Overproduction of the Yeast STE12 Protein Leads to Constitutive Transcriptional Induction

Haploid a and alpha cells of the yeast Saccharomyces cerevisiae respond to the pheromones alpha- and a-factor, respectively, by increasing transcription of inducible genes, arresting cell division, and forming cell-surface projections. These responses are dependent on the activity of several genes, including STE12, whose product binds to the pheromone response element located within the regulatory DNA sequences of inducible genes. We assayed the effects of overproducing the STE12 protein in both STE+ cells, as well as ste2, ste7, and ste11 mutant cells. We find that overproduction leads to increased transcription of pheromone-inducible genes and is able to suppress the mating defect of the ste mutants. These results suggest that one effect of pheromone treatment may be to increase the ability of the STE12 protein to activate transcription. In addition, we observed that cells cannot tolerate very high levels of the STE12 protein, and many arrest in G1 with a large size and morphological changes. Thus, constitutively high-level transcription of pheromone-inducible genes causes cells to display some features similar to treatment with pheromone.