Search

The structural behavior of the nucleosome core particles in the range of solvent Na+ concentration from 10.45 to 0.45 mM has been studied by small-angle neutron and synchroton radiation X-ray scattering, sedimentation, atomic absorption spectroscopy, density measurements, and circular dichroism. With decreasing salt concentration, the appearance of a scattering peak that is assignable to interparticle interactions, an intraparticle structural transition, a decrease in the sedimentation velocity of the particle, and a release of bound Na+ ions from the particle are all observed concurrently when the ratio of solvent Na+ ions per particle is below approximately 1000. These observations are interpreted to indicate that a release of bound Na+ ions from the particle brings about structural rearrangements and weakens the electrostatic shielding of the particle, and this introduces long-range repulsive ordering of the particle in low-salt solution. Analyses of the scattering data indicate that the rearrangement within the core particle in low-salt solution is slight, changing the particle's shape slightly from cylindrical to a more spherical form by moving the center of the mass of the DNA somewhat inward with accompanying small decreases in the radii of gyration of both the DNA and the histones.

The kinetic studies of chemical modifications of the lysine and arginine residues of the nucleosome core particle from chicken erythrocytes with large molar excesses of 2,4,6-trinitrobenzenesulfonic acid and 2,3-butanedione, respectively, were performed over 20 mM-2.0 M NaCl. Each modification reaction was divided into the two, a rapid and an extremely slow, pseudo-first-order processes, and the numbers of the exposed and buried residues of the respective amino acids were determined. At very low ionic strength (20 mM boric buffer) all residues were inaccessible to the reagents. Between 0.3 and 0.6 M NaCl the basic amino acid residues in a nucleosome core particle were classified into the following three groups: (1) 42 arginine and 78 lysine residues in the N-terminal and C-terminal regions of the histones that are exposed to solvent, (2) a small number of arginine residues (approximately less than 14 Arg) that are strongly bound to the DNA phosphates, and (3) 48 arginine and 38 lysine residues buried in the globular region of the histone octamer. The results suggest that a small number of arginine residues play an essential role in the folding of DNA into a nucleosome core particle.