The response of Schizosaccharomyces pombe towards the oxyanions selenate [Se(VI)] and dichromate [Cr(VI)] was investigated in order to establish the involvement of the yeast ATP sulfurylase in their reduction. An ATP sulfurylase-defective/selenate-resistant mutant of S. pombe (B-579 Se(R) -2) and an ATP sulfurylase-active/selenate-sensitive strain of S. pombe (B-579 Se(S)) were included in this study. The inhibitory effect of Se(VI) and Cr(VI) oxyanions on growth and bioaccumulation was measured. The sensitive strain showed natural sensitivity to selenate while the resistant mutant tolerated a 100-fold higher concentration of selenate. These results indicate that selenate toxicity to microorganisms is connected with the reduction of selenate to selenite. Both strains showed similar sensitivity to Cr(VI) and in this study there was no evidence that ATP sulfurylase participates in the reduction process of Cr(VI).
Selenate-resistant mutants were obtained from several strains of Schizosaccharomyces pombe. The obtained mutants all belonged to the same genetic complementation group. They were low in sulphate uptake activity and in ATP sulphurylase activity. They grew on medium containing sulphite, thiosulphate, cysteine or glutathione but not methionine as the sole source of sulphur. From these results, the mutants were concluded to carry mutations in the ATP sulphurylase gene. Inability of the mutants to utilize methionine as a sulphur source is rationalized by the absence of the reverse transsulphurylation pathway in this organism; wild type strains must utilize methionine as a sulphur source after it is degraded to give rise to sulphate.
Sulphur plays an important role in yeasts, especially in the biosynthesis of methionine and cysteine. The inorganic sulphur source, sulphate, is taken up by the cells via the sulphate-permease(s). After its transport, it is activated and subsequently reduced to sulphide or serves as a donor for sulphurylation reactions. Selenate anion (SeO4(2-)), which has the same metabolic pathway as sulphate, is toxic for the cells of Schizosaccharomyces pombe. We isolated selenate resistant mutants which cannot utilize sulphate, therefore they need organic sulphur source for growth. One of the selenate resistant mutants was successively transformed with S. pombe genomic libraries and the gene complementing the selenate resistance was identified as that of coding for the ATP-sulphurylase enzyme.
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