Condensation of dinucleosomes by individual subfractions of H1 histone

Dinucleosomes purified from micrococcal nuclease digests of steer kidney nuclei were stripped of H1 histone by exposure to 0.50 M NaCl. They were then formed in a complex with individual subfractions of calf thymus H1 histone by dialysis of histone-dinucleosome mixtures from 0.50 M NaCl to concentrations of NaCl between 0 M and 0.08 M; between 0.30 M and 0.10 M the complexes precipitated, and so were not included in the study. The presence of H1 in the complexes was shown to cause an asymmetrical, ordered condensation as revealed by distortions of the circular dichroic spectrum of the DNA. The distortions were negligible at 0.04 M NaCl and below, and increased as a function of ionic strength between 0.05 M and 0.08 M. The degree of distortion of the spectrum, and therefore the nature of dinucleosome condensation, differed greatly from one H1 subfraction to the next. One of the three subfractions tested had almost no effect on the circular dichroism in comparing its dinucleosome complex to H1-depleted dinucleosomes. The other subfractions to different degrees produced large distortions that resulted in spectra that were of the psi type at the higher salt concentration.