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 rap1ts strains provides evidence that RAP1 functions in both transcriptional activation and silencing of mating-type genes. Several observations indicate that rap1ts strains are defective in the expression of MAT-alpha, whose upstream activation sequence (UAS) contains a RAP1-binding site. At nonpermissive temperatures, decreases in MAT-alpha steady-state transcript levels can be detected in MAT-alpha rap1ts strains. Furthermore, these strains are deficient in alpha-pheromone production and simultaneously express at least two a-specific genes. These phenotypes can be reversed by replacing the RAP1-binding site at MAT-alpha with a binding site for the GAL4 transcriptional activator. Certain rap1ts alleles have an opposite effect on the silent mating-type locus HMR, which becomes partially derepressed at nonpermissive temperatures.