Yeast pheromone response pathway: characterization of a suppressor that restores mating to receptorless mutants.

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Abstract
Saccharomyces cerevisiae haploid cells, alpha and a, mate after being appropriately stimulated by the pheromone secreted by the opposite cell type (a-factor and alpha-factor, respectively). The binding of a pheromone to its receptor is a signal that initiates a series of intracellular changes that lead to the specific physiological alterations required for mating. To identify components of the signal transduction pathway, we sought pseudorevertants that restored mating competence to receptor mutants (MAT alpha ste3::LEU2). The suppressor srm1-1 was isolated as a recessive mutation that conferred temperature-sensitive growth to all strains and mating ability to MAT alpha ste3::LEU2 strains at the nonpermissive temperature. In addition, when srm1-1 mutants were shifted to the nonpermissive temperature, they exhibited two phenotypes characteristic of pheromone response, induction of FUS1 transcription and accumulation of cells in the G1 phase of the cell cycle. The srm1-1 mutation also suppressed a deletion of the alpha-factor-receptor gene in a cells. Together, these phenotypes suggest that the wild-type SRM1 product is a component of the pheromone response pathway. Deletion of STE4 or STE5, which are required in both haploid cell types for mating and response to pheromone, was not suppressed by srm1-1, suggesting that the SRM1 product may function before the STE4 and STE5 products. SRM1 is an essential gene and is expressed in both haploid cell types as well as in the product of their mating, a/alpha diploids. Homozygous srm1-1 a/alpha diploids were temperature sensitive although they did not arrest in G1. Thus, the SRM1 product may also have a role in the vegetative life cycle of cells.