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UBC Theses and Dissertations

Multiple mechanisms regulate the activity of Gal 4 Rohde, John Roy


The GAL regulon of Saccharomyces cerevisiae provides a model for the study of eukaryotic gene regulation. Expression of the GAL genes is dependent on the transcriptional activator Gal4. Normally, Gal4 is inhibited by the negative regulator Gal80. Galactose binds to the inducer protein Gal3 which then becomes capable of interacting with Gal80. This causes a conformational change in the Gal4-Gal80 complex that allows Gal4 to activate transcription. Induction of the GAL genes is also associated with phosphorylation of the activator Gal4. Data presented in this thesis demonstrate that Gal4 phosphorylation is required for sensitive response to the inducer galactose. Phosphorylation of Gal4 is dispensible for induction in cells which possess fully functional alleles of GAL3; however, phosphorylation of Gal4 at S699 is absolutely required for the process of "long term adaptation" whereby yeast are able to induce the GAL genes in the absence of Gal3. S699 phosphorylation on Gal4 occurs independently of the Gal3-galactose signaling mechanism. The phosphorylation of Gal4 at S699 is mediated by the RNA polymerase II holoenzyme component SrblO and represents a genetically non-epistatic mechanism of induction to that mediated by Gal3-galactose. In contrast, SrblO and Gal4 phosphorylation at S699 are genetically epistatic with each other. These data suggest that induction of the GAL genes is controlled by two separate signaling mechanisms: one which is galactose specific and acts through Gal3, and a second signal which controls the activity of the transcriptional activator Gal4 through phosphorylation. The major implication of these observations is that multiple signaling pathways converge on a single transcriptional activator through holoenzyme-associated Cdks to regulate its activity, thereby allowing the appropriate level of transcription to match the cells environment and physiological status.

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