I. Fractions of PM2 closed circular DNA which renature upon neutralization of strongly alkaline solutions. The ethidium bromide-mediated renaturation of closed circular DNAs denatured in strongly alkaline solutions. II. Hydrodynamic properties of supercoiled closed circular DNAs at negative and positive superhelix densities. Occurrence of regions with single-stranded character in closed circular DNAs as a function of superhelix density

Part I. Closed circular duplex PM2 form I DNA was shown to have a constant fraction, about 7-8%, which renatured to its native form I conformation after exposure to strongly alkaline conditions and neutralization of the alkaline medium on ice, regardless of the pH (above 12.2) to which the DNA sample was exposed. The remainder of the molecules remain denatured. This "snapback" phenomenon was later confirmed in an independent study but was not further investigated. Further research on this phenomenon has been carried out in the present study. [...] Part II. The dependence of the initial rate of introduction of the first single-chain scission (initial nicking rate) into covalently closed circular duplex DNA by the single strand-specific nuclease from Pseudomonas BAL 31 on the superhelix density of the DNA ([delta o]) at values of [delta o] < 0 has been examined. This was done by reacting highly purified Ps. nuclease with samples of closed circular phage PM2 DNA artificially made so that they contained different numbers of negative superhelical turns. [...]