DNA-histone interactions in nucleosomes.

We have utilized micrococcal nuclease digestion and thermal denaturation studies to investigate the binding of DNA to the histone core of the nucleosome. We conclude that a total of approximately 168 base pairs (bp) of DNA can interact with the histone core under appropriate solution conditions, even in the absence of lysine-rich histones. The interactions in this total length of DNA can be divided into three classes: (a) approximately 22 bp at the ends is bound only at moderate ionic strength. It is easily displaced, and its removal yields the 146 bp core particle. (b) approximately 46 bp near the ends of the core DNA are quite weakly bound to the core, and are displaced at quite moderate temperatures. (c) The remaining central 100 bp are strongly bound, and interact with all of the sites on the histones which strongly protect DNA against DNAse I digestion. A theoretical analysis of the cleavage of nucleosomal DNA by DNAse I has been used to develop evidence that the pattern of protection offered by the histone core is very similar in nuclei to that in isolated core particles.