title: Spermidine-condensed phi X174 DNA cleavage by micrococcal nuclease: torus cleavage model and evidence for unidirectional circumferential DNA wrapping.
abstract: Spermidine-condensed phi X174 replicative form (RF) II DNA was digested with micrococcal nuclease to yield seven 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 [Eickbush, T. H. & Moudrianakis, E. N. (1978) Cell 13, 295-306] is shown to be inconsistent with our empirical solution results. We propose a continuous, circumferential DNA wrapping model in which a significant fraction of the collapsed circular phi X174 RFII DNA molecules form regular toruses comprised of seven complete, unidirectional double-helical wraps.
mesh_terms: D010584:Bacteriophage phi X 174 / Q000648:ultrastructure; D004270:DNA, Circular / Q000378:metabolism; D004279:DNA, Viral / Q000378:metabolism*; D008836:Micrococcal Nuclease / Q000378:metabolism; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D013095:Spermidine / Q000378:metabolism*
chemical_list: D004270:DNA, Circular; D004279:DNA, Viral; D008836:Micrococcal Nuclease; D013095:Spermidine