Tn1525, a kanamycin R determinant flanked by two direct copies of IS15

We have isolated plasmid pIP112 (IncI1) from Salmonella panama and characterized by restriction endonucleases analysis and by recombinant DNA techniques a transposable element designated Tn1525. This 4.44 kilobase (kb) transposon confers resistance to kanamycin by synthesis of an aminoglycoside phosphotransferase (3') (5") type I and contains two copies of IS15 (1.5 kb) in direct orientation. The modular organisation of Tn1525 offers the possibility for intramolecular homologous recombination between the two terminal direct repeats and thus accounts for the in vivo structural lability of plasmid pIP112: instability of kanamycin resistance and tandem amplification of the kanamycin determinant. Other transposons mediating resistance to kanamycin by the same enzymatic mechanism were analysed by agarose and polyacrylamide gel electrophoresis, following digestion with restriction endonucleases, and by Southern hybridizations. These comparisons indicate that, although the structural genes for the phosphotransferases are homologous, Tn1525 differs from Tn903 and Tn2350 and is closely related but distinct from Tn6. Using the same techniques Tn1525 was detected on plasmids belonging to different incompatibility groups and originating from various species of Gram-negative clinical isolates. These results indicate that Tn1525 is representative of a new family of class I composite transposons already spread in diverse pathogenic bacterial genera.