Second EcoRI fragment of F capable of self-replication

The cloning of fragments of F' plasmid deoxyribonucleic acid produced by restriction endonuclease EcoRI has revealed that fragment f7, not previously suspected to have replicative properties, is able to replicate autonomously. The ability of f7 to replicate was observed when it was cloned with fragments coding for resistance to either kanamycin or streptomycin and sulfonamide. Such f7 miniplasmids have been obtained from an F'lac+ and two F'gal+ temperature-sensitive mutant plasmids and from the unmutated F plasmid. Plasmids containing both f5 and f7 fragments were also obtained. Expression of resistance to "female-specific" bacteriophages requires that f5 and f7 be present in the same plasmid since cells containing separate f5 and f7 plasmids are not resistant to bacteriophage phi II. f7 plasmids were less stable than miniplasmids containing f5, particularly at fast growth rates. The bearing of these results on the isolation and behavior of temperature-sensitive F mutants is discussed.