Induction of the yeast alpha-specific STE3 gene by the peptide pheromone a-factor

The yeast Saccharomyces cerevisiae exhibits two mating types, a and alpha. Efficient mating of a and alpha cells requires the action of peptide pheromones secreted by each cell type. For example, a cells secrete a-factor, which alters the physiology of alpha cells, thereby preparing those cells for mating. To investigate the mechanism by which the pheromones act on the target cells, we have examined the effect of a-factor on expression of the STE3 gene, a gene which is required for mating by alpha cells and which is expressed only in alpha cells. We have monitored STE3 expression by two assays: RNA production from the chromosomal STE3 locus and beta-galactosidase activity produced from a plasmid-borne STE3-lacZ gene fusion. By both assays we show that a-factor induces a rapid increase in STE3 expression. Induction of STE3 RNA occurs even if protein synthesis is blocked by cycloheximide. Using temperature-sensitive cell division cycle mutants, we have also shown that induction occurs in cells arrested at several discrete positions in the cell cycle. These results demonstrate (1) that induction of STE3 expression by a-factor is a primary response to the pheromone, and (2) that alpha cells are capable of responding to a-factor regardless of their position in the cell cycle.