High-resolution proton-magnetic-resonance studies of chromatin core particles.

The binding of histones in chromatin core particles and in core particles depleted of histones H2A and H2B has been studied by high-resolution proton nuclear magnetic resonance (NMR) at 270 MHZ. At low ionic strengths it is shown that histones H3 and H4 are bound in the core particle. Further, whereas the apolar regions of H2A and H2B are also bound to the core particle, the basic N-terminal and C-terminal regions are more mobile and give rise to sharp resonances in the NMR spectrum of the core particle. Between 0.3 and 0.6 M NaCl there is further release of basic regions of histones H3 and H4 from the complex. The dissociation of the core particle between 0.6 and 2.0 M NaCl is accompanied by the release of the structured apolar regions of the histones as evidenced by the appearance of a complex aromatic spectrum and perturbed upfield ring-current-shifted methyl resonances. Arginine residues are implicated in the binding between histones and DNA and 69% of these residues are found in the apolar regions of the histones. The interactions between histones and DNA in the core particle thus involves H3 and H4 and the apolar regions of H2A and H2B. It is suggested that these basic regions of H2A and H2B have binding sites outside the core particle.