Regulation of the transcriptional repressor Sum1 by the meiosis-specific kinase Ime2 in Saccharomyces cerevisiae
Meiosis is the process through which the haploid gametes required for sexual reproduction are produced. In the budding yeast Saccharomyces cerevisiae meiosis is coupled to spore formation and requires the coordinated transcription of meiosis-specific genes that fall into three broad temporal classes: early, middle, and late genes. IME2 is an early gene that encodes a Cdk-like protein kinase which carries out critical functions in meiosis, being involved in the regulation of S phase, as well as early and middle gene transcription. The middle genes are expressed after prophase and are required for the meiotic divisions and initiation of the spore morphogenesis pathway. The MSE (middle sporulation element) is found in the promoter of most middle genes and is bound by either the transcriptional repressor Sum1, which is active during mitosis and early meiosis, or the transcriptional activator Ndt80. Genetic evidence shows a link between Ime2 and Sum1 that indicates Ime2 may be responsible for alleviating Sum1-dependent repression. Here we demonstrate that Ime2 directly phosphorylates Sum1 on T306 both in vitro and in vivo. A Sum1 phosphomimic mutant strain progresses through meiosis with altered kinetics and forms a high number of spores containing only two asci, indicating that Ime2 phosphorylation of Sum1 upregulates middle gene transcription. Previous work has shown that Sum1 is stabilized and middle genes are repressed by the recombination checkpoint, which protects genomic integrity by preventing chromosome missegregations. We show that extensive phosphomodifications that are added to the Ime2 protein after S phase are prevented in a genetic model of checkpoint arrest, suggesting that Ime2 itself is downregulated by the recombination checkpoint. The phosphorylation of Sum1-T306 is not downregulated by the checkpoint, but instead is stabilized, suggesting that Ime2 regulation is not the mechanism used by the checkpoint to maintain Sum1 repression of middle genes. We also describe the Ime2 consensus sequence, RPXT/S, which we demonstrate can be used to identify previously unknown Ime2 substrates.