Determinants of H3K4me pattern establishment

A hallmark of genes transcribed by RNA polymerase II (RNApII) is a gradient of histone H3 lysine 4 (H3K4) methylation. Various factors differentially bind to H3K4me3 near promoters, H3K4me2 just downstream, and H3K4me1 further downstream to modulate gene expression. Set1/COMPASS, the single S. cerevisiae H3K4 methyltransferase, binds transcribing RNApII, but COMPASS may also be allosterically regulated by specific subunits and histone H2B ubiquitylation. To ask whether differential H3K4 methylation is determined by regulated activity at specific gene locations or by the amount of time COMPASS spends near the nucleosome, ChIP-Seq analyses was performed in cells with altered transcription elongation rates or with Set1 fused to RNApII. Our results support a simple model where higher H3K4 methylations result from both increased duration and frequency of COMPASS proximity to the nucleosome.