Control of yeast cell type by the mating type locus: Positive regulation of the α-specific STE3 gene by the MATα 1 product

The mating type locus (MAT) determines the three yeast cell types, a, α, and a/α. It has been proposed that alleles of this locus, MATa and MATα, encode regulators that control expression of unlinked genes necessary for mating and sporulation. Specifically, the α1 product of MATα is proposed to be a positive regulator of α-specific genes. To test this view, we have assayed RNA production from the α-specific STE3 gene in the three cell types and in mutants defective in MATα. 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 a cells produce STE3 RNA, whereas a and a/a cells do not. Furthermore, matα 1 mutants do not produce STE3 RNA, whereas matα 2 mutants do. These results show that the STE3 gene, required for mating only by α cells, is expressed only in α cells. They show also that production of RNA from the STE3 gene requires the α1 product of MATα. Thus α1 positively regulates at least one α-specific gene by increasing the level of that gene's RNA product.