Physicochemical properties of salt-soluble, unsheared chromatin. Hydrodynamic model calculations supporting a...

Hydrodynamic model calculations of a special, salt-soluble chromatin fraction were performed on the basis of former experimental data. Using the models of an extended helix and of a cylinder fitted to this helix we conclude that in 0.1 M (NH4)2SO4 this chromatin has the conformation of a nucleosome double-fiber helix, i.e. a structure in which two helices share a common axis. Besides this helical structure, a stretched, linear double-fibrillar form can be derived from sedimentation data on the basis of a simple cylinder model. From the kinetic behaviour of experimental sedimentation coefficients which decrease shortly after chromatin extraction a conformational transition of the helical to the fibrillar form is postulated. At 0.2 M (NH4)2SO4 where histone H1 is released, these double-structures are supposed to dissociate into two single molecules with similar dimensions.