Some Characteristics of the Resistance Transfer Factor (RTF) Episome as Determined by Inactivation with Tritium, P32, and Gamma Radiation

The resistance transfer factor (RTF) episome was studied by measuring its inactivation by Co⁶⁰ gamma radiation, by incorporated P³², and by tritium incorporated as tritium-labeled thymine. The D₃₇ for Co⁶⁰ irradiation was 7 to 9 × 10⁴ rad. Growth of the bacteria harboring the RTF in BUdR (bromouracil deoxyriboside) increased the sensitivity of the RTF to the gamma radiation. The RTF was markedly inactivated by tritium after growth of the host (thymine requiring) bacteria in tritium-labeled thymine, thus further establishing the presence of thymine in the genome of the RTF. Assuming the efficiency of inactivation by P³² to be 10%, the phosphorus content of the RTF was estimated to be about 2 × 10⁵ P atoms/episome. The data suggest the RTF contains double stranded DNA with a molecular weight of the order of 3 to 8 × 10⁷.