CONTROL OF SACCHAROMYCES-CEREVISIAE CARBOXYPEPTIDASE-S (CPS1) GENE-EXPRESSION UNDER NUTRIENT LIMITATION

Expression of the vacuolar carboxypeptidase S (CPS1) gene in Saccharom yces cerevisiae is regulated by the availability of nutrients. Enzyme production is sensitive to nitrogen catabolite repression; i.e. the pr esence of ammonium ions maintains expression of the gene at a low leve l. Transfer of ammonium-glucose pre-grown cells to a medium deprived o f nitrogen causes a drastic increase in CPS] RNA level provided that a readily usable carbon source, such as glucose or fructose, is availab le to the cells. Derepression of the gene by nitrogen limitation is cy cloheximide-insensitive. Neither glycerol, ethanol, acetate nor galact ose support derepression of CPS1 expression under nitrogen starvation conditions. Non-metabolizable sugar analogs (2-deoxyglucose, 6-methyl- glucose or glucosamine) do not allow derepression of CPS1, showing tha t the process is energy-dependent. Production of carboxypeptidase yscS also increases several-fold when ammonium-pregrown cells are transfer red to media containing glucose and a non-readily metabolizable nitrog en source such as proline, leucine, valine or leucyl-glycine. Analysis of CPS1 expression in RAS2+ (high cAMP) and ras2 mutant (low cAMP) st rains and in cells grown at low temperature (23-degrees-C) and in beat -shocked cells (38-degrees-C) shows that steady-state levels of CPS1 m RNA are not controlled by a low cAMP level-signalling pathway.