INTERACTION OF THE HISTONE (H3-H4)(2) TETRAMER OF THE NUCLEOSOME WITHPOSITIVELY SUPERCOILED DNA MINICIRCLES - POTENTIAL FLIPPING OF THE PROTEIN FROM A LEFT-HANDED TO A RIGHT-HANDED SUPERHELICAL FORM

We have studied the ability of the histone (H3-H4)(2) tetramer, the ce ntral part of the nucleosome of eukaryotic chromatin, to form particle s on DNA minicircles of negative and positive superhelicities, and the effect of relaxing these particles with topoisomerase I. The results show that even modest positive torsional stress from the DNA, and in p articular that generated by DNA thermal fluctuations, can trigger a ma jor, reversible change in the conformation of the particle. Neither a large excess of naked DNA, nor a crosslink between the two H3s prevent ed the transition from one form to the other. This suggested that duri ng the transition, the histones neither dissociated from the DNA nor w ere even significantly reshuffled. Moreover, the particles reconstitut ed on negatively and positively supercoiled minicircles look similar u nder electron microscopy. These data agree best with a transition invo lving a switch of the wrapped DNA from a left-to a right-handed superh elix, It is further proposed, based on the left-handed overall superhe lical conformation of the tetramer within the octamer [Arents, G., Bur lingame, R, W., Wang, B. C., Love, W. E. & Moudrianakis, E. N. (1991) Proc. Natl. Acad. Sci. USA 88, 10148-10152] that this change in DNA to pology is mediated by a similar change in the topology of the tetramer itself, which may occur through a rotation (or a localized deformatio n) of the two H3-H4 dimers about their H3-H3 interface. Potential impl ications of this model for nucleosome dynamics in Fire are discussed.