Nucleosome structure and conformational changes.

We have used a variety of chemical probes to measure the accessibility of DNA on the surface of the nucleosome. We review these results, and describe new experiments which show that T4 phage DNA can form complexes with the core histones, possessing the properties of normal nucleosomes. Since T4 DNA is largely occupied by glucose residues in the major groove, this suggests (as did earlier probe experiments) that the major groove is not filled with histone amino acid side chains. We also report results of recent measurements which appear to show that only a few strong charge interactions are involved in the attachment of the terminal 20 nucleotide pairs at each end of nucleosome core DNA. We speculate on the possible functional significance of the accessibility of DNA revealed by all of these experiments. We have also examined conformational changes induced in nucleosomes at high ionic strength (0.5-0.7M NaCl). The frictional coefficient is found to undergo a small increase in this region, not consistent with models in which the nucleosome is completely unfolded, but possibly reflecting the dissociation of terminal DNA from the nucleosome surface.