Glutathione (␥-L-glutamyl-L-cysteinylglycine) is an important antioxidant molecule, helping to buffer the cell against free radicals and toxic electrophiles. Expression of the yeast GSH1 gene, encoding the first enzyme involved in glutathione biosynthesis, ␥-glutamylcysteine synthetase, is regulated by oxidants and the heavy metal cadmium at the level of transcription. We present evidence that the transcription factors involved in controlling the network of sulfur amino acid metabolism genes are also responsible for regulating GSH1 expression in response to cadmium. In particular the transcription factors Met-4, Met-31, and Met-32 are essential for cadmium-mediated regulation of gene expression, whereas the DNA-binding protein Cbf1 appears to play a negative role in controlling GSH1 expression.Glutathione (␥-L-glutamyl-L-cysteinylglycine) is an important molecule that plays a major role in protecting cells against damage caused by oxidants, heavy metals, and pesticides (1). Glutathione can act as as a free radical scavenger, with the redox-active sulfhydryl group reacting with oxidants to produce oxidized glutathione (GSSG). In response to oxidative stress caused by compounds such as H 2 O 2 and superoxide aniongenerating agents (for example menadione), the yeast Saccharomyces cerevisiae induces stress responses that result in protection against subsequent toxic levels of oxidants (2-6). We and others (7-10) have shown that glutathione can protect S. cerevisiae against oxidative stress and is therefore an important antioxidant molecule in this organism. In addition to its role as an antioxidant, there is also evidence that glutathione can act as a storage compound for both sulfur and nitrogen (11,12).It is also apparent that much of the regulation of the oxidative adaptive stress responses in yeast appears to be at the level of transcription (4, 13-15). We have previously demonstrated (13) that exposure of yeast cells to oxidants results in an increase in the steady state level of GSH1 mRNA, which encodes ␥-glutamylcysteine synthetase, the first enzyme in the GSH biosynthetic pathway. Furthermore, expression of the GSH1 gene has also been shown to be dependent upon the transcription factor Yap1 (13, 16). The expression of the gene encoding yeast glutathione reductase (GLR1) has also been shown to be regulated by oxidants and dependent upon the Yap1 protein (17). The pattern of gene expression controlled by the Yap1 protein has also been investigated using microarrays and high resolution two-dimensional gel electrophoresis (18,19). We reported that GSH1 gene expression was also inducible by the heavy metal cadmium (20). Glutathione is known to be important in countering cadmium toxicity; indeed, gsh1 mutants are hypersensitive to cadmium (21, 22). Moreover, yap1 mutants are defective in regulation of GSH1 expression and are also sensitive to cadmium (16, 23). There is also clear evidence for a direct link between metal ions and oxidant resistance/sensitivity. Mutations in a number of genes give rise to either res...

The budding yeast Saccharomyces cerevisiae has been shown to possess a number of discrete but overlapping adaptive stress responses. We show here that yeast has an adaptive stress response towards mercury and that this response overlaps to some extent with the H 2 O 2 and cadmium-inducible stress responses. Expression of the yeast GSH1 gene, encoding c-glutamylcysteine synthetase, is known to be regulated by hydrogen peroxide; in this study we show that expression of a GSH1-lacZ reporter gene is shown to be regulated by exposure to heavy metals, such as mercury and cadmium. Other redoxactive metals, including copper and iron, were found not to induce GSH1 expression. We show that mercury-mediated regulation of the GSH1 gene is not by the same mechanism used by cadmium. Moreover, our experiments suggest the possibility that the oxidative stress produced by mercury exposure is similar to that produced by treatment with H 2 O 2 , consistent with our finding that the Yap1 protein is also involved in the response of yeast towards mercury.

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The budding yeast Saccharomyces cerevisiae has been shown to possess a number of discrete but overlapping adaptive stress responses. We show here that yeast has an adaptive stress response towards mercury and that this response overlaps to some extent with the H(2)O(2) and cadmium-inducible stress responses. Expression of the yeast GSH1 gene, encoding gamma-glutamylcysteine synthetase, is known to be regulated by hydrogen peroxide; in this study we show that expression of a GSH1-lacZ reporter gene is shown to be regulated by exposure to heavy metals, such as mercury and cadmium. Other redox-active metals, including copper and iron, were found not to induce GSH1 expression. We show that mercury-mediated regulation of the GSH1 gene is not by the same mechanism used by cadmium. Moreover, our experiments suggest the possibility that the oxidative stress produced by mercury exposure is similar to that produced by treatment with H(2)O(2), consistent with our finding that the Yap1 protein is also involved in the response of yeast towards mercury.