Summary: The tetracycline-resistant strains lu121 and fr16017, recently isolated in West Germany, each harbour a plasmid; that of the former (pLU121) has a mol. wt of 31.5 x 10 and that of the latter (pFR16017) has a mol. wt of 33 x 10. Conjugation and DNA-DNA hybridization studies have shown that both plasmids are self-transmissible and carry tetracycline-resistance genes. The purified plasmid DNA of strain lu121 transformed a sensitive strain to tetracycline resistance. The two R factors are closely related to the plasmid specifying ampicillin resistance (pKRE5367). Electron microscope DNA heteroduplex analysis indicated that pLU121 and pFR16017 probably carry the tetracycline-resistance transposon TnD and that pKRE5367 probably carries the ampicillin-resistance transposon TnA. There is more than one integration site for the insertion which probably represents TnD in pFR16017. All three plasmids have a similar plasmid core and could have a common evolutionary origin.


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