A two-stage molecular model for control of mini-F replication.

It is known that mini-F replication requires production of a 29,000-Da protein, protein E, and origin of replication sequences mapping around 45. kb. Further, control of replication is determined by two genes, copA and copB. In the present work a description is given of the cloning of an F restriction fragment containing the amino terminal portion of the protein E gene, repE, and associate promoter activity. It is shown that expression of this promoter is negatively regulated in trans by sequences taken from the F replication region of copA+ plasmids. However, the same sequences taken from six different copA- plasmids failed to repress expression of the promoter. Since prior studies have shown that copA+ determines a repressor of replication, it is now suggested that the above results are an accounting of where this repressor works. A hypothesis is also proposed to explain control of F replication by the copA and copB regulatory genes.