The switch in the helical handedness of the histone (H3-H4)2 tetramer within a nucleoprotein particle requires a reorientation of the H3-H3 interface.

It has recently been proposed that the histone (H3-H4)2 tetramer undergoes structural changes, which allow the particle to accommodate both negatively and positively constrained DNA. To investigate this process, we modified histone H3 at the H3-H3 interface, within the histone (H2A-H2B-H3-H4)2 octamer or the histone (H3-H4)2 tetramer, by forming adducts on the single cysteine of duck histone H3. We used three sulfhydryl reagents, iodoacetamide, N-ethylmaleimide, and 5,5'-dithiobis(2-nitrobenzoic acid). Torsionally constrained DNA was assembled on the modified histones. The H3 adducts, which have no effect on the structure of the nucleosome, dramatically affected the structural transitions that the (H3-H4)2 tetrameric nucleoprotein particle can undergo. Iodoacetamide and N-ethylmaleimide treatment prevented the assembly of positively constrained DNA on the tetrameric particle, whereas 5, 5'-dithiobis(2-nitrobenzoic acid) treatment strongly favored it. Determination of DNA topoisomer equilibrium after relaxation of the tetrameric nucleoprotein particles with topoisomerase I demonstrated that the structural transition occurs without histone dissociation. Incorporation of H2A-H2B dimers into the tetrameric particle containing modified or unmodified cysteines allowed nucleosomes to reform and blocked the structural transition of the particle. We demonstrate the importance of the histone H3-H3 contact region in the conformational changes of the histone tetramer nucleoprotein particle and the role of H2A-H2B in preventing a structural transition of the nucleosome.