Chromatin structure. Nuclease digestion profiles reflect intermediate stages in the folding of the 30-nm fiber rather than the existence of subunit beads.

Conditions have been found for the isolation of rat liver nuclei which maintain chromatin in its native state and suppresses endogenous nuclease activity. Chromatin prepared in this way can be dispersed into buffers containing various concentrations of monovalent or divalent cations so that the 30-nm fiber is either totally or partially decondensed. When probed with micrococcal nuclease, the digestion profiles show that although the 30-nm fiber can be cleaved periodically to generate superbead-like particles this only occurs under certain ionic conditions when the fiber is partially decondensed. It is likely that this cleavage pattern reflects the transient exposure of specific nuclease sensitive sites as the 30-nm fiber condenses, rather than the existence of a specific subunit of a beaded 30-nm fiber. The periodicity of these nuclease-sensitive sites appear to be related to the asymmetric distribution of histone H1 molecules along the length of the fiber.