Replication of colicin E1 plasmid DNA in minicells from a unique replication initiation site

Replicating DNA molecules of the colicin E1 plasmid isolated from minicells are cleaved at a single site by R1 restriction endonuclease (EcoR1). Electron microscopic measurements of the replicating molecules treated with the endonuclease indicate that (a) replication is initiated at a site between 14% and 20% of the distance from the EcoR1 endonuclease cleavage site; and (b) extensive replication of most molecules occurs in one direction from the initiation site, although a limited amount of replication in the opposite direction may occur. Single-stranded regions at one or both replication forks, involving one or both DNA strands, can be frequently found in replicating molecules.