Identification and characterization of a second copy number control gene in mini-F plasmids

We previously reported the existence of a series of chemically induced trans recessive copy-number mutations (cop) for mini-F plasmids and the existence of a similar series of cop mutations induced by insertion of the ampicillin resistance transposon Tn3. In this paper we describe the experiments showing that these two series of mutations are in different genes. Briefly, the experiments show that the one mutant series can complement the other, that the mutations map in distinct but adjacent regions, that the copy numbers of double mutants are the products of the copy numbers determined by the single mutations, and that Tn3 does not elevate copy number by a polar effect on the adjacent cop gene defined by chemical mutagenesis. We term the latter gene copA and the gene mutated by Tn3, copB. We also demonstrate here that copB mutations are recessive to the wild type allele. Further, we have characterized copB by deletion and recombinational analysis as the series of five 19- to 22-base-pair directly repeated sequences that had previously been designated incC-that is, one of the incompatibility genes. The evidence for this conclusion is that plasmids lacking two, three or five direct repeats have their copy number elevated proportionately. Possible mechanisms for copB control of replication are discussed.