Dynamics and equilibria of nucleosomes at elevated ionic strength.

We have prepared chicken erythrocyte nucleosomes lacking proteins other than the inner histones and containing long DNA. Our nucleosomes' DNA has mean length +/- SD = 190 +/- 15 base pairs. No DNA less than 155 base pairs is present. Nucleosome stability in salt was examined by boundary and band sedimentation and by particle gel electrophoresis. We find the following. (i) A second species is slowly generated by treatment with salt. This species sediments with S20,w = 5.5 S (as does purified mononucleosomal DNA), is not associated with histones, and electrophoretically migrates as mononucleosomal DNA. We conclude it is free DNA. Thus, salt causes nucleosomes to dissociate, independently of either noncore proteins, or of any nucleosome population with DNA less than 146 base pairs. (ii) Dissociation is reversible, and is enhanced by nucleosome dilution. Thus, it appears to follow the law of mass action. (iii) The equilibrium extent of dissociation increases with salt. A second effect of salt is a fast, reversible 10% decrease in S20,w of the nucleosomes left intact. From hydrodynamic calculations, this is consistent with either a slight unfolding of the entire nucleosome, or an unbinding of the terminal DNA regions from the histone core.