The Structure of Chromatin Core Particles and the Higher Order Structure of Chromatin

ABSTRACT Small-angle neutron scattering studies of chromatin core particles suggest a structure comprising a nearly spherical histone core (of ~3.2 nm radius and with which is associated very little water) and DNA (associated with ~60 nm 3 of water) wound so that it touches the outer surface of the particle at all points which are distant ~5.5 nm from the centre. The best fit to the neutron data is very approximately 3 nm between the irregular turns of DNA so that the particle, in agreement with low resolution crystallography data, looks like an~oblate ellipsoid with axes 11, 11 and~5.5 nm. NMR studies of the core particles show sharp proton resonances which are consistant with signals coming from the N-terminal and C-terminal regions of H2A and H2B together with the N-terminal region of H3. This work suggests that some of the basic regions of histones H2A and H2B interact with regions outside the local core particle. The latter may belong to linker DNA, adjacent nucleosomes or non-histone proteins. Multimers of chromatin nucleosomes have been prepared by light micrococcal nuclease digestion of chicken erythrocyte nuclei in the presence of 0.15 M NaCl. Neutron scattering studies at low ionic strengths indicate that multimers are stretched in the form of a rod, with DNA mostly, but by no means entirely, at the periphery. On increasing the ionic strength by addition of Na + or Mg ++ ions the string of nucleosomes folds into a much fatter structure which becomes very tightly packed in 2 mM Mg ++ . The cross-section radius of gyration under these conditions is 10.5 ± 5 nm and the radial distribution function for the cross section fits a model of a rather solid rod of diameter ~30 nm. The arrangement of nucleosomes in chromatin higher order structure in the environment of the cell nucleus is yet to be established.