Putting the pieces together: histone H2B ubiquitylation directly stimulates histone H3K79 methylation

Bat3 facilitates H3K79 dimethylation by DOT1L and promotes DNA damage-induced 53BP1 foci at G1/G2 cell-cycle phases

The methyltransferase DOT1L methylates histone H3 at K79 to facilitate specific biological events. H3K79 dimethylation (H3K79-2Me) by DOT1L influences the DNA damage response by promoting 53BP1 recruitment to DNA damage sites; however, it is unclear if this methylation is required as 53BP1 interacts with dimethylated H4 (H4K20-2Me) with a much higher affinity. We demonstrate that H3K79-2Me, while negligible during S-phase, is required for ionizing radiation (IR)-induced 53BP1 foci formation during G1/G2-phases when H4K20-2Me levels are low. Further, we describe an essential role for HLA-B-associated transcript 3 (Bat3) in regulating this process in U2OS cells. Bat3 co-localizes with DOT1L at histone H3, and Bat3 knockdown results in decreased DOT1L-H3 interaction and H3K79-2Me, leading to a reduction in IR-induced 53BP1 foci formation, defects in DNA repair and increased sensitivity to IR. We demonstrate that a conserved Bat3 ubiquitin-like motif and a conserved DOT1L ubiquitin-interacting motif promote DOT1L-Bat3 interaction to facilitate efficient H3K79-2Me and IR-induced 53BP1 foci formation during G1/G2-phases. Taken together, our findings identify a novel role for Bat3 in regulating DOT1L function, which plays a critical role in DNA damage response.

Novel trans-tail regulation of H2B ubiquitylation and H3K4 methylation by the N terminus of histone H2A

Chromatin is regulated by cross talk among different histone modifications, which can occur between residues within the same tail or different tails in the nucleosome. The latter is referred to as trans-tail regulation, and the best-characterized example of this is the dependence of H3 methylation on H2B ubiquitylation. Here we describe a novel form of trans-tail regulation of histone modifications involving the N-terminal tail of histone H2A. Mutating or deleting residues in the N-terminal tail of H2A reduces H2B ubiquitylation and H3K4 methylation but does not affect the recruitment of the modifying enzymes, Rad6/Bre1 and COMPASS, to genes. The H2A tail is required for the incorporation of Cps35 into COMPASS, and increasing the level of ubiquitylated H2B in H2A tail mutants suppresses the H3K4 methylation defect, suggesting that the H2A tail regulates H2B-H3 cross talk. We mapped the region primarily responsible for this regulation to the H2A repression domain, HAR. The HAR and K123 of H2B are in close proximity to each other on the nucleosome, suggesting that they form a docking site for the ubiquitylation machinery. Interestingly, the HAR is partially occluded by nucleosomal DNA, suggesting that the function of the H2A cross talk pathway is to restrict histone modifications to nucleosomes altered by transcription.