Condensation of the chromatin gel by cations

Abstract

Calf thymus chromatin gel, containing strongly bound nonhistone proteins, was used to study the effect of easily removable and tightly bound cations on the condensation of chromatin. The chromatin volume was found to be linearly dependent on the reciprocal square root of the concentration of easily removable cations (Tris·H + + Na + and Mg 2+) except for the initial stages of condensation (up to 7–10 mM monovalent and 0.15–0.2 mM divalent cations). The effect of Mg 2+ at the initial stage of condensation was not reproduced by Na + and vice versa. At higher concentrations the effects of Na + and Mg 2+ were additive. The removal of tightly bound divalent cations by a treatment of the chromatin gel with 1,10-phenanthroline led to an approx. 50% increase in the volume of the chromatin gel, which was maintained at each concentration of easily removable cations. The 1,10-phenanthroline-caused decondensation of the chromatin gel was reversed by Ca 2+ but not by Mg 2+, Zn 2+ and Cu 2+. The chromatin gel pretreated with Ca 2+ was not further decondensed by 1,10-phenanthroline.