The mercury(II)-resistance transposons Tn502 and Tn512 were sequenced and shown to be members of the Tn5053 family. They are currently the sole representatives from the clinical setting and were obtained from geographically disparate Pseudomonas aeruginosa strains. The family is comprised of six novel transposons that display genetic and structural variability that has arisen in different ways. The mer and tni arms of the transposons can be differently combined, suggesting that chimeric interchanges have occurred, possibly mediated by the TniR resolvase. The mer modules within the mer arms are remnants of Tn21/Tn501-like transposons that inserted into a tni-containing ancestral transposon. The mer modules themselves are polymorphic and that of Tn502 is a new type. It includes the putative urf2M gene, the 3'-end of which expresses a protein and hence is a bone fide gene (tniM). Homologues of tniM occur beyond the Tn5053 family and include Tn21 tnpM. Other studies have implicated tniM and tnpM in transposition. Based on sequence and compositional differences, members of the environmentally widespread Tn5053 family constitute a different lineage from the related and clinically successful intI-containing Tn402 family.