Interaction of histone H1 with superhelical DNA. Sedimentation and electron microscopical studies at low salt concentration.

Complexes of histones H1 with superhelical SV40 DNA obtained by direct mixing were studied in 0.1 SSC buffer corresponding to 0.02 M Na+. Depending on the molar input ratio H1/DNA three classes of sedimenting species were observed: (1) a component sedimenting similar to superhelical DNA with a sedimentation coefficient s2o,w of 25 S observable up to 335 Mol H1/Mol DNA (w/w = 2); (2) a component with s2o,w = 120 S appearing at 135 Mol H1/Mol DNA and (3) growing amounts of heterogeneous aggregates greater than 1000 S. Electron micrographs revealed the 25 S component to consist of double-fibers formed from one DNA molecule and the 120 S component to consist of bundles of several such double-fibers. The aggregates represent cable-like structures. The addition of ethidium bromide to 25 S complexes induces the formation of bundles, if H1 is present in a quantity which alone is not sufficient to bring about this effect. This result indicates that ethidium bromide effects a redistribution of H1 molecules and that H1 is responsible for the bundle formation.