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  • Title: Spermidine-condensed phi X174 DNA cleavage by micrococcal nuclease: torus cleavage model and evidence for unidirectional circumferential DNA wrapping.
    Author: Marx KA, Reynolds TC.
    Journal: Proc Natl Acad Sci U S A; 1982 Nov; 79(21):6484-8. PubMed ID: 6216482.
    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.
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