Histone octamer acetylation affects the free energy of nucleosome formation

Nucleosomes, histone octamer-DNA complexes, form the fundamental repeating units of eukaryotic chromatin. Numerous post-translational modifications of histone octamers are found \emph{in vivo} and are known to play roles in gene regulation and DNA repair, but the molecular functions of these modifications are not well understood. In this study we consider the effects of acetylating histone protein H3 residues Lys$^{115}$ and Lys$^{122}$. These modifications reduce the positive surface charge of the histone octamer at contact points with the negatively charged DNA phosphate backbone and add steric bulk in the dyad region. We report results from competitive reconstitutions that show the free energy of nucleosome formation between wild-type and modified histone octamer binding to a strong nucleosome positioning sequence is reduced. These results suggest that these modifications may be involved in nucleosome assembly and disassembly.