Internal structure of discrete nucleohistone complexes which form in vitro under conditions of physiological ionic strength.

Three discrete histone-DNA complexes assemble spontaneously when the four core histones are mixed with DNA under conditions which are close to physiological (0.15 M NaCl, pH 8). These species include the (H2A,H2B) dimeric complex (P1), the (H2A2,H2B2,H3,H4) hexameric complex (P2) and the nucleosome core complex (P3). This report compares several properties of these complexes with the properties of nucleosome cores assembled at high ionic strength (0.6 M NaCl). Based on histone-histone cross-linking studies, CD spectra, and thermal denaturation experiments, P1 is structurally similar to the subnucleosomal (H2A,H2B) fragment isolated from nuclease-digested chromatin. P1, P2, P3, and high salt-assembled nucleosome cores can all incorporate (H2A,H2B) pairs which have been previously cross-linked. Although the CD spectra and thermal denaturation profiles of P2 and P3 are closely related to those of nucleosome cores assembled in 0.6 M NaCl, cross-linking studies indicate that the arrangement of the histones in P2, and in a proportion of the P3 particles assembled in 0.15 M NaCl, are significantly different from their arrangement in nucleosome cores assembled in 0.6 M NaCl. The single cysteine residue on the H3 of P2 is accessible to the solvent. The two fluorescently labeled cysteine residues in a large proportion of the P3 particles assembled in 0.15 M NaCl are in a different orientation with respect to each other than the same residues in nucleosome cores assembled at high ionic strength.