Novel Gal3 proteins showing altered Gal80p binding cause constitutive transcription of Gal4p-activated genes in Saccharomyces cerevisiae

Gal4p-mediated activation of galactose gene expression in Saccharomyces cerevisiae normally requires both galactose and the activity of Gal3p. Recent evidence suggests that in cells exposed to galactose, Gal3p binds to and inhibits Gal80p, an inhibitor of the transcriptional activator Gal4p. Here, we report on the isolation and characterization of novel mutant forms of Gal3p that can induce Gal4p activity independently of galactose. Five mutant GAL3(c) alleles were isolated by using a selection demanding constitutive expression of a GAL1 promoter-driven HIS3 gene. This constitutive effect is not due to overproduction of Gal3p. The level of constitutive GAL gene expression in cells bearing different GAL3(c) alleles varies over more than a fourfold range and increases in response to galactose. Utilizing glutathione S-transferase-Gal3p fusions, we determined that the mutant Gal3p proteins show altered Gal80p-binding characteristics. The Gal3p mutant proteins differ in their requirements for galactose and ATP for their Gal80p-binding ability. The behavior of the novel Gal3p proteins provides strong support for a model wherein galactose causes an alteration in Gal3p that increases either its ability to bind to Gal80p or its access to Gal80p. With the Gal3p-Gal80p interaction being a critical step in the induction process, the Gal3p proteins constitute an important new reagent for studying the induction mechanism through both in vivo and in vitro methods.