Integration of Chloramphenicol-Resistance Genes of an R Factor into Various Sites of an Escherichia coli Chromosome

Abstract

The chloramphenicol-resistance (Cml^r) determined by an unstable and nontransmissible R factor, R9-4, was lost at high frequency during the subculturing of the R9-4⁺ bacteria. Two stable Cml^r strains were isolated after long term storage of Escherichia coli K12 harboring R9-4. The cml genes governing Cml^r were found to be integrated into the E. coli chromosome: one of them linked closely with met B locus [9] and the other did not. When the R9-4 was introduced by transduction into an HfrC strain and a sexual cross was done between HfrC R9-4⁺ and F^-, Cml^r of R9-4 was transferred at low frequency and the cml genes integrated into the chromosomes were easily obtained in the F^- exconjugants. It was found that they had been integrated into various regions of the E. coli chromosome, near lac, ara, mtl, and gal genes.