Ion binding and chromatin condensation.

Abstract

1. The binding of 45Ca2+ to hen erythrocyte chromatin has been studied to help elucidate how cations induce a reversible condensation of this chromatin. 2. As the unbound Ca2+ of the medium rises from 0.5 to 4 mM, Ca2+ is bound to the chromatin with a stability constant of approx. 3.1 and a saturation value of 0.25 Ca2+ per DNA phosphate, or one-half the value for pure DNA. Condensation of the chromatin is half complete when this binding of calcium is roughly half complete. Hence the transition from the uncondensed to the condensed state occurs as repulsion between the free DNA phosphates of erythrocyte chromatin is neutralised by bound cations. Genetically active chromatin may be maintained in an uncondensed state in living cells by the presence of different negative groups that remain unneutralised at the unbound cation concentrations of the cell. 3. That only one-half of the calcium binding sites of DNA are masked in erythrocyte chromatin supports recent models of chromatin structure in which the DNA double helix is wound round a core of histones. 4. Competition for calcium binding sites in the chromatin by other cations was also studied.