Isolation of circular viral and complementary strand DNA from bacteriophage f1 duplex replicative-form DNA.

A general method has been developed for the large scale isolation of intact, circular, single-stranded DNA molecules of each strand from supercoiled duplex DNA. The method involves the conversion of the supercoiled duplex DNA to singly nicked, relaxed duplex DNA; denaturation of the duplex DNA; separation of circular DNA molecules from linear DNA molecules; and separation of circular plus and minus strands. All separations involve zone sedimentation. No isopycnic gradient centrifugation is required. The last step in the purification, the separation of plus and minus strands, can be easily adapted for small scale analytical measurements of the amounts of plus and minus strand DNA.