Properties of DNA of bacteriophage N1, a DNA with reversible circularity

The DNA of N1 bacteriophage (Naylor and Burgi, 1956), like that of its host, Micococcus lysodeikticus, has high guanine plus cytosine content (Scaletti and Naylor, 1959). We report here the results of some further physicochemical studies of the properties of this bacteriophage DNA. The most important results are that the DNA, as prepared by lysis of the phage, is linear but cyclizes when suitably annealed, and that the circular molecules revert to a linear form on heating and quenching. Thus, the molecules contain cohesive ends just as does the DNA of bacteriophage λ. The buoyant density indicates a base composition of 64(±1)% GC. The molecular weigth, as estimated from the sedimentation velocity and by electron-microscope length measurements, is 33(±1.5) × 10⁶. There appear to be no inherent single-strand breaks in the DNA. A method is described for positive staining with uranyl ion of DNA samples prepared for electron microscopy by the basic protein film technique (Kleinschmidt and Zahn, 1959). This modification makes shadowing unnecessary.