Mutation and identification of the F plasmid locus determining resistance to acridine orange curing

A region in plasmid F responsible for resistance to acridine orange curing has been identified. Insertion of the transposable element for ampicillin resistance Tn3 at the 45.8- or 46.35-kilobase coordinate in mini-F plasmids results in resistance to curing by acridine orange and also results in increased plasmid copy numbers. In contrast, copy number mutants of mini-F induced by chemical mutagenesis are sensitive to acridine-induced loss of plasmid. Hence, the Tn3-induced acridine orange resistance does not represent a suppression of sensitivity because of elevated plasmid copy numbers. General hypotheses to explain our results are presented.