RAP1 protein activates and silences transcription of mating-type genes in yeast

RAP1 is a sequence-specific DNA-binding protein essential for cell growth. The occurrence of RAP1-binding sites in many promoter regions, the mating-type gene silencer elements, and telomeres suggests that RAP1 has multiple functions in the cell. To assess its role in transcription, temperature-sensitive mutations in RAP1 were generated. Analysis of rap1(ts) strains provides evidence that RAP1 functions in both transcriptional activation and silencing of mating-type genes. Several observations indicate that rap1(ts) strains are defective in the expression of MATα, whose upstream activation sequence (UAS) contains a RAP1-binding site. At nonpermissive temperatures, decreases in MATα steady-state transcript levels can be detected in MATα rap1(ts) strains. Furthermore, these strains are deficient in α-pheromone production and simultaneously express at least two a-specific genes. These phenotypes can be reversed by replacing the RAP1-binding site at MATα with a binding site for the GAL4 transcriptional activator. Certain rap1(ts) alleles have an opposite effect on the silent mating-type locus HMR, which becomes partially derepressed at nonpermissive temperatures.