Chromatin compaction under mixed salt conditions: Opposite effects of sodium and potassium ions on nucleosome array folding

It is well known that chromatin structure is highly sensitive to the ionic environment. However, the combined effects of a physiologically relevant mixed ionic environment of K + , Mg 2+ and Na + , which are the main cations of the cell cytoplasm, has not been systematically investigated. We studied folding and self-association (aggregation) of recombinant 12-mer nucleosome arrays with 177 bp DNA repeat length in solutions of mixtures of K + and Mg 2+ or Na + and Mg 2+ . In the presence of Mg 2+ , the addition of sodium ions promotes folding of array into 30-nm fibres, whereas in mixtures of K + and Mg 2+ , potassium ions abrogate folding. We found that self-association of nucleosome arrays in mixed salt solutions is synergistically promoted by Mg 2+ and monovalent ions, with sodium being slightly more efficient than potassium in amplifying the self-association. The results highlight the importance of a mixed ionic environment for the compaction of chromatin under physiological conditions and demonstrate the complicated nature of the various factors that determine and regulate chromatin compaction in vivo .