Structural aspects of histone complexes and nucleosomes revealed by the accessibility of cysteine side chains.

Chemical modification with 5,5'-dithiobis-(2-nitrobenzoic acid) shows that histone H3 cysteines 110 (chicken) or 96 and 110 (calf) are completely protected in native chromatin and core particles but become unmasked simultaneously during a salt induced dissociation. In whole histone extracted from chicken erythrocyte chromatin, H3 Cys-110 residues experience a uniform environment at 2 M NaCl and pH 5.5. In whole histone extracted under the same conditions from calf thymus, H3 Cys-96 provides a more accessible thiol group than Cys-110. At higher pH values a conformational heterogeneity is induced causing a partial exposure of both cysteine side chains. The kinetic approach described in the present work provides a highly sensitive means to probe the homogeneity of H3 containing protein complexes.