Nucleosome core particles differing in their levels of histone acetylation have been formed on a closed circular DNA that contains a tandemly repeated 207 bp nucleosome positioning sequence. The effect of acetylation on the linking number per nucleosome particle has been determined. With increasing levels of acetylation, the negative linking number change per nucleosome decreases from -1.04 +/- 0.08 for control to -0.82 +/- 0.05 for highly acetylated nucleosomes. These results indicate that histone acetylation has the ability to release negative supercoils previously constrained by nucleosomes into a closed chromatin loop and in effect function as a eukaryotic gyrase.