Histone H1·DNA complexes: sedimentation analysis of structural transitions

Abstract

The structural properties of complexes of histone H1 with linear DNA were investigated by means of analytical velocity sedimentation techniques. Independent of the method of complex preparation, with increasing H1/DNA input ratio up to 0.9 w/w two cooperative structural transitions were observed at 20 mM Na +, each leading to a more compact structure with higher sedimentation coefficient. The first structural transition (occurring at H1/DNA input ratios of 0.2–0.3 w/w) may be described by a decrease of the apparent persistence length of DNA, suggesting a bending of the DNA double helix in the H1·DNA complex. The second structural transition (at H1/DNA≈0.6 w/w) obviously is caused by the formation of symmetrically backfolded DNP double chains as previously seen by electron microscopy. These double fibres are characterized by cooperative binding of H1 and by a high linear packing density of the histone. Experiments performed in the presence of urea indicate that the globular domain of H1 contributes to the stabilization of the DNP double fibre structure.