Phosphorylation of histone H3.3 at serine 31 promotes p300 activity and enhancer acetylation.

The histone variant H3.3 is enriched at enhancers and active genes, as well as repeat regions such as telomeres and retroelements, in mouse embryonic stem cells (mESCs) ^1-3 . Although recent studies demonstrate a role for H3.3 and its chaperones in establishing heterochromatin at repeat regions ^4-8 , the function of H3.3 in transcription regulation has been less clear ^9-16 . Here, we find that H3.3-specific phosphorylation ^17-19 stimulates activity of the acetyltransferase p300 in trans, suggesting that H3.3 acts as a nucleosomal cofactor for p300. Depletion of H3.3 from mESCs reduces acetylation on histone H3 at lysine 27 (H3K27ac) at enhancers. Compared with wild-type cells, those lacking H3.3 demonstrate reduced capacity to acetylate enhancers that are activated upon differentiation, along with reduced ability to reprogram cell fate. Our study demonstrates that a single amino acid in a histone variant can integrate signaling information and impact genome regulation globally, which may help to better understand how mutations in these proteins contribute to human cancers ^20,21 .