Cations and the accessibility of chromatin to nucleases

When rat liver nuclei prepared with polyamines as stabilising cations are digested with DNAase II, release of both inactive chromatin and Mg-soluble, active chromatin is greatly reduced, in comparison to digestion of liver nuclei prepared with Mg2+ as stabilising cation. Chromatin release from polyamine stabilised nuclei is also inhibited relative to Mg-stabilised nuclei following digestion with micrococcal nuclease under two very different cation conditions. Nuclei prepared with polyamines and monovalent ions as stabilising cations exhibit properties intermediate between these two extremes with both nucleases. These effects are due to residual binding of polyamines to chromatin, which is thus maintained in a condensed state, inaccessible to nucleases. Since polyamine binding is not easily reversed, concentrations of polyamines and other cations must be rigidly controlled in experiments on chromatin structure if artefacts are to be avoided. The significance of these findings to the nature and properties of active chromatin within the intact nucleus is considered.