H3 phosphorylation-dependent structural changes in chromatin. Implications for the role of very lysine-rich histones

Contrary to native H1/H5-containing chromatin where phosphorylation induces local structural changes affecting chromatin condensation, in stripped fibers phosphorylation of the totality of H3 molecules does not affect significantly chromatin conformation and DNA-protein interactions. Modification of H3 causes only a slight increase of flexibility of nucleosomal chains, despite important changes in histone topography revealed by immunochemical reactivity studies. We suggest that phosphorylation may only induce into the system the potential for dynamic change by modulating histone-histone interactions within and between nucleosomes, probably as a result of conformational change in the H3 protein. The signal for structural change would come from one or other factors (very lysine-rich histones, non-histones) that influence internucleosomal interactions at very specific locations in the chromatin, probably through protein-protein contacts. So, phosphorylation may modify a direct interaction between the N-terminal basic tail of H3 and very lysine-rich histones.