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Chromatin-targeting domains and the divergence of histone H3 McBurney, Kristina Lillian


NuA3 is one of the major histone H3 HATs in yeast, as its catalytic subunit, Sas3, is responsible for acetylation of K14 and 23. The only characterized chromatin-targeting domain within the HAT is the PHD finger of Yng1, which associates with H3K4me3 and directs NuA3 to the 5’ ends of genes. We examined the genome-wide localization of Sas3, and found that it strongly correlates with H3K36me3. We demonstrated that recruitment of NuA3 to chromatin is dependent on methylation of both H3K4 and K36, and have implicated a novel member of the NuA3 complex – Pdp3 – as being responsible for this interaction. This likely occurs through its PWWP domain, which is a known H3K36me3-interactor in other proteins. In combination with the PHD finger of Yng1, this provides a mechanism by which NuA3 is recruited across the entirety of transcribed genes. In addition to its PHD finger and PWWP domain, NuA3 also contains the YEATS domain of Taf14. This is a conserved eukaryotic domain of unknown function present exclusively in transcription-related complexes. Although evidence exists suggesting that YEATS domains in other proteins interact directly with histones, its role in the NuA3 complex has remained elusive. We confirmed that the YEATS domain functions in chromatin-targeting of NuA3, and that it interacts directly with H3K9, 18, and 27 acetylated peptides. Finally we showed that NuA3 recruitment is dependent on Gcn5. This work describes a novel mechanism by which acetylation by one HAT targets further acetylation by another, and provides an additional mechanism for recruitment of NuA3. Finally, we explored the functional divergence of residues within histone H3 in yeast and humans. We showed that, while amino acids that define histone H3.3 are dispensable for yeast growth, substitution of residues within the histone H3 α3 helix with their human counterparts resulted in a severe growth defect. Furthermore, these mutations resulted in altered nucleosome positioning, both in vivo and in vitro, which was accompanied by an increased preference for nucleosome positioning sequences. Taken together, this suggests that divergent residues within the histone H3 α3 helix play differing roles in chromatin regulation between yeast and metazoans.

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