Salt-and histone H1-induced structural changes of reconstituted minichromosomes

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.