Generation of miniplasmids from copy number mutants of the R plasmid NR1

Abstract

Small, closed circular deoxyribucleic acid molecules, called miniplasmids, were observed in Escherichia coli harboring copy number mutants of the R plasmid NR1 after growth in medium containing tetracycline. The level of tetracycline resistance conferred by the copy mutant plasmids was lower (3 to 6 μg/ml) than that conferred by NR1 (100 μg/ml). The presence of the miniplasmid enhanced the level of tetracycline resistance conferred by the copy mutant. Miniplasmids of molecular weights 4 x 106 to 13 x 106 were found. They carried no antibiotic resistance markers and could be eliminated by growth in the presence of chloramphenicol and/or streptomycin-spectinomycin. Studies with the restriction endonucleases EcoRI and SalI indicated that the miniplasmids are derived from the region of the copy mutant plasmids that contains the origin for replication of the resistance transfer factor. There were ~12 copies of the miniplasmid per chromosome, compared with 3 to 6 copies of the copy mutants of NR1. The miniplasmids appeared to be incompatible with the copy mutant plasmids.