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UBC Theses and Dissertations

Investigating histone methylation in yeast : regulation of H3K4me3, and the role of the methyl-histone binding domains of Isw1b Johnson, Ian


Chromatin structure is regulated in part by the post-translational modification of histones. Histone methylation is highly conserved amongst eukaryotes, and is arguably one of the best characterized indicators of whether a gene is repressed or active. There are several unique states of histone methylation, each capable of specific downstream effects through the recruitment of highly specific methyl-histone binding domains and their associated chromatin-altering protein complexes. Histone H3 lysine 4 tri-methylation (H3K4me3) is a well known mark of actively transcribed genes, and co-localizes with histone H3 lysine 14 acetylation (H3K14ac), another mark of actively transcribed genes. The discovery that H3K4me3 is lost when H3K14 is substituted with another residue, led to the possibility of cross-talk between H3K4me3 and H3K14ac. The first part of this thesis demonstrates that H3K4me3 is indeed dependent on H3K14ac. Furthermore, we go on to show for the first time, that H3K14ac protects H3K4me3 from demethylation by the histone demethylase Jhd2. Though the mechanisms by which methyl-histone binding domains recognize methylated chromatin have been well studied, the specific physiological roles of the numerous methyl-histone binding domains have yet to be investigated. Isw1 is a highly conserved catalytic subunit of several ATP-dependent chromatin-modifying complexes. One of these complexes, Isw1b, has two putative methyl-histone binding domains, the PHD finger of Ioc2, and the PWWP domain of Ioc4. The second part of this thesis investigates the role that these domains play in the localization of the Isw1b complex to a specific region of the genome. Though we were unable to demonstrate a role for the PHD finger of Ioc2, we did demonstrate that the PWWP domain of Ioc4 is involved in chromatin localization. Additionally we found that Ioc2’s ability to bind chromatin is negatively affected by association with Ioc4 in the Isw1b complex, though the significance of this finding has yet to be determined.

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