Studies on Resistance Transfer Factor Deoxyribonucleic Acid in Escherichia coli

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Abstract
A variant of the derepressed R factor, R1, which does not contain any of the drug resistance markers, and represents, in large part, the resistance transfer factor (RTF) was studied in Escherichia coli . RTF deoxyribonucleic acid (DNA) was specifically labeled in a female cell after conjugation. Physical characterization of the molecule showed that RTF possessed an average molecular weight of 50 × 10 6 daltons and a buoyant density of 1.709 g/cm 3 . By comparison to R1, we calculate that the region of DNA carrying the drug resistance genes is therefore about 20% of the R1 molecule and has a buoyant density of approximately 1.716 g/cm 3 . These results support the hypothesis that the single species of R-factor DNA observed in E. coli represents a composite of the 1.709 and 1.716 g/cm 3 replicons seen in Proteus .