Yeast stress responses
- S. Hohmann, W. H. Mager
Phytochelatins protect plants and the yeast S. pombe against cadmium toxicity. They chelate cadmium which is then transported into the vacuole. They are produced enzymatically from glutalhione. We have obtained a cadmium-sensitive mutant of S. pombe that produces almost no phytochelatins but high amounts of glutathione upon treatment with cadmium. In vitro assays of phytochelatin synthetase showed activity only in the wild-type cells, not in cells of the mutant. The mutant could be complemented by the gene encoding glutathione-synthetase (GSH2). Cadmium resistance and phytochelatin production were restored in the transformant. The mutated allele of the gsh2-gene was isolated from the mutant and sequenced. A single base-pair exchange was found that lead to a single change of the amino-acid sequence. The mutant could also be complemented by two different cDNAs from Arabidopsis thaliana. One of these cDNAs coded for the GSH2 protein, the other one for a protein unknown so far. The A. thaliana gsh2 eDNA restored phytochelatin production only poorly, the other one to a very high level. From these data we conclude that the GSH2 protein of S. pombe is not only necessary for glutathione synthesis but also for phytotochelatin production. As the gsh2 eDNA of A. thaliana complemented the mutation to a far lesser extend than the eDNA encoding a protein with no similarity to enzymes from the glutathione biosynthetic pathway, a defect in glutathione biosynthesis in the mutant can be ruled out. The role of the gsh2 gene in phytochelatin production will be discussed.