single nucleotide polymorphisms are presented.