The effect of salt on oligocation-induced chromatin condensation

Condensation of model chromatin in the form of fully saturated 12-mer nucleosome arrays, induced by addition of cationic ligands (ε-oligolysines with charge varied from +4 to +11), was studied in a range of KCl concentrations (10-500mM) using light scattering and precipitation assay titrations. The dependence of EC₅₀ (ligand concentration at the midpoint of the array condensation) on C_KCl displays two regimes, a salt-independent at low C_KCl and a salt-dependent at higher salt concentrations. In the salt-dependent regime EC₅₀ rises sharply with increase of C_KCl. Increase of ligand charge shifts the transition from the salt-independent to salt-dependent regime to higher salt. In the nucleosome array system, due to the partial neutralization of the DNA charge by histones, a lower oligocation concentration is needed to provoke condensation in the salt-independent regime compared to the related case of DNA condensation by the same cation. In the physiological range of salt concentrations (C_KCl=50-300mM), K⁺ ions assist array condensation by shifting EC₅₀ of the ε-oligolysines to lower values. At higher C_KCl, K⁺ competes with the cationic ligands, which leads to increase of EC₅₀. Values of salt-dependent dissociation constant for the ε-oligolysine-nucleosome array interaction were obtained, by fitting to a general equation developed earlier for DNA, describing the dependence of EC₅₀ on dissociation constant, salt and polyelectrolyte concentrations.