Disassembly of chromatin into approximately equal to 50 kb units by detergent.

Nuclei isolated from higher eukaryotic cell lines were directly analyzed by field inversion gel electrophoresis. Brief incubation of nuclei with ionic detergents yielded a single band between 50-100 kb. The apparent fragment size decreased to approximately equal to 50 kb after proteinase digestion. The latter treatment alone induced less regular, less than or equal to 50 kb fragmentation. DNA extracted from detergent and proteinase-treated nuclei also appeared in a band of about 40 kb. Embedding into agarose plugs did not protect nuclei, as opposed to cells, from detergent-induced fragmentation. The phenomenon is strikingly analogous to the double-strand DNA cleavage reactions mediated by topoisomerase II. Our data are compatible with any of the following interpretations: 1.) regularly spaced protein bridges, probably involving topoisomerase II, maintain or control continuity of chromosomal DNA in certain states of higher eukaryotic cells. 2.) The DNA might become accessible to a putative endonuclease at regularly spaced sites upon detergent treatment of isolated nuclei.