Chloramphenicol transposons found in Salmonella naestved and Escherichia coli of domestic animal origin.

Salmonella naestved strain AHI-21, of calf origin, harbors a conjugative R plasmid of group H1, pTE21, which encodes resistance to chloramphenicol (Cm), tetracycline, streptomycin, and sulfadimethoxine. Escherichia coli strain AHI-1, of pig origin, also harbors a conjugative R plasmid of group I alpha, pTE1, which encodes resistance to chloramphenicol and trimethoprim. When either of these R plasmids coexisted with a nonconjugative plasmid, pMK1, which is a composite plasmid of ColE1 and a kanamycin transposon (Tn5), transposition of the Cmr gene into pMK1 occurred independently of the host recA gene function, indicating that both R plasmids contained Cm transposons, Tn3351 and Tn3352. Electron microscopic analysis of self-annealed and heteroduplex molecules showed that they were of approximately 1.7 megadaltons in size and were inserted within the ColE1 loop region of pMK1. However, inverted repeat structures were not seen in these two Cm transposons. Restriction enzyme cleavage analysis showed that both Tn3351 and Tn3352 were indistinguishable in their cleavage patterns, suggesting that they were almost identical in deoxyribonucleic acid sequence despite the difference in their origin. These results suggest that the reciprocal transposition of the Cmr gene might have occurred between Salmonella and E. coli in nature.