(3H)-imipramine binding and plasmid replication sites in Escherichia coli

Binding of (3H)-imipramine to F'lac plasmid-carrying E.coli was studied. The binding was saturated at 500 mM imipramine in the case of F'lac cells, and the imipramine binding of plasmid-carrying bacteria was inhibited by several tricyclic compounds, such as acridine orange, desimpramine and promethazine. Methylene blue enhanced the (3H)-imipramine binding of bacteria. Binding of radioactively labelled pBR322 plasmid DNA to the internal membrane was more efficient in the presence of plasmid-eliminating drugs than in that of their ineffective derivatives. It is concluded that a structural association exists between imipramine binding and plasmid replication sites in bacteria. The plasmid DNA is assumed to bind irreversibly to its replication site in the presence of the drug and this process can lead to plasmid elimination.