In the budding yeast Saccharomyces cerevisiae, the preservation of the mating competent haploid (a or alpha) and the mating incompetent diploid (a/alpha) is necessary to prevent aneuploidy. Once haploid cells respond to pheromone, the mating-specific signal transduction pathway is activated, and the MAP kinase Fus3 phosphorylates two specific repressor proteins Rst1 and Rst2 (also known as Dig1 and Dig2) to promote Ste12-dependent transcription of mating-specific genes. In contrast, diploid cells cannot mate because genes that encode components of the mating pathway are repressed through the combined action of the Mata1-Matalpha2 and Matalpha2-Mcm1 repressors. Surprisingly, repression of Ste12 by Rst1 and Rst2 is essential for diploid sterility. Homozygous deletion of both RST1 and RST2 (rst-) causes a/alpha diploid cells constitutively to express a-specific genes and mate preferentially as a-cells. This phenotype is sensitive to Ste12 dosage, as removal of one copy of STE12 completely reduces the ectopic activation of a-specific genes. The Matalpha2-Mcm1 complex, which normally represses a-specific genes, is defective in rst- diploids because Matalpha2 is destabilized in rst- diploids, possibly as a consequence of its relocalization from the nucleus to the cytoplasm. This study finds that Rst1 and Rst2 are necessary for the a/alpha diploid cell type. Rst1 and Rst2 are required in order to prevent the amplification of a robust Ste12 transcriptional programme that appears to over-ride Matalpha2-dependent repression of haploid and a-specific genes.
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