Molecular nature of the drug-resistance factors of the enterobacteriaceae

Treatment of bacterial cells harboring this R-factor with either acridine dyes or ultraviolet light was shown to prevent the replication of the episome, so that the episomic DNA was eliminated from such “cured” cells.

Abstract

A drug-resistance episome (R-factor) carrying the determinants of resistance to streptomycin, chloramphenicol, tetracycline and sulfonamide was transferred from an Escherichia coli donor to Proteus mirabilis, E. coli and Serratia marcescens. DNA was isolated from the recombinant strains and analyzed by CsCl density-gradient centrifugation. The R-factor was shown to consist of two species of DNA, the base composition of which is 68% and 52% G + C, respectively. The molecular weight of the R-factor is 25 million daltons; the 58% component of the R-factor comprises about 80% of the episomic DNA, the other 20% being 52% in G + C content. Treatment of bacterial cells harboring this R-factor with either acridine dyes or ultraviolet light was shown to prevent the replication of the episome, so that the episomic DNA was eliminated from such "cured" cells. In addition, it was found that the E. coli K12 substrain (W677) used in these studies carried a naturally occurring DNA species of 64% G + C which was shown to be a plasmid or episome of unknown function. Evidence is presented that there is only one copy of the R-factor per chromosome in E. coli and S. marcescens. In P. mirabilis, however, there appear to be about ten copies of the R-factor per chromosome.