Structural changes of soluble rat liver chromatin induced by the shift in pH from 7 to 9

Soluble rat liver chromatin was studied at pH 7 and at pH 9. In order to remove selectively non-histone components or non-histone components and histone H1, fractionation of chromatin was performed at pH 7 and pH 9 at different ionic strengths. The salt-dependent condensation of the fractionated chromatin was analysed in the electron microscope. There is no difference between the appearance of H1-depleted chromatin at poH 7 and pH 9. In H1-containing chromatin the shift from pH 7 to pH 9 leads to the following morphological changes: a) at very low ionic strength the nucleosomes unravel partially or totally and the zigzag-shaped fibres disappear in favour of beads-on-a-string; b) with increasing ionic strength the filaments condense into fibres, however, these fibres appear distorted and clearly less ordered than at pH 7. There is no indication of a release or displacement of histone H1. The pH-effect is completely reversible. The data suggest a pH-induced change in the mode of action of histone H1 in the formation of nucleosome beads and higher order chromatin structures.