Increasing the ionic strength of rat liver chromatin solutions above 0.4 M causes increasing viscosity. This indicates transformation of the compact chromatin molecules to more elongated forms. In the range of 0.4-0.5 M ionic strength histone H1 is dissociating continuously from the chromatin and the quaternary structure chromatin unravels. At ionic strength higher than 0.5 M the viscosities of chromatin solutions are furthermore increasing due to structural deformation. Near 0.7 M ionic strength the core histones H2A and H2B begin to dissociate from the chromatin, and the opening of the nucleosome cores leads to increasing elongation of the chromatin molecules.
Equilibrium dialysis studies and sedimentation experiments with soluble rat liver chromatin and radioactive 45Ca support the hypothesis that there is a specific binding of Ca2+ to chromatin molecules. At low ionic strength the binding constant is about 12 1/mᴍ indicating that more than half of the negative charges of the chromatin molecules are neutralized due to the binding of Ca2+ at appropriate concentrations. This effect possibly accounts for low concentration of Ca2+ and other divalent cations being effective in inducing compact higher order structures of chromatin. Monovalent cations are not specifically bound, but high concentrations cause unspecific shielding of the charges on the chromatin molecules and thus structural transformation. Even under these high salt conditions a specific binding of Ca2+ to chromatin occurs. The binding constant is, however, only 0.4 1/mᴍ due to the lower effective concentration of the charged chromatin molecules which is reduced by unspecific neutralizing.
Recent observations and hypotheses on the structure of chromatin are reviewed. Elementary "subunit" for higher structural orders is the nucleosome, consisting of a histone octamer and doublehelical DNA wrapped around it. During the last years details of the nucleosomal structure could be deduced down to a resolution of 2 nm. In the chromatin fiber, built up by (mono-)nucleosomes, the superhelical DNA has a tertiary structure, from which structures of still higher order (quaternary structure of DNA) can be formed. The correlation of these structures to the terms euchromatin and heterochromatin are discussed. Finally, some functional aspects, especially transcription and replication, are discussed in view of the new knowledge of chromatin structure.
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