GLUCOSE-INDUCED REGULATORY DEFECTS IN THE SACCHAROMYCES-CEREVISIAE-BYP1 GROWTH INITIATION MUTANT AND IDENTIFICATION OF MIG1 AS A PARTIAL SUPPRESSOR

Saccharomyces cerevisiae byp1-3 mutants displayed a long lag phase when shifted from a nonfermentable carbon source to a medium containing glucose. The byp1-3 mutation also caused several defects in regulatory phenomena which occur during the transition from the derepressed state to the repressed state. As opposed to wild-type cells, the addition of glucose to cells of the byp1-3 mutant grown on nonfermentable carbon sources did not induce a cyclic AMP signal. Fructose-2,6-bisphosphate formation and inactivation of fructose-1,6-bisphosphatase were severely delayed, but trehalase activation was not affected. In addition, the induction of pyruvate decarboxylase both at the level of activity and that of transcription was very slow compared with that in wild-type cells. These pleiotropic defects in glucose-induced regulatory phenomena might be responsible for the very long lag phase of byp1-3 cells and the inability of ascospores to initiate growth after germination on glucose media. Screening of a yeast gene library for clones complementing the byp1-3 phenotype resulted in the isolation of a truncated form of the previously described zinc finger transcription repressor MIG1. The entire MIG1 gene and the truncated form suppressed even on a single-copy vector the growth initiation defect but not the regulatory abnormalities of the byp1-3 mutant. MIG1 is not allelic to byp1-3.