Overproduction of the GAL1 or GAL3 protein causes galactose-independent activation of the GAL4 protein: evidence for a new model of induction for the yeast GAL/MEL regulon.

The transcriptional activation function of the Saccharomyces cerevisiae GAL4 protein is modulated by the GAL80 and GAL3 proteins. In the absence of galactose, GAL80 inhibits the function of GAL4, presumably by direct binding to the GAL4 protein. The presence of galactose triggers the relief of the GAL80 block. The key to this relief is the GAL3 protein. How GAL3 and galactose activate GAL4 is not understood, but the long-standing notion has been that a galactose derivative formed by catalytic activity of GAL3 is the inducer that interacts with GAL80 or the GAL80-GAL4 complex. Here we report that overproduction of the GAL3 protein causes constitutive expression of GAL/MEL genes in the absence of exogenous galactose. Overproduction of the GAL1 protein (galactokinase) also causes constitutivity, consistent with the observations that GAL1 is strikingly similar in amino acid sequence to GAL3 and has GAL3-like induction activity. Cells lacking the GAL10-encoded UDP-galactose-UDP-glucose epimerase retained the constitutivity response to overproduction of GAL3, making it unlikely that constitutivity is due to endogenously produced galactose. A galactose-independent mechanism of constitutivity is further indicated by the inducing properties of two newly created galactokinaseless alleles of GAL1. On the basis of these data, we propose a new model for galactose-induced activation of the GAL4 protein. This model invokes galactose-activation of the GAL3 and GAL1 proteins which in turn elicit an alteration of the GAL80-GAL4 complex to activate GAL4. This model is consistent with all the known features of the system and has important implications for manipulating GAL4-dependent transcriptional activation in vitro.