| Diploid cells of yeast Saccharomyces cerevisae undergo meiosis and sporulation in response to starvation for glucose and nitrogen. This process is accompanied by the sequential expression of early, middle and late meiosis-specific genes. Ume6p, a DNA binding protein controls expression of early meiotic genes (EMG) both negatively and positively. It represses EMG expression when nutrients are present through the association with the Sin3p/Rpd3p histone deacetylase complex. It activates EMG expression upon starvation through the function of Ime1p, a meiosis-specific activator. It is not clear how Ume6p is switched from a repressor to an activator and how Ume6p activates EMG expression.;In my thesis, I showed that the Ume6p activation domain (the N-terminal region of Ume6p) interacts with Ime1p and Rim11p, a GSK3 homolog in yeast and a positive regulator for EMG expression. Both interactions are induced under starvation condition and Ime1p/Ume6p interaction depends on Rim11p while Rim11p/Ume6p interaction does not require Ime1p. I further proved that Ume6p is phosphorylated in vivo in response to starvation signals. Ume6p mutant derivatives that fail to interact with Ime1p or Rim11p and promote EMG expression also fail to be phosphorylated in vivo. Rim11p is not the only kinase that phosphorylates Ume6p. Mck1p, another GSK3 homolog in yeast and also a positive regulator of EMG expression also phosphorylates Ume6p in vivo and probably at the same site as Rim11p. Rim15p, a kinase involved in RAS2-cAMP pathway is also required for Ume6p in vivo phosphorylation in meiosis. Besides EMG, Ume6p also controls expression of some non-meiosis-specific genes, e.g. INO1, a gene involved in phospholipid biosynthesis. I showed Ume6p, Rim11p, Mck1p and Rim15p all affect INO1 expression but phosphorylation at N-terminal region is not required for this function of Ume6p.;My work also indicated the weak sporulation in ume6Delta strain depends on functions of Ime1p and Rim11p. I also carried out a genetic screen for mutants that can activate EMG expression in the presence of nutrients. I was able to isolate two such mutants and some interesting features of these mutants were analyzed. |
