Purification and properties of Intracellular Lamba DNA Rings

Intracellular lambda DNA has been purified from bacteria infected with lambda phage. The prediction from sedimentation properties that one intracellular form of lambda DNA is a twisted, closed-circular molecule has been confirmed by observation in the electron microscope. Two unique forms of lambda DNA are present in infected cells: twisted, closed-circular molecules (component I) and open-circular molecules (component II). One single-strand scission is sufficient to convert component I to component II. Both components I and II, either native or denatured, are able to infect spheroplasts. The infective entity present after denaturation of component I is a molecule in which strand separation has not occurred. That present after denaturation of component II is a single-strand ring; single-stranded linear molecules are not infective. The ionic strength dependence of the sedimentation rate of components I and II and phage lambda DNA has been studied. For comparative purposes, similar studies have been made with φX174 replicative form. A new consequence of DNA circularity is evident from these studies