Mg 2+-induced transition to strongly cooperative binding of histone H1 to linear DNA

Abstract

By addition of Mg 2+ ions to histone H1-DNA complexes formed at 20mM NaCl a transition to strongly cooperative binding of histone H1 to DNA is induced. In the analytical ultracentrifuge, above a critical Mg 2+ concentration of about 0.05 mM, the single component representing the original H1-DNA complex is replaced by two components: a higher order H1-DNA complex type characterized by a much higher sedimentation coefficient, and a slow-sedimenting component consisting of essentially H1-free DNA above 0.1 mM Mg 2-. The fast complex diappears upon removal of Mg 2+, showing that the process is reversible, and also upon addition of urea. Electron microscopy shows the cooperatively formed H1-DNA complexes to appear predominatly as loosely twisted cable rings in unfixed specimens, and as strongly condensed circular structures of different diameter, but approximately uniform thickness (of about 12nm) after fixation with glutaraldehyde. Besides these higher order structures, only single fibres indistinguishable from control DNA may be seen; individual double fibres which, in the absence of Mg 2+, represent the predominant H1-DNA complexe structure at about 0.4–0.8 w/w H1/DNA are no longer visible. The transition to strongly cooperative binding of H1 occurs at approximately the same Mg 2+ concentration which is known to induce the folding of the 10 nm nucleosome chain into the 30nm solenoid structure of chromatin.