Histone H2B ubiquitylation controls processive methylation but not monomethylation by Dot1 and Set1

Methylation is a relatively stable histone modification, yet regulation of the transition between mono-, di-, and trimethylation of lysine (K) residues may control dynamic processes such as transcription and DNA repair. Identifying factors that regulate the ability of methyltransferases to perform successive rounds of methylation on the same lysine residue is important for understanding the functions of histone methylation. Previous reports have indicated that ubiquitylation of histone H2B K123 is required for methylation of lysines 4 and 79 of histone H3 by the methyltransferases Set1 and Dot1, respectively. In contrast, by using chromatin immunoprecipitation and mass spectrometry, we find that ubiquitylation of H2B-K123 is dispensable for monomethylation of H3-K4 and H3-K79 but is required for the transition from monomethylation to subsequent methylation states. Dot1 binding to chromatin occurs normally in the absence of H2B-K123 ubiquitylation, suggesting that ubiquitylation does not regulate enzyme recruitment but does regulate the processive activity of the histone methyltransferase.