Worden, E. J.; Hoffmann, N.; Hicks, C.; Wolberger, C.

Methylation of histone H3, lysine 79 (H3K79), by Dot1L is a hallmark of actively transcribed genes that depends on monoubiquitination of H2B at lysine 120 (H2B-Ub), and is a well-characterized example of histone modification cross-talk that is conserved from yeast to humans. The mechanism by which H2B-Ub stimulates Dot1L to methylate the relatively inaccessible histone core H3K79 residue is unknown. The 3.0 [A] resolution cryo-EM structure of Dot1L bound to ubiquitinated nucleosome reveals that Dot1L contains binding sites for both ubiquitin and the histone H4 tail, which establish two regions of contact that stabilize a catalytically competent state and positions the Dot1L active site over H3K79. We unexpectedly find that contacts mediated by both Dot1L and the H4 tail induce a conformational change in the globular core of histone H3 that reorients K79 from an inaccessible position, thus enabling this side chain to project deep into the active site in a position primed for catalysis. Our study provides a comprehensive mechanism of cross-talk between histone ubiquitination and methylation and reveals an unexpected structural plasticity in histones that makes it possible for histone-modifying enzymes to access residues within the nucleosome core.