Dynamics of the interactions of histones H2A,H2B and H3,H4 with torsionally stressed DNA.

The interactions of histones H2A,H2B and H3,H4 with closed circular DNA maintained in either a positively or negatively coiled state have been studied. The interactions were assayed by measuring the rate at which negative stress was stored in the DNA by the histones and by the salt concentration sufficient to cause dissociation on sucrose gradients. Additional experiments were performed in which DNAs of substantially different molecular weights and opposite topological states were mixed with the histones in order to study histone mobility under varied conditions. This mobility was characterized by separating the complexes on sucrose gradients and by analyzing the DNA's topological state after topoisomerase I treatment. Histones H3,H4 were found to differ substantially from histones H2A,H2B with regard to the DNA topology with which they prefer to interact. The results are consistent with a model in which transcription-induced positive stress in advance of the RNA polymerase unfolds the nucleosome to facilitate the release of H2A,H2B. The data are also consistent with a model in which histones H3,H4 remain associated with the DNA during polymerase passage and serve as a nucleation site for the reassociation of H2A,H2B. The rapid production of transcription-induced negative stress in the wake of a polymerase would have substantial importance in facilitating the reassociation of histones H2A,H2B.