THE RAS1 FUNCTION OF SCHIZOSACCHAROMYCES-POMBE MEDIATES PHEROMONE-INDUCED TRANSCRIPTION

Loss of ras1+ function renders fission yeast cells unable to undergo morphological changes in response to mating pheromones, whereas cells carrying activated mutations in ras1 are hyper-responsive. This has led to the suggestion that the ras1 gene product plays a role in mating pheromone signal transduction. Using partially purified M factor we demonstrate that the mat1-Pm gene, which controls entry into meiosis, is transcribed in response to a pheromone signal. Strains mutated in the ras1 gene or in ste6, the fission yeast homologue of Ras protein GDP/GTP exchange factor, are unable to induce transcription of mat1-Pm in response to M factor. Furthermore, an activated ras1(val17) mutant exhibits a stronger induction of the mat1-Pm transcript. However, transcription still depends on nitrogen deprivation as well as on the presence of pheromone, showing that activation of the Ras1 protein alone does not substitute for any of these signals. The pat1-114 mutant bypasses the ras1/ste6 checkpoint, suggesting that activation of ras1 contributes to inactivation of the pat1 protein kinase.