Control of yeast cell type by the mating type locus: positive regulation of the alpha-specific STE3 gene by the MAT alpha 1 product.

The mating type locus (MAT) determines the three yeast cell types, a, alpha, and a/alpha. It has been proposed that alleles of this locus, MATa and MAT alpha, encode regulators that control expression of unlinked genes necessary for mating and sporulation. Specifically, the alpha 1 product of MAT alpha is proposed to be a positive regulator of alpha-specific genes. To test this view, we have assayed RNA production from the alpha-specific STE3 gene in the three cell types and in mutants defective in MAT alpha. The STE3 gene was cloned by screening a yeast genomic clone bank for plasmids that complement the mating defect of ste3 mutants. Using the cloned STE3 gene as a probe, we find that alpha cells produce STE3 RNA, whereas a and a/alpha cells do not. Furthermore, mat alpha 1 mutants do not produce STE3 RNA, whereas mat alpha 2 mutants do. These results show that the STE3 gene, required for mating only by alpha cells, is expressed only in alpha cells. They show also that production of RNA from the STE3 gene requires that alpha 1 product of MAT alpha. Thus alpha 1 positively regulates at least one alpha-specific gene by increasing the level of that gene's RNA product.