Site-specific integration of genes into hot spots for recombination flanking aadA in Tn21 transposons.

Tn21-related transposons are widespread among bacteria and carry various resistance determinants at preferential sites, hs1 and hs2. In an in vivo integrative recombination assay it was demonstrated that these hot spots direct the integration of aminoglycoside resistance genes like aadB from Klebsiella pneumoniae and aacAI from Serratia marcescens, in a recA- background. The maximum required recognition sequence which must be present in both the donor and recipient plasmids is 5' CTAAAACAAAGTTA 3' (hs2). The double-site-specific recombination occurred with a frequency of 10(-5)-10(-6). The resulting structures include not only replicon fusion products but also more complex structures carrying two copies of the donor plasmid or simply the donor gene flanked by hs elements. hs1 and hs2 are thought to act as recognition sites for a transacting site-specific recombinase. By the use of Tn21 deletion derivatives, it has been shown that the recombinase is not encoded by Tn21. This new integrative recombination system is involved in the acquisition of new genes by Tn21-related transposons and their spread among bacterial populations.