AbstractExpression of the vacuolar carboxypeptidase S (CPS1) gene inSaccharomyces cerevisiaeis regulated by the availability of nutrients. Enzyme production is sensitive to nitrogen catabolite repression; i.e. the presence of ammonium ions maintains expression of the gene at a low level. Transfer of ammonium–glucose pre‐grown cells to a medium deprived of nitrogen causes a drastic increase inCPS1RNA level provided that a readily usable carbon source, such as glucose or fructose, is available to the cells. Derepression of the gene by nitrogen limitation is cycloheximide‐insensitive. Neither glycerol, ethanol, acetate nor galactose support derepression ofCPS1expression under nitrogen starvation conditions. Non‐metabolizable sugar analogs (2‐deoxyglucose, 6‐methyl‐glucose or glucosamine) do not allow derepression ofCPS1, showing that the process is energy‐dependent. Production of carboxypeptidase yscS also increases several‐fold when ammonium‐pregrown cells are transferred to media containing glucose and a non‐readily metabolizable nitrogen source such as proline, leucine, valine or leucyl‐glycine. Analysis ofCPS1expression inRAS2+(high cAMP) andras2mutant (low cAMP) strains and in cells grown at low temperature (23°C) and in heat‐shocked cells (38°C) shows that steady‐state levels ofCPS1mRNA are not controlled by a low
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