How protein binding sensitizes the nucleosome to histone H3K56 acetylation.

The nucleosome, the fundamental gene-packing unit comprising an octameric histone protein core wrapped around by DNA, has a flexible structure that enables dynamic gene regulation mechanisms. Histone lysine acetylation at H3K56 removes a positive charge from the histone core where it interacts with the termini of the nucleosomal DNA and acts as a critical gene regulatory signal that is implicated in transcription initiation and elongation. The predominant proposal on the biophysical role of H3K56 acetylation (H3K56ac) is that weakened electrostatic interaction between DNA termini and the histone core results in facilitated opening and subsequent disassembly of the nucleosome. However, this effect alone is too weak to account for the strong coupling between H3K56ac and its regulatory outcomes. Here we utilized a semi-synthetically modified nucleosome with H3K56ac in order to address this discrepancy. Based on the results, we propose an innovative mechanism by which the charge neutralization effect of H3K56ac is significantly amplified via enzyme binding. We employed three-color single-molecule fluorescence resonance energy transfer (smFRET) to monitor the opening rate of nucleosomal DNA termini induced by binding of histone chaperone Nap1. We observed an elevated opening rate upon H3K56ac by 5.9 folds which is far larger than 1.5 folds as previously reported for the spontaneous opening dynamics in the absence of Nap1. Our proposed mechanism successfully reconciles this discrepancy based on that DNA opening for Nap1 binding must be larger than the average spontaneous opening. This is a novel mechanism that can explain how a small biophysical effect of histone acetylation results in a significant change in enzyme binding rate.