Control of gene expression at the MAT(alpha) locus in Saccharomyces cerevisiae

Donnamarie Giesman, University of Pennsylvania


There are three cell types in yeast, two haploid, a and $\alpha$, and one diploid, a/$\alpha$. The two haploid cell types represent opposite mating types and can conjugate to form the diploid cell type. Cell type in yeast is determined by the allele present at the $MAT\alpha$ locus. The two genes at $MAT\alpha$, $MAT\alpha$1 and $MAT\alpha$2, encode proteins which regulate cell type specific gene expression. The bidirectional transcription of $MAT\alpha$1 and $MAT\alpha$2 is regulated solely by a 14 base pair sequence near the center of the intergenic region. This upstream activating sequence (UAS) is both necessary and sufficient to activate transcription, and corresponds to the consensus binding site for the RAP1 protein. The $MAT\alpha$ UAS binds a protein found in crude extracts from yeast, and is protected from DNaseI digestion in the presence of these cell extracts. Point mutations in the $MAT\alpha$ UAS result in altered mating behavior and a decrease in the level of transcription at $MAT\alpha$. Two of these point mutations generate a population of cells which mate as both a and a cells (bimate). A third point mutation completely abolishes $\alpha$-mating. Overexpression of the RAP1 gene suppresses these mating defects and increases transcription from mutant $MAT\alpha$ UAS elements. A temperature sensitive allele in the RAP1 gene (rap1$\sp{ts}$) causes $\alpha$-specific mating defects at the semi-permissive temperature, but no decrease in the level of $MAT\alpha$ transcription. Cells mutant at both the $MAT\alpha$ UAS and RAP1 mate strictly as a cells and do not produce $MAT\alpha$ RNAs. Mutations in a third gene, SPT13, affect mating behavior in both a and $\alpha$ cells. SPT13 mutants have a reduced level of transcription at $MAT\alpha$. Overexpression of the wild type SPT13 gene does not suppress the mating defects of $MAT\alpha$ UAS point mutants. It does partially suppress both the temperature-dependent lethality and the $\alpha$-specific mating defect of cells mutant at RAP1. Interestingly, the rap1$\sp{ts}$ mutation suppresses the sporulation defect of diploids homozygous for a point mutation in SPT13. RAP1 and SPT13 perform multiple functions in the yeast cell and may interact in several pathways including, but not limited to, the mating pathway.

Subject Area

Molecular biology|Genetics

Recommended Citation

Giesman, Donnamarie, "Control of gene expression at the MAT(alpha) locus in Saccharomyces cerevisiae" (1991). Dissertations available from ProQuest. AAI9125651.