Transposon Tn5090 of plasmid R751, which carries an integron, is related to Tn7, Mu, and the retroelements

Abstract

Integrons confer on bacterial plasmids a capability of taking up antibiotic resistance genes by integrase-mediated recombination. We show here that integrons are situated on genetic elements flanked by 25-bp inverted repeats. The element carrying the integron of R751 has three segments conserved with similar elements in Tn21 and Tn5086. Several characteristics suggest that this element is a transposon, which we call Tn5090. Tn5090 was shown to contain an operon with three open reading frames, of which two, tniA and tniB, were predicted by amino acid similarity to code for transposition proteins. The product of tniA (559 amino acids) is a probable transposase with 25% amino acid sequence identity to TnsB from Tn7. Both of these polypeptides contain the D,D(35)E motif characteristic of a protein family made up of the retroviral and retrotransposon IN proteins and some bacterial transposases, such as those of Tn552 and of a range of insertion sequences. Like the transposase genes in Tn552, Mu, and Tn7, the tniA gene was followed by a gene, tniB, for a probable ATP-binding protein. The ends of Tn5090, like those of most other elements producing D,D(35)E proteins, begin by 5'-TG and also contains a complex structure with four 19-bp repeats at the left end and three at the right end. Similarly organized repeats have been observed earlier at the termini of both Tn7 and phage Mu, where they bind their respective transposases and have a role in holoenzyme assembly. Another open reading frame observed in Tn5090, tniC, codes for a recombinase of the invertase/resolvase family, suggesting a replicative transposition mechanism. The data presented here suggest that Tn5090, Tn7, Tn552, and Mu form a subfamily of bacterial transposons which in parallel to many insertion sequences are related to the retroelements.