Abstract
Ubiquitylation of H2B on lysine 120 (H2Bub) is associated with active transcriptional elongation. H2Bub has been implicated in histone cross talk and is generally regarded to be a prerequisite for trimethylation of histone 3 lysine 4 (H3K4me3) and H3K79 in both yeast and mammalian cells. We performed a genome-wide analysis of epigenetic marks during muscle differentiation, and strikingly, we observed a near-complete loss of H2Bub in the differentiated state. We examined the basis for global loss of this mark and found that the H2B ubiquitin E3 ligase, RNF20, was depleted from chromatin in differentiated myotubes, indicating that recruitment of this protein to genes substantially decreases upon differentiation. Remarkably, during the course of myogenic differentiation, we observed retention and acquisition of H3K4 trimethylation on a large number of genes in the absence of detectable H2Bub. The Set1 H3K4 trimethylase complex was efficiently recruited to a subset of genes in myotubes in the absence of detectable H2Bub, accounting in part for H3K4 trimethylation in myotubes. Our studies suggest that H3K4me3 deposition in the absence of detectable H2Bub in myotubes is mediated via Set1 and, perhaps, MLL complexes, whose recruitment does not require H2Bub. Thus, muscle cells represent a novel setting in which to explore mechanisms that regulate histone cross talk.