[Study of structural changes in chromatin in the presence of mono- and divalent cations by means of flow linear dichroism].

The chromatin structure in solution was studied by the flow linear dichroism method (LD) in a wide range of ionic strengths. It was found that the increasing of the ionic strength from 0.25 mM EDTA, pH 7.0 to 100 mM NaCl leads to a manifold decrease of the LD amplitude of chromatin and inversion of the LD sign from negative to positive at 2 mM NaCl. Chromatin exhibits a positive LD maximum value at 10-20 mM NaCl. These data enable us to conclude that in very low ionic strength (0.25 mM EDTA-2mM NaCl) the nucleosome discs are oriented with their flat faces parallel to the chromatin filament axis. Increasing ionic strength up to 20 mM NaCl leads to reorientation of the nucleosome discs and to formation of chromatin structure with nucleosome flat faces more or less perpendicular to the fibril axis. A conformational transition of that kind is not revealed in H1-depleted chromatin. This testifies to the fact that the histone H1 is responsible for the higher order structure of chromatin. It was found that divalent cations do not induce the inversion of the LD sign of chromatin.