The F plasmid ccd autorepressor is a complex of CcdA and CcdB proteins

The ccd operon of plasmid F produces three proteins, CcdA, CcdB, and RepD. Prior research has established that the operon is autorepressed and that at least CcdB, but not RepD, is required for autorepression. A role for CcdA in autorepression was suggested but not clearly shown. We now present a series of biochemical experiments which show that both CcdA and CcdB proteins are required for maximal formation of protein-ccd operator complexes. We also show that CcdA and CcdB are present in a complex whether or not ccd operator is present. The clear implication is that autorepressor is a complex of CcdA and CcdB. We also map the start site of the ccd transcript thus providing the first experimental evidence for the location of the ccd promoter.