Differences in the condensation of chromatin by individual subfractions of histone H1: implications for the role of H1(0) in the structural organization of chromatin.

The effectiveness of histone H1 subfractions H1-1 and H1(0) in inducing the ordered condensation of chromatin was examined by thermal denaturation, circular dichroism, electric birefringence, orientation mechanism, and orientational relaxation time measurements. Soluble rat liver chromatin was stripped of H1 by dissociation in 500 mM NaCl and long fragments of chromatin were subsequently reassociated with purified individual H1 subfractions for ratios of 1 and 2 mol of H1 per nucleosome. H1 subfractions behave differently with respect to their interactions with DNA in chromatin: although the orientation mechanisms of reconstituted chromatins are identical, H1(0) induces a less efficient protection of DNA than H1-1, as shown by nuclease digestion and by the length of free extended linker DNA determined by electric birefringence. This corresponds to a more extended structure of H1(0)-reconstituted chromatin as judged by the value of relaxation time. One can imagine that the replacement of H1 by H1(0) leads to a different structure or stability of the chromatin, confering a certain degree of flexibility of this region. This may be related to the functional role of H1(0) in DNA replication or transcription and may explain metabolic and evolutionary differences among H1 subfractions as recently suggested by Lennox [Lennox, R. W. (1984) J. Biol. Chem. 259, 669-672]. The extent of condensation when H1-depleted chromatin is overloaded with histones is probably a function of the electrostatic interactions between the basic C-terminal tails of histones and chromatin. Electric birefringence also reveals differences between native and reconstituted chromatins that are overlooked by several other criteria.