Reversibility of Structural Rearrangements in Mononucleosomes Induced by Ionic Strength

Using fluorescence microscopy of single particles with Förster resonance energy transfer recording, structural rearrangements that occurred in nucleosomes formed on the 603 DNA template at high ionic strength were studied. Within the range of 0.7–1.3 M KCl, large-scale changes occurred in the nucleosome structure, including the formation of at least two states differing in the degree of DNA unwrapping from the histone octamer and affecting from 13 to 35 and more pairs of nucleotides. Content of the fraction of nucleosomes with modified structure varied from 60% at 0.7 M KCl to 100% at 1.3 M KCl. Preservation of the association between core histones and DNA in the new conformational states ensured reversibility of structural changes when KCl concentration was reduced to a physiological level. Reversibility was ~100% upon transition from 0.7 M to 0.15 KCl and decreased to ~50% upon transition from 1.3 M to 0.15 KCl.