Salt-induced release of DNA from nucleosome core particles

At elevated salt concentrations, the structure of chromatin is destabilized. This paper is concerned with the processes by which DNA is released from nucleosome core particles in free, uncomplexed form. Our experiments indicate that the DNA release reaction has distinctly different characteristics below and above approximately 0.75 M NaCl. Below this concentration of salt, release of the histones from the DNA is highly cooperative, so that no dissociation intermediates are even seen. Above this salt concentration, histone release is not so cooperative; H2A and H2B are released from the DNA more readily than are H3 and H4. This results in an apparently heterogeneous population of (H2A, H2B)-depleted intermediate species sedimenting at rates between that of free DNA and that of intact core particles. Dissociation of core particles at NaCl concentrations below 0.75 M is readily reversible. Reassociation of DNA and histones from higher salt concentrations is nearly quantitative if carried out by gradual decrease of salt concentration, but rapid dilution to low salt results in the formation of a fraction of metastable nucleosome multimers. To help organize our description of the DNA release process, we introduce a stability diagram for the core particle, defined with respect to the independent variables of salt concentration and particle concentration. We draw upon our own experimental work and also upon the work of several other laboratories. We distinguish five major regions in this diagram.