The Mating-Type Proteins of Fission Yeast Induce Meiosis by Directly Activating mei3 Transcription

Abstract

ABSTRACT Cell type control of meiotic gene regulation in the budding yeast Saccharomyces cerevisiae is mediated by a cascade of transcriptional repressors, a1-α2 and Rme1. Here, we investigate the analogous regulatory pathway in the fission yeast Schizosaccharomyces pombe by analyzing the promoter of mei3 , the single gene whose expression is sufficient to trigger meiosis. The mei3 promoter does not appear to contain a negative regulatory element that represses transcription in haploid cells. Instead, correct regulation of mei3 transcription depends on a complex promoter that contains at least five positive elements upstream of the TATA sequence. These elements synergistically activate mei3 transcription, thereby constituting an on-off switch for the meiosis pathway. Element C is a large region containing multiple sequences that resemble binding sites for M c , an HMG domain protein encoded by the mating-type locus. The function of element C is extremely sensitive to spacing changes but not to linker-scanning mutations, suggesting the possibility that M c functions as an architectural transcription factor. Altered-specificity experiments indicate that element D interacts with P m , a homeodomain protein encoded by the mating-type locus. This indicates that P m functions as a direct activator of the meiosis pathway, whereas the homologous mating-type protein in S. cerevisiae (α2) functions as a repressor. Thus, despite the strong similarities between the mating-type loci of S. cerevisiae and S. pombe , the regulatory logic that governs the tight control of the key meiosis-inducing genes in these organisms is completely different.