Histone H3 lysine 4 mono-methylation does not require ubiquitination of histone H2B

The yeast Set1-complex catalyzes histone H3 lysine 4 (H3K4) methylation. Using N-terminal Edman sequencing, we determined that 50% of H3K4 is methylated and consists of roughly equal amounts of mono, di and tri-methylated H3K4. We further show that loss of either Paf1 of the Paf1 elongation complex, or ubiquitination of histone H2B, has only a modest effect on bulk histone mono-methylation at H3K4. Despite the fact that Set1 recruitment decreases in paf1delta cells, loss of Paf1 results in an increase of H3K4 mono-methylation at the 5' coding region of active genes, suggesting a Paf1-independent targeting of Set1. In contrast to Paf1 inactivation, deleting RTF1 affects H3K4 mono-methylation at the 3' coding region of active genes and results in a decrease of global H3K4 mono-methylation. Our results indicate that the requirements for mono-methylation are distinct from those for H3K4 di and tri-methylation, and point to differences among members of the Paf1 complex in the regulation of H3K4 methylation.     

Ubiquitination of histone H2B by Rad6 is required for efficient Dot1-mediated methylation of histone H3 lysine 79

Dot1 is a non-SET domain protein that methylates histone H3 at lysine 79, a surface-exposed residue that lies within the globular domain. In the context of a nucleosome, H3 lysine 79 is located in close proximity with lysine 123 of histone H2B, a major site for ubiquitination by Rad6. Here we show that Rad6-mediated ubiquitination of H2B lysine 123 is important for efficient methylation of lysine 79, but not lysine 36, of histone H3. In contrast, lysine 79 methylation of H3 is not required for ubiquitination of H2B. Our study provides a new example of trans-histone regulation between modifications on different histones. In addition, it suggests that Rad6 affects telomeric silencing, at least in part, by influencing methylation of histone H3.     

Histone crosstalk between H2B monoubiquitination and H3 methylation mediated by COMPASS

COMPASS, the yeast homolog of the mammalian MLL complex, is a histone H3 lysine 4 (H3K4) methylase consisting of Set1 (KMT2) and seven other polypeptides, including Cps35, the only essential subunit. Histone H2B monoubiquitination by Rad6/Bre1 is required for both H3K4 methylation by COMPASS, and H3K79 methylation by Dot1. However, the molecular mechanism for such histone crosstalk is poorly understood. Here, we demonstrate that histone H2B monoubiquitination controls the binding of Cps35 with COMPASS complex. Cps 35 is required for COMPASS' catalytic activity in vivo, and the addition of exogenous purified Cps35 to COMPASS purified from a Deltarad6 background results in the generation of a methylation competent COMPASS. Cps35 associates with the chromatin of COMPASS-regulated genes in a H2BK123 monoubiquitination-dependent but Set1-independent manner. Cps35 is also required for proper H3K79 trimethylation. These findings offer insight into the molecular role of Cps35 in translating the H2B monoubiquitination signal into H3 methylation.