The globular region of histone H5 is equally accessible to antibodies in relaxed and condensed chromatin

The accessibility of histone H5 in chromatin was examined with monoclonal antibodies recognizing several epitopes of the globular region (GH5) of the histone (Rózalski, M., Lafleur, L., and Ruiz-Carrillo, A. (1985) J. Biol. Chem. 260, 14379-14385). The stoichiometry of the chromatin-antibody complexes indicated that while 0-86% of the H5 molecules were able to react, depending on the particular epitope, the extent of antibody binding to relaxed chromatin (in 5 mM KCl) and condensed chromatin (in 100 mM KCl or 0.35 mM MgCl2) was virtually identical. This indicates that the topography of H5 does not change during the conformational transition of chromatin. The data suggest that H5 is not completely internalized in the 30-nm fiber or that the fiber is flexible enough to allow full exposure of the GH5 epitopes. Several control experiments, including monoclonal antibody binding, sedimentation analysis, DNase II digestion, and glutaraldehyde cross-linking, showed that epitope accessibility is not due to H5 exchange or to perturbation of the chromatin fiber. The accessibility of GH5 suggests ways in which inactive chromatin may be unfolded in vivo.