Bidirection replication from a unique origin in a mini-F plasmid.

Replicating molecules of the mini F-kanamycin resistance plasmid, pML31, derived from F'lac, have been isolated from Escherichia coli as covalently-closed circular DNA molecules. These molecules were examined in the electron microscope after digestion with either EcoRI or BamHI restriction endonuclease. The structure of the majority of the molecules replicating was consistent with a bidirectional mode of replication starting at a unique origin on the F-fragment. This origin is located approximately 2.3 kilobases from one of the EcoRI sites. Orientation of the F-fragment relative to the physical map of F showed the position of the origin to be at 42.6 kilobases. A small proportion of molecules appeared to be replicating unidirectionally in either direction from this origin. Termination of replication of pML31 apparently occurs in the fragment containing the locus for kanamycin resistance in a unique region opposite the origin in the circular DNA molecule.