REGULATION OF SULFATE ASSIMILATION IN SACCHAROMYCES-CEREVISIAE

We examined how the activity of O-acetylserine and O-acetylhomoserine sulphydrylase (OAS/OAH) SHLase of Saccharomyces cerevisiae is affected by sulphur source added to the growth medium and genetic background o f the strain. In a wild-type strain, the activity was repressed if met hionine, cysteine or glutathione was added to the growth medium. Howev er, in a strain deficient of cystathionine gamma-lyase, cysteine and g lutathione were repressive, but methionine was not. In strains deficie nt of serine O-acetyltransferase (SATase), OAS/OAH SHLase activity was low regardless of sulphur source and was further lowered by cysteine and glutathione, but not by methionine. From these observations, we co ncluded that S-adenosylmethionine should be excluded from being the ef fector for regulation of OAS/OAH SHLase. Instead, we suspected that S. cerevisiae would have the same regulatory system as Escherichia coli for sulphate assimilation; i.e. cysteine inhibits SATase to lower the cellular concentration of OAS which is required for induction of the s ulphate assimilation enzymes including OAS/OAH SHLase. Subsequently, w e obtained data supporting this speculation.