SPREAD AND EVOLUTION OF NATURAL PLASMIDS HARBORING TRANSPOSON TN5

The presence of transposon Tn5 was studied in 730 Enterobacteriaceae s trains from clinical and sewage origin. From these strains, twenty-fiv e conjugative plasmids harboring transposon Tn5 were isolated. These p lasmids were compared with pJR67 and pRYC119, the only previously stud ied plasmids harboring Tn5. A phylogenetic tree of the evolution of al l different plasmids was proposed. Irrespective of their bacterial hos t and geographical place of isolation, some of the plasmids were shown to be identical. All of them can be included in only eight different prototypical plasmid species. Twenty-two plasmids (88%) carried an Inc Il incompatibility determinant as judged from DNA hybridization experi ments. The presence of some other common resistance genes suggested th at these plasmids are descendants of a common ancestor. These IncIl pl asmids could be grouped in six prototypical species. The results prese nted here suggest that Tn5 spread in nature may be dependent on the co njugative ability of the IncI plasmids harboring the transposon, rathe r than on the efficiency of Tn5 transposition between different replic ons.