MULTIPLE MECHANISMS PROVIDE RAPID AND STRINGENT GLUCOSE REPRESSION OFGAL GENE-EXPRESSION IN SACCHAROMYCES-CEREVISIAE

Expression of the GAL genes of Saccharomyces cerevisiae is induced dur ing growth on galactose by a well-characterized regulatory mechanism t hat relieves Ga180p inhibition of the Ga14p transcriptional activator. Growth on glucose overrides induction by galactose. Glucose repressio n acts at three levels to reduce GAL1 expression: (i) it reduces the l evel of functional inducer in the cell; (ii) it lowers cellular levels of Ga14p by repressing GAL4 transcription; and (iii) it inhibits Ga14 p function through a repression element in the GAL1 promoter. We quant ified the amount of repression provided by each mechanism by assaying strains with none, one, two, or all three of the repression mechanisms intact. In a strain lacking all three repression mechanisms, there wa s almost no glucose repression of GAL1 expression, suggesting that the se are the major, possibly the only, mechanisms of glucose repression acting upon the GAL genes. The mechanism of repression that acts to re duce Ga14p levels in the cell is established slowly (hours after gluco se addition), probably because Ga14p is stable. By contrast, the repre ssion acting through the upstream repression sequence element in the G AL1 promoter is established rapidly (within minutes of glucose additio n). Thus, these three mechanisms of repression collaborate to repress GAL1 expression rapidly and stringently. The Mig1p repressor is respon sible for most (possibly all) of these repression mechanisms. We show that for GAL1 expression, mig1 mutations are epistatic to snf1 mutatio ns, indicating that Mig1p acts after the Snf1p protein kinase in the g lucose repression pathway, which suggests that Snf1p is an inhibitor o f Mig1p.