AFM analysis of changes in nucleosome wrapping induced by DNA epigenetic modification

The wrapping and unwrapping of the nucleosome, which is a fundamental packing unit of chromatin, are tied to the regulation of gene expression. The accessibility of DNA within nucleosomes is controlled not only by chromatin-remodeling molecules, but also by chemical modifications of histones and DNA. Understanding the structural changes of a nucleosome during epigenetic modifications is a key to unravel the mechanisms of gene regulation. Here, we reconstituted nucleosomes using methylcytosine- or hydroxymethylcytosine-substituted DNA, and analyzed their morphological features by atomic force microscopy (AFM). Our results indicate that cytosine methylation induces overwrapping of the DNA around the histone octamer, whereas cytosine hydroxymethylation has a lesser effect on the overwrapping of the DNA. These results suggest that two types of DNA modification yield different wrapping states of nucleosomes, which may contribute to the compaction and relaxation of the chromatin structure.