Origin of replication of pBR345 plasmid DNA.

A small (approximately 1100 base pairs) ColE1-type plasmid, pBR345, was constructed from plasmid pMB1 by a series of in vitro recombinant manipulations. Approximately 9% of the supercoiled pBR345 DNA obtained from cultures amplified with chloramphenicol appears to be replicative intermediates with replicating "eye" structures of uniform size. Results obtained from electron microscopy and biochemical analyses have enabled us to localize the origin of replication at the same position as that reported for ColE1. A sequence of 420 nucleotides surrounding this origin has been determined. A comparison between this sequence and the one determined for the origin of replication of ColE1 is presented.