We have studied the ability of the histone (H3-H4)2 tetramer, the central part of the nucleosome of eukaryotic chromatin, to form particles 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 particular that generated by DNA thermal fluctuations, can trigger a major, reversible change in the conformation of the particle. Neither a large excess of naked DNA, nor a crosslink between the two 113s prevented the transition from one form to the other. This suggested that during the transition, the histones neither dissociated from the DNA nor were even significantly reshuffled. Moreover, the particles reconstituted on negatively and positively supercoiled minicircles look similar under electron microscopy. These data agree best with a transition involving a switch of the wrapped DNA from a leftto a right-handed superhelix. It is further proposed, based on the left-handed overall superhelical conformation of the tetramer within the octamer [Arents, G., Burlingame Nucleosome dynamics is a necessary requirement of DNA function in chromatin, including transcription, replication, or repair. It has long been thought to be mediated by the tripartite organization of the histone octamer, made of an (H3-H4)2 tetramer bound with two H2A-H2B dimers (1). Thermodynamic studies of octamer assembly and disassembly showed that the forces holding the tetramer and dimers together are of a different nature and much stronger than the forces binding the dimers to the tetramer (1, 2), inspite of an extensive dimer-tetramer interface. This interface is disrupted in the first step of octamer disassembly.The crystal structure of the nucleosome core particle (3), and even more so, of the histone octamer (4), subsequently confirmed this tripartite organization. Moreover, one of the H2A-H2B dimers in the crystal structure of the core particle appears significantly displaced from its original position (3), which probably reflects the effects of crystal-packing forces and the relative ease with which the dimers can move relative to the tetramer. Such dimer lability may also be important in vivo, as is emphasized by the observations of an H2A-H2B deficit in nucleosome cores originating from transcriptionally active chromatin (5), and of H2A-H2B exchange with the endogenous histone pool upon chromatin transcription (6-8).Here we provide evidence for a potential conformational flexibility of the (H3-H4)2 tetramer, a tribute to the role of this tripartite organization of the histone octamer in nucleosome dynamics. This conformational flexibility is suggested by the observation of a similar affinity of the tetramer for negatively and positively supercoiled DNA minicircles, which apparently arises from the ability of the wrapped DNA to switch from a left-to a right-handed superhelix. This transition is found to require only modest positive torsional stress in the DNA, and can be triggered by DN...

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