Spermidine-condensed phi X174 DNA cleavage by micrococcal nuclease: torus cleavage model and evidence for unidirectional circumferential DNA wrapping.

Abstract

Spermidine-condensed .vphi.X174 replicative form (RF) II DNA was digested with micrococcal nuclease to yield 7 identifiable DNA bands forming an arithmetic fragment-length series. The DNA monomer unit length was found to be 780 .+-. 80 base pairs. This result is most consistent with a proposed model for micrococcal nuclease cleavage of a DNA torus organized by the unidirectional, circumferential wrapping of B-geometry DNA. By a topological consideration, the blunt-end-rod-fusion model for torus formation is shown to be inconsistent with the empirical solution results. A continuous, circumferential DNA wrapping model in which a significant fraction of the collapsed circular .vphi.X174 RFII DNA molecules form regular toruses comprised of 7 complete, unidirectional double-helical wraps is proposed.