No abstract

It is shown that the release of the slightly lysine-rich histones f2a2 and f2b by 0.4 M ammonium sulfate from conventionally isolated chromatin is diminished in comparison to the lysed nuclei. The change in extractability is further demonstrated by the application of ethidium bromide. At a molar input ratio of 0.09 (moles ethidium bromide/moles nucleotide) and 0.4 M ammonium sulfate the slightly lysine-rich histones are released from the chromatin to 70 - 80% if the lysed nuclei are used. At 0.1 M ammonium sulfate ethidium bromide effected also a release of 50 % of histone f1. Comparable effects could not be observed with chromatin prepared in a conventional way but instead a tendency towards loss of histone f3 in the presence of ethidium bromide was observed.

No abstract

By moving boundary sedimentation it is shown that the interaction of H1 histone with superhelical circular SV40 DNA results in the formation of giant heterogeneous aggregates. The size of these aggregates grows with increasing H1 concentration. s20,w values of some 10 000 S were measured. As compared with open relaxed circular DNA a preferential interaction of superhelical DNA with H1 histone is observed, irrespective of the sign of the superhelical turns which was reversed by the addition to DNA of ethidium bromide. The addition to the H1 complexed aggregates of ethidium bromide effects a progressive breakdown of the aggregates. Furthermore, the superhelicity of DNA is not changed by the addition of small amounts of H1 histone.

Structural changes of reconstituted SV 40 minichromosomes have been studied in relation to the salt concentration and addition of histone H1 by sedimentation and electron microscopy. Sedimentation data are represented as functions of the NaCl concentration and the Debye-Hückel electrostatic screening radius 1/alpha. The latter representation which proved to provide more information revealed three structural states of the SV 40 reconstitutes which can be additionally characterized by electron microscopy as follows: Expanded or relaxed conformation including free DNA spacers between the nucleosomes at low salt concentration (approx. 0.001 M-0.05 M NaCl), increasing condensation at moderate salt concentration (approx. 0.05 M-0.3 M NaCl) and expansion of this condensed state above approx. 0.3 M NaCl. The condensation of the reconstitutes at moderate salt concentration does not require the presence of histone H1. H1 seems to stabilize the condensed state against electrostatic expansion. The condensation might be promoted by salt-dependent conformational changes of naked superhelical DNA as revealed by sedimentation measurements.