Date of Award
Doctor of Philosophy
Molecular Biology, Microbiology and Biochemistry
Covalent modifications of histones play a critical role in many important biological processes such as transcription, DNA repair and recombination. Among the major modifications known so far, histone H3 acetylation at lysines 9 and 14 (H3K9/14), monoubiquitination of histone H2B at lysine123 (H2BK123) and H3 lysine 4 methylation (H3K4) are among the more studied ones. The importances of these modifications have been further stressed by its connection to various human diseases including cancers. Previous biochemical studies have shown that H2BK123 ubiquitination is mandatory for methylation at histone H3K4. However, little is known about the regulatory mechanisms of H3K4 methylation by H2B ubiquitination in vivo. Thus, the prime focus of this study is to understand the factors involved in the regulation of H2B ubiquitination, the regulatory mechanisms of the cross-talk between H2BK123 ubiquitination and H3K4 methylation and the role of these covalent modifications in transcriptional regulation under physiological conditions. Here in this study, I have shown that Ubp8p, a histone deubiquitinase, is a bona fide subunit of SAGA (Spt3-Ada-Gcn5 acetyltransferase) co-activator complex and selectively regulates both di and trimethylation of histone H3K4 at the core promoter of a SAGA-dependent gene in vivo. However, over the open reading frames for a subset of constitutive genes H2B ubiquitination selectively upregulates only H3K4 trimethylation but not dimethylation. Moreover, such an upregulation of H3K4 trimethylation has no impact on the RNA Polymerase II (RNAPII) recruitment and hence transcription of the respective genes. Interestingly, at an inducible gene, histone H2B ubiquitination promotes transcription elongation independently of H3K4 methylation. Furthermore, this study also demonstrates for the first time, the molecular mechanism for the cross-talk between H2B ubiquitination and H3K4 methylation in vivo. Evidently a COMPASS subunit, Cps35p, is necessary for the trans-tail cross talk between histones H2B and H3. Finally, this study also shows that Sgf73p, a SAGA subunit, is required for SAGA recruitment at the promoters of several SAGA dependent genes and facilitates transcription in both HAT-dependent and HAT-independent manner. Collectively, the results from this study not only provide deep insights into the regulatory mechanisms of H2B ubiquitination and H3K4 methylation (and their role in transcription) but also give a new functional dimension to SAGA subunit, Sgf73p, under physiological conditions. Given the role of histone acetylation, ubiquitination and methylation in many human diseases, the results from this study is of tremendous clinical value unveiling new therapeutical targets.
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