CHARACTERIZATION OF NGG1, A NOVEL YEAST GENE REQUIRED FOR GLUCOSE REPRESSION OF GAL4P-REGULATED TRANSCRIPTION

The GAL1-10 genes of Saccharomyces cerevisiae are regulated by the int eraction of cis- and trans-acting factors which facilitate activated t ranscription in galactose but not in glucose medium. By selecting muta tions that allow expression of a defective gal1-10-his3 hybrid promote r, we have identified a novel gene, NGG1, which is required for glucos e repression of the GAL10-related his3-G25 promoter. ngg1 was identifi ed as a recessive null mutation that in the presence of a gal80 backgr ound resulted in a 300-fold relief of glucose repression for the his3- G25 promoter. This compared with a 20-fold and negligible relief of re pression in gal80 and ngg1 strains, respectively. Deletion analysis of the his3-G25 promoter showed a correlation between the number of GAL4 p binding sites and the relative level of NGG1p activity. Relief of gl ucose repression by NGG1 was dependent on the presence of GAL4, but wa s independent of the GAL4 promoter. In addition, NGG1p activity was se en for a promoter construct containing independent GAL4p binding sites . These results suggest that NGG1p acts to inhibit GAL4p function in g lucose medium. We have cloned NGG1 by complementation and found that i t contains an open reading frame of 2106 bp which could encode a prote in with a molecular weight of 79 230.