A STE12-ALLELE HAVING A DIFFERENTIAL EFFECT ON A VERSUS ALPHA-CELLS

The transcriptional activator ste12p is a key component of the yeast p heromone response pathway: phosphorylated as a consequence of signal t ransduction, it activates transcription of genes that promote mating a nd the subsequent fusion of the two cell types a and alpha. Activation by Ste12p requires three types of protein-protein interaction between DNA-binding activator proteins: (1) Ste12p by itself can induce non-c ell-type-specific genes involved in mating; (2) cooperation of the tra nsactivator Mcm1p with Ste12p induces a-specific genes; and (3) format ion of a complex of the activator proteins Mcm1p and alpha 1 (a transc riptional activator of alpha-specific genes) with Ste12p is believed t o induced alpha-specific genes. We isolated and characterized a partia lly functional ste12 allele (ste12-T50), that is defective only in the activation of alpha-specific genes. ste12-T50 was isolated as a secon d-site mutation conferring the a mating phenotype on mat alpha 2 mutan t cells. In mat alpha 2 cells, where due to the lack of repressor, alp ha 2, both sets of cell-type-specific genes are expressed. ste12-T50 a pparently tips the balance in favor of a-specific gene expression. Thu s, mat alpha 2 ste12-T50 cells mate like a cells. Additional ste12 mut ants that confer the a mating phenotype on mat alpha 2 cells have also been isolated.