The yeast C-type cyclin Ume3p/Srb11p and its cyclin-dependent kinase partner Ume5p/Srb10p repress the transcription of several genes required for meiotic recombination or meiosis I nuclear division. To relieve this repression, Srbllp is destroyed early in meiosis, prior to the first meiotic division. This report identifies two roles for Srb11p in regulating meiotic development. First, SRB11 is required for the normal exit from the mitotic cell cycle prior to meiotic induction. Specifically, mutants lacking SRB11 (srb11delta) uncouple bud growth from chromosome segregation, producing small buds with nuclei. The bud growth defect is most likely due to the failure of srb11delta mutants to reestablish polarized actin fibers at the bud tip following exposure to sporulation medium. Second, Srb11p is required for the efficient execution of meiosis I. srb11delta mutants either exhibited a delay in performing meiosis I and meiosis II or skipped meiosis I entirely. This meiotic defect is not due to the activation of the recombination or spindle assembly checkpoint pathways. However, the expression of several meiotic genes is delayed and reduced in the mutant strains. These results suggest a positive role for Srb10-Srb11p in regulating the transcription program. This model is supported by the finding that overexpression of the meiotic inducer IME2 partially restored the ability of srb11 mutants to perform meiosis I. In conclusion, these findings indicate that Srb11p is required for both entry into and execution of the meiotic program, thus describing multiple roles for a C-type cyclin in the regulation of a developmental pathway.
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