Budding yeast mate in the Gl phase of the cell cycle. Cells of each mating type produce mating pheromones that interact with cell surface receptors on cells of the opposite mating type. Binding of pheromone to receptors triggers a differentiative program including altered gene transcription, altered cell morphology, and Gl cell cycle arrest. The START regulatory point in the Gl phase of the cell cycle marks a sharp transition from sensitivity to mating pheromones to resistance until the next Gl phase. We are interested in the molecular basis of this commitment event, which integrates conjugation and the cell cycle. The FAR1 gene is essential for mating pheromone arrest. We have found that FAR1 degradation is cell-cycle regulated, and degradation correlates with phosphorylation. This control and FAR1 transcriptional control result in significant accumulation of Far1 protein only in the pre-START Gl phase of the cell cycle. Cell cycle regulation of FAR1 thus can contribute to the transition from mating pheromone sensitivity to resistance at START. This implies a negative control by cell-cycle progression on machinery involved in cell cycle arrest and mating. We have begun genetic and biochemical analysis of the basis for the control of Far1 degradation. This analysis is based on our observations that Far1 phosphorylation precedes its degradation, and that an N-terminal deletion mutant of Far1 blocks degradation. We have identified an additional independent negative control of the overall pheromone signalling pathway. Transcriptional induction by mating factor of genes involved in mating is largely blocked at about the time of START, possibly by specific Cln/Cdc28 protein kinase complexes. We wish to characterize this control, first genetically and ultimately biochemically.
