Nucleas action on chromatin: evidence for discrete, repeated nucleoprotein units along chromatin fibrils

The time course of the fragmentation of calf thymus chromatin by DNase II (deoxyribonucleate 3'-oligonucleotidohydrolase, EC 3.1.4.6) has been examined by sedimentation of chromatin digests through linear (5-20%) sucrose gradients. The action of nuclease is decidedly nonrandom, ultimately producing roughly equal amounts of acid-soluble oligonucleotides and 11S nucleoprotein particles. The 11S particles contain double-stranded DNA that is approximately 400 A or 120 base-pairs long, as measured by electron microscopic examination of deproteinized samples, and is maintained in a compact conformation within the intact particles. In addition, 15S nucleoprotein particles containing predominantly 800-A lengths of DNA have been isolated from less extensively digested chromatin. Evidence is presented which indicates that the 11S particles are fundamental structural units that are arranged in tandem along certain regions of chromatin fibrils. Preliminary experiments with different nucleases and with chromatin from different mammalian species indicate that these results are a natural consequence of the arrangement of DNA and proteins in mammalian chromatin and are not peculiar to the system described in detail.