ROLE OF O-ACETYLHOMOSERINE SULFHYDRYLASE IN SULFUR AMINO-ACID SYNTHESIS IN VARIOUS YEASTS

Mutants defective in O-acetylhomoserine sulfhydrylase (OAH-SHLase) wer e obtained in five yeast strains representative of different yeast gen era: Saccharomyces cerevisiae, Kluyveromyces lactis, Yarrowia lipolyti ca, Schizosaccharomyces pombe and Trichosporon cutaneum. In vitro, in all five strains, the enzyme also had O-acetylserine (GAS) sulfhydryla se activity so it is a 'bifunctional' OAH/OAS-SHLase (Yamagata, 1989). The enzyme was only found to be essential in S. cerevisiae (OAH SHLas e-negative mutants are auxotrophs). Its impairment in K, lactis caused a slower growth rate and a decrease of the sulfur amino acid pool. In T. cutaneum only the pool was affected whereas in Y. lipolytica and S . pombe the lesion caused no change in the growth rate nor in the pool . In all strains where OAH SHLase-negative mutants were prototrophs, a monofunctional OAS sulfhydrylase was detected. The results indicate t hat OAH SHLase may play different physiological roles in various yeast s.