Regulation of the Yeast HO Gene

Saccharomyces cerevisiae has two haploid cell types (a and α) which can mate to form a third cell type: the a/α diploid. Cell type is primarily determined by the information encoded at the mating-type (MAT) locus. A haploid carries either the MATa or the MATα allele, and the diploid carries both. Master regulator genes, encoded by these two MAT alleles, affect the expression of many genes and thereby cause cell type-specific differentiation. Two genes from MATα (called α1 and β2) and one gene from MATa (a1) are involved in regulating cell-type-specific processes. The α1 gene product induces transcription of α-specific genes, while α2 turns off a-specific genes. In a cells, and indeed in cells lacking active MAT genes, a-specific genes are constitutively expressed. When the a/α diploid is formed, a1 and α2 both act to turn off α1 transcription, to repress haploid specific gene expression, and to induce diploid specific...