Accessibility of histone H1(0) and its structural domains to antibody binding in extended and folded chromatin.

The aim of this work was to study the accessibility of histone H1(0) and its structural domains to antibody binding in high molecular mass chromatin fragments of different conformations. Three types of specific antibody populations were used: (1) anti-H1(0) which reacted with antigenic determinants situated along the whole polypeptide chain, (2) anti-GH5 or anti-GH1(0) which recognized epitopes located in the globular region of H1(0) and (3) anti-C-tail antibodies reacting specifically with fragment 99-193 of the protein molecule. The immunoreactivity of the chromatin-bound antigen was investigated by solid-phase ELISA performed on glutaraldehyde-cross-linked chromatin and by an inhibition assay carried out with native chromatin in solution. The results of both methods were unidirectional and showed that: (1) the accessibility of H1(0) did not change with the compaction of the fiber; (2) the G-domain was not accessible to antibodies either in the relaxed or in the condensed state of the fragments, (3) the binding of the C-terminus-specific antibodies was different for isolated monosomes and for the chromatin fiber and (4) the degree of exposure of the epitopes of H1(0) in chromatin was much less than that of histone H1.