Nucleosome structural changes due to acetylation.

We have measured the helical repeat of acetylated nucleosomes reconstituted with HeLa cell histone octamers and the 5 S RNA gene positioning sequence from Xenopus borealis. The hydroxyl radical footprinting experiments show that the helical repeat of nucleosome-bound DNA is unchanged upon acetylation, and that the extent of DNA-protein contacts remains unaltered. These results, combined with previous determinations of the linking number change in minichromosomes as a function of the extent of acetylation, are analyzed with surface linking theory. We demonstrate that the shape of the nucleosome changes upon acetylation, in such a way as to decrease the number of times the DNA winds around the nucleosome central axis.