THE MATING-TYPE PROTEINS OF FISSION YEAST INDUCE MEIOSIS BY DIRECTLY ACTIVATING MEI3 TRANSCRIPTION

Cell type control of meiotic gene regulation in the budding yeast Sacc haromyces cerevisiae is mediated by a cascade of transcriptional repre ssors, alpha 1-alpha 2 and Rme1. Here, we investigate the analogous re gulatory pathway in the fission yeast Schizosaccharomyces pombe by ana lyzing the promoter of mei3, the single gene whose expression is suffi cient 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 upstre am of the TATA sequence. These elements synergistically activate mei3 transcription, thereby constituting an on-off switch for the meiosis p athway. 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 facto r. Altered-specificity experiments indicate that element D interacts w ith P-m, a homeodomain protein encoded by the mating-type locus. This indicates that P-m functions as a direct activator of the meiosis path way, whereas the homologous mating-type protein in S. cerevisiae (alph a 2) functions as a repressor. Thus, despite the strong similarities b etween the mating-type loci of S. cerevisiae and S, pombe, the regulat ory logic that governs the tight control of the key meiosis-inducing g enes in these organisms is completely different.