|M.Sc Student||Edry Iris|
|Subject||The Choice between Transcriptional Repression and|
Activation of Early Meiosis-Specific Genes in
Budding Yeast Saccharomyces cerevisiae
|Department||Department of Biology||Supervisor||Professor Emeritus Yona Kassir|
Saccharomyces cerevisiae diploid cells exit the mitotic cell cycle and enter meiosis upon nitrogen depletion in the absence of glucose. During mitotic growth, the early meiosis-specific genes (EMG) are silent due to the recruitment of the histone deacetylase, Sin3/Rpd3 repression complex, to their promoters by Ume6. Under meiotic conditions these genes are expressed due to the recruitment by Ume6 of the transcriptional activator, Ime1. Ume6/Sin3/Rpd3 complex is formed under meiotic conditions, nevertheless, it does not repress transcription of EMG. The goal of this study was to determine how in the presence of the repression complex EMG are transcribed. We show that artificial recruitment of Sin3 or Rpd3 to a heterologous reporter gene causes repression, but only in the presence of glucose. Relief of repression of Rpd3 depends on both Ime1 and Ume6, suggesting that Ume6 might tether Ime1 to Rpd3. In the presence of glucose Ime1 is not expressed, and its ectopic expression does not suffices for association with Ume6, explaining why relief of repression depends on the absence of glucose. Similarly, relief of repression of Sin3 is also dependent on Ime1, in agreement with the observation that the repression ability of Sin3 depends on Rpd3. On the other hand, Ume6 seems to have a distinct, repressing effect on Sin3, suggesting that Ume6 might recruit an additional repression complex to Sin3. These results suggest that Ime1 might posses two functions, it is required to relieve repression of Rpd3, and to activate transcription. Furthermore, these imply that induction of transcription of silent genes is a two step mechanism that depends on active relief of repression.