Leann Howe

The enrichment of histone acetylation within transcribed chromatin was first observed in the 1960s, and how specific histones are acetylated has been a central question of chromatin biology ever since. One mechanism for specificity is through the targeted recruitment of histone acetyltransferases (HATs) to transcribed chromatin, and we first focused on recruitment of the NuA3 HAT complex in S. cerevisiae. NuA3 is known to bind to cotranscriptional histone methylation through two domains: the PHD finger in Yng1 and the PWWP domain in Pdp3, which in vitro bind to H3K4 and H3K36 methylation, respectively. While the in vitro binding has been well characterized, the relative in vivo contributions of these histone methylation marks in targeting NuA3 is unknown. Here, through genome-wide colocalization and mutational interrogation, we demonstrate that the PHD finger of Yng1 and the PWWP domain of Pdp3 independently target NuA3 to H3K4 and H3K36 methylated chromatin, respectively. Interestingly however, the simple presence of NuA3 is insufficient to ensure the acetylation of associated nucleosomes, suggesting a secondary level of regulation that does not involve control of HAT-nucleosome interactions.Next we studied targeting of histone acetylation itself, focusing on the causality of the relationship between histone acetylation and RNAPII transcription. Through genome-wide analysis of mammalian cell culture and budding yeast, we reveal that the preponderance of histone acetylation is tightly linked with RNAPII occupancy, and, in S. cerevisiae, chemically or genetically altering RNAPII localization results in a corresponding change in histone acetylation. These findings show that histone acetylation is primarily targeted through RNAPII as a consequence of transcription. Importantly, several lines of evidence suggest that RNAPII does not promote acetylation by simple HAT targeting. First, we show that HAT occupancy is a poor predictor of histone acetylation. Second, NuA4 recruitment to upstream activation sequences of either Taf1 (TFIID) enriched or depleted promoters does not result in acetylation in the absence of transcription. Collectively, these data suggest that the activity of HATs is regulated post-recruitment by a mechanism that is dependent on RNAPII.

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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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DNA in the eukaryotic cell is packaged into a structure called chromatin. Chromatin is a dynamic structure that regulates access to DNA in response to environmental stimuli. Two widely conserved mechanisms that influence chromatin structure are the addition of post-translational modifications (PTMs) to histones and other chromatin-associated proteins, and the replacement of canonical histones with histone variants.Histone acetylation is catalyzed by histone acetyltransferases (HATs). HATs are comprised of a catalytic subunit, and associated proteins. Genetic analysis of the yeast HATs has shown that the combined deletion of the two HAT genes, GCN5 and SAS3, results in an inviable strain of yeast. In this thesis, I show that the inviability of the gcn5Δsas3Δ mutant is due to a combined failure to acetylate both histone H3 and the chromatin-remodeler protein Rsc4. Further, I show that acetylation of Rsc4 is catalyzed by Gcn5 in a HAT complex-independent manner.The linker histone, H1, is associated with higher-order chromatin structure; it has been shown that removal of H1 is required to allow access to DNA. In this work, I show that deletion of the linker histone rescues the growth of a conditional gcn5Δ sas3Δ mutant expressing a temperature-sensitive version of Sas3. Further, I present the incorporation of the histone variant Htz1 as an additional mechanism for mobilizing the linker histone away from the +1 nucleosome. I, also, provide data that corroborates evidence suggesting that the yeast linker histone binds a single nucleosome.Another histone variant found in many eukaryotes is histone H3.3, which is primarily incorporated into transcriptionally active regions in chromatin. In this dissertation, we created a series of human-yeast histone hybrids and tested their ability to rescue yeast lacking both endogenous copies of histone H3. Our data shows that the two human histone H3 variants, H3.1 and H3.3, are functionally interchangeable for growth in most nutrient conditions, confirming that the four amino acids that are different between H3.1 and H3.3 are not necessary to create transcriptionally permissive chromatin. Finally, we present evidence that three yeast H3 C-terminal domain amino acids play an important role in regulating the interactions of yeast H3.

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The presence of nucleosomes over vast regions ofgenome negatively influencestranscription creating a need for temporal and structural regulation of chromatin. Thedefaulttranscription-repressive state can be countered by addition of post-translationalmodifications by chromatin modifiersor chromatin alterationbyhistone chaperones.Chaperones alterchromatin structurebeforetheRNAPIIpassageand restore itafterwards. Howmodifiers and chaperones function within chromatin is an area ofintense research.Herewe show how two complexes, the yeastFACTand NuA3contribute tochromatinfunction.Yeast Facilitates Chromatin Transactions (yFACT) is a histone chaperone thatmaintains chromatin structure.The model of yFACT functionin vivo is asubject of muchdebate. Weprovide evidencethat yFACT acts by stably binding and alteringnucleosomes. We alsopresenttheEM structure of yFACT associated withnucleosomes.We find that yFACT-associated nucleosomes are hyper-acetylated and show evidencefor it being an effect of a direct interaction between yFACT and NuA3.At the same time, acetylation ofthe H3K56 residue bythe histone acetyltransferaseRtt109, acts to recruit yFACT to chromatin through a nucleosome-dependent mechanism.To determinethe distribution of yFACT-associated nucleosomesweconstructeda map of yFACT-nucleosomelocalization atsingle-nucleosomeresolution. We showthat while yFACT-bound nucleosomes are distributed thought thegenomethey are alsopositionedoverthecanonical Nucleosome Depleted Regions (NDR).The yFACT-boundnucleosomesare positionedaround TATA-elements andNhp6-target sequencesgenome-wide.Deletionof NHP6A/Bleads toloss ofchromatinat these loci. Our worksuggeststhe first ever sequence-dependent mechanism of histone chaperoneactioninSaccharomyces cerevisiae.We also examined NuA3 recruitment to chromatin andshowedthat Yng1, asubunit of NuA3 with a known affinity for H3K4me3 is a bivalent protein.Whileaspreviously shown,the C-terminal PHD finer of Yng1 binds to H3K4me3, the N-terminusof Yng1 canalsobind to unmodified chromatin.Although these motifscan bindindependently, together they increase the apparent association of Yng1 withchromatin. Yng1 binding to chromatin is regulated by the HDA1 complex.

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No abstract available.