Regulation of middle meiotic gene expression by the Sum1 repressor in Saccharomyces cerevisiae
Meiosis is a cellular process used by sexually reproducing organisms to generate gametes. Meiotic development in yeast (sporulation) is a transcriptionally driven program that consists of three temporally distinct classes of genes termed early, middle, and late. Budding yeast meiosis is an excellent model system to study cellular differentiation programs and transcriptional cascades. The transition from early to middle gene expression is a key regulatory step required for pachytene-exit, meiosis and sporulation. Middle promoters are regulated by DNA elements termed middle sporulation elements (MSEs) that are bound by the Sum1 repressor in vegetative cells and the Ndt80 activator as cells exit prophase. NDT80 is itself a middle gene activated by a positive auto-regulatory loop. Understanding how this regulatory switch occurs is the subject of my work. Previous work suggests that Sum1 regulates meiosis solely through the NDT80 promoter. A copper inducible form of Ndt80 was used to disentangle the effect of Sum1 on Ndt80. Our data suggest that Sum1 does indeed regulate meiosis through genes other than NDT80. Published literature suggests that this switch at MSEs from Sum1 to Ndt80 occupancy occurs as a result of Ndt80 competing with Sum1 for binding. In other work, we discovered an Ndt80-independent pathway for Sum1 removal. This pathway requires phosphorylation of Sum1 on residue T306 by the meiosis-specific kinase Ime2. This requirement is bypassed by deletion of Hst1, a Sir2 homologue, known to bind Sum1. Collectively, this work demonstrates that Sum1 removal from DNA occurs in a meiosis-specific pathway that does not require competition with Ndt80, and is consistent with Sum1 being a key brake that controls meiotic development. Finally, I show that Sum1 protein moves from the nucleus to the cytoplasm and back during the meiotic program. This change in sub-cellular localization is a novel means of Sum1 repressor activity regulation.