The spatial and temporal expression of specific gene sets is critical for the execution of complex differentiation programs. The long-term goal of this study is to mechanistically define transient transcription in the context of the budding yeast meiotic differentiation program. Many genes required for the meiotic landmark events are repressed during mitotic cell division but then transiently induced during development in temporal waves termed "early", "middle" and "late". Ume6p binds "early" meiotic gene promoters and mediates their vegetative repression by recruiting both histone deacetylase (HDAC) and chromatin remodeling complexes. We have recently discovered that early meiotic gene induction requires Ume6p destruction by the Cdc20p- directed anaphase promoting complex/cyclosome (APC/C) ubiquitin ligase. Although well known for its role in controlling the G2 ->M transition in mitotic cells, this is the first report that APC/CCdc20 targets a transcription factor for destruction. In addition, Ume6p destruction is restricted to cells entering meiosis, even though APC/CCdc20 is active (and Ume6p is present) during mitotic cell division. The meiotic inducer Ime1p provides a trigger to promote Ume6p destruction thus suggesting a new mechanism by which APC/C substrate selection is redirected within the context of a differentiation program. While searching for potential meiosis-specific destruction signals, we discovered that Ume6p is a substrate of the Gcn5p histone acetyltransferase (HAT) complex called SAGA. The well-studied acetylation and deacetylation of nucleosomes maintains chromatin in "open" and "closed" configurations, respectively. However, the acetylation of transcription factors has not been demonstrated previously in yeast and has only been described for a few transactivators in mammalian systems. To our knowledge, Ume6p is the first transcriptional repressor found to be acetylated. Interestingly, preliminary results point to a role for acetylation in both preventing Ume6p DNA binding ability and enhancing its degradation. These findings suggest a new model for Gcn5p-dependent transcriptional activation through direct inhibition of Ume6p repressor function. Following induction, meiotic gene expression and the execution of landmark events are coupled by a series of checkpoint systems. Preliminary results indicate that Ume6p destruction is prevented upon activation of the DNA damage checkpoint. The mechanism by which Ume6p is protected from destruction following checkpoint activation, or the role of additional checkpoint pathways, is unknown. To understand the molecular mechanisms by which Ume6p-dependent repression is relieved upon meiotic induction, and how repression is reestablished in response to checkpoint pathways, the following aims are proposed:
Aim1. Dissect the molecular mechanisms directing developmental re-tasking of the APC/C.
Aim2. Determine the role that acetylation plays in Ume6p activity and regulation.
Aim3. Identify and characterize the meiotic pathways that mediate Ume6p destruction.
During normal development, gene expression is precisely controlled to provide the correct gene product at the correct time. Failure to execute the proper gene expression program can lead to developmental defects or embryonic lethality. This proposal describes experiments to understand how these transcription programs are regulated when cells undergo development.
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