Studies of Nucleosome Structure

The nucleosome is the elementary DNA-protein unit responsible for the organization of chromatin (for re-view, see Kornberg 1977; McGhee and Felsenfeld 1980). Investigations in this laboratory have focused on elucidating the structure of the nucleosome at atomic resolution and determining how it is assembled in the higher-order structure of chromatin. Accounts of this work have been reported recently (Richmond et al. 1981; Klug 1982). In this paper we provide a brief sum-mary of our present understanding of nucleosome struc-ture at an overall resolution of about 20,h,. Our ap-proach to obtaining crystals that are ordered to a resolu-tion beyond 6 A and our progress toward solving the current crystals at 7-A resolution are also presented. The path of the DNA superhelix and the course of the double-helical backbones should be clearly visible in the electron density map at 7-A resolution. On the basis of the available diffraction data, the individual phosphate atoms of the DNA, as well as aspects of the histone structure, should be well defined. Thus, we xpect hat the points of contact of the histones along the DNA superhelix will be evident.