ADH2 EXPRESSION IS REPRESSED BY REG1 INDEPENDENTLY OF MUTATIONS THAT ALTER THE PHOSPHORYLATION OF THE YEAST TRANSCRIPTION FACTOR ADR1

In Saccharomyces cerevisiae, expression of the ADH2 gene is undetectab le during growth on glucose. The transcription factor ADR1 is required to fully activate expression when glucose becomes depleted. Partial a ctivation during growth on glucose occurred in cells carrying a consti tutive allele of ADR1 in which the phosphorylatable serine of a cyclic AMP (cAMP)-dependent protein kinase phosphorylation site had been cha nged to alanine. When glucose was removed from the growth medium, a su bstantial increase in the level of this constitutive expression was ob served for both the ADH2 gene and a reporter construct containing the ADR1 binding site. This suggests that glucose can block ADR1-mediated activation independently of cAMP-dependent phosphorylation at serine 2 30. REG1/HEX2/SRN1 was identified as a potential serine 230-independen t repressor of ADH2 expression. Yeast strains carrying a deletion of t he REG1 gene, reg1-1966, showed a large increase in ADR1-dependent exp ression of ADH2 during growth on glucose. A smaller increase in ADR1-i ndependent expression was also observed. Additionally, an increase in the level of ADR1 expression and posttranslational modification of the ADR1 protein were observed. When the reg1-1966 allele was combined wi th various ADR1 constitutive alleles, the level of ADH2 expression was synergistically elevated. This indicates that REG1 can act independen tly of phosphorylation at serine 230. Our results suggest that glucose repression in the presence of ADR1 constitutive alleles occurs primar ily through a REG1-dependent pathway which appears to affect ADH2 tran scription at multiple levels.