The 23 kilobase multiple drug resistance r-determinant (r-det) of the R plasmid NR1 is an IS1-mediated transposon, Tn2671. Drug-resistant transductants isolated after infection with bacteriophage P1::Tn2671 derivatives carry the intact r-det in their chromosomes. Independently isolated transductants carry the r-det at different locations on the chromosome. From the chromosome, Tn2671 can transpose to various locations on the phage P7 genome. Throughout these processes, r-det is maintained as a stable unit. Various possible molecular mechanisms, which all might contribute with characteristic frequencies to the transposition of Tn2671, are discussed. The results presented are relevant to the understanding of mechanisms for a wide spreading of drug resistance genes.


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