Eukaryote DNA is organised into the more compact nucleosome by wrapping 147bp of DNA around a histone octamer core. The N-terminal tails of the histones protrude through the DNA and can be modified by a variety of enzymes. Acetylation of Histone 4 Lysine 16 (H4K16ac) is an important modification associated with an increase in transcription, and in flies is an important component of the doseage compensation system. It is also unique amongst histone modifications in that it has been directly associated with chromatin decompaction. H4K16ac has been linked to development through its Histone Acetyltransferase, MOF. Deletion of MOF in mice leads to mass chromatin defects, and embryonic lethality prior to the blastocyst stage. I set out to understand the role of H4K16ac in differentiating Embryonic Stem cells (ES cells) and chromatin compaction in vivo. I generated a ChIP-seq profile for H4K16ac in undifferentiated ES cells, and after 3 days of retinoic acid (RA) differentiation. This revealed an association of H4K16ac with the promoters of transcribed genes in pluripotent ES cells, followed by loss H4K16ac on ES cell specific genes and gain of the modification on differentiation specific genes. There were some silent genes in ES cells, however, which were acetylated on their promoters. Through this study I also found that H4K16ac and MOF mark active enhancers in ES cells, along with H3K4me1 and H3K27Ac and p300. H4K16ac did not mark a known regulatory region in limb cells, and it is possible that it marks active enhancers only of ES cells. Furthermore, I looked at the compaction state large regions (>100kb) which lost H4K16ac upon differentiation by FISH, to determine if loss of H4K16ac could predict compaction. The regions selected showed no change in compaction state between UD and D3 cells, meaning that loss of H4K16ac does not directly lead to chromatin compaction in vivo. However loss of H4K16ac may be necessary for any subsequent compaction, or the change in compaction may take place at nucleosomal level. Finally, I attempted both to overexpress and reduce the level of MOF in ES cells. I was unable to manipulate the level of MOF in this cell type in either direction; expression of endogenous MOF was silenced after very little time, and stable MOF shRNA cell lines showed no reduction in levels of MOF. Therefore, potentially, dosage of MOF/H4K16ac in this cell type is critical. This study may help to understand the significance of H4K16ac in ES cell differentiation and chromatin compaction.

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