Characterizing the Role of SUMO in Transcriptional Repression in S. cerevisiae
Small ubiquitin-like modifier, SUMO, is a member of the ubiquitin-like protein family. Similar to ubiquitin, SUMO is conjugated to target proteins as a post-translational modification via an E1, E2, and E3 enzymatic cascade, in a process known as SUMOylation. SUMOylation leads to altered target protein localization, half-life, or protein-protein interactions. Bursts of SUMOylation following stresses (SUMO Stress Response / SSR) have been documented, however what triggers the SSR, the kinetics of the response, and the cellular components necessary to mediate the response have not been well characterized. Additionally, SUMO has been widely implicated in transcriptional regulation and chromatin organization, although the nature of its role remains unclear. This thesis aims to: 1) complete a comprehensive characterization of the SSR, and 2) elucidate the true role of SUMO in transcriptional regulation and chromatin organization. To this end, I characterized the SSR following a variety of stresses and time courses. I then used mutants and inhibitors to demonstrate that the SSR is triggered by coordinated transcriptional changes (such as in response to a stress). RNA-Seq of a SUMO system mutant identified 162 transcripts which show a SUMO-dependent transcriptional de-repression following SUMO system inactivation. These transcripts are primarily inducible genes which are normally induced under specific conditions. Notably, these mutants were still able to activate and inactivate transcription in response to a stress, demonstrating that SUMO is not required for acute transcriptional activation or inactivation. Subsequently I investigated possible mechanisms for SUMO-dependent transcriptional repression. Firstly, I used pulldowns and site directed mutagenesis to demonstrate that Rlf2, a component of the histone chaperone CAF-1, is able to bind SUMO in S. cerevisiae. This raises the possibility that CAF-1 plays a role in SUMO-mediated transcriptional repression. Secondly, I used ATAC-Seq to demonstrate that SUMO system inhibition may increase chromatin openness, which could lead to the observed transcriptional de-repression. Together, these data indicate that SUMO may be involved in maintaining chromatin in a closed state; however, further research is needed to elucidate the mechanism. These experiments assist in understanding the role of SUMOylation and the consequences of using SUMO as a therapeutic target.
chromatin, RNA-seq, stress response, SUMO, transcription
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