The complete nucleotide sequence of an 8447 bp-long mercury-resistance transposon (Tn5053) has been determined. Tn5053 is composed of two modules: (i) the mercury-resistance module and (ii) the transposition module. The mercury-resistance module carries a mer operon, merRTPFAD, and appears to be a single-ended relic of a transposon closely related to the classical mercury-resistance transposons Tn21 and Tn501. The transposition module of Tn5053 is bounded by 25 bp terminal inverted repeats and contains four genes involved in transposition, i.e. tniA, tniB, tniQ, and tniR. Transposition of Tn5053 occurs via cointegrate formation mediated by the products of the tniABQ genes, followed by site-specific cointegrate resolution. This is catalysed by the product of the tniR gene at the res region, which is located upstream of tniR. The same pathway of transposition is used by Tn402 (Tn5090) which carries the integron of R751. Transposition genes of Tn5053 and Tn402 are interchangeable. Sequence analysis suggests that Tn5053 and Tn402 are representatives of a new family of transposable elements, which fall into a recently recognized super-family of transposons including retroviruses, insertion sequences of the IS3 family, and transposons Tn552 and Tn7. We suggest that the tni genes were involved in the dissemination of integrons.

SummaryDNA sequence database search revealed that most of Tn5053/Tn402 family transposons inserted into natural plasmids were located in putative res regions upstream of genes encoding various resolvase-like proteins. Some of these resolvase genes belonged to Tn3 family transposons and were closely related to the tnpR genes of Tn1721 and a recently detected Tn5044. Using recombinant plasmids containing fragments of Tn1721 or Tn5044 as targets in transposition experiments, we have demonstrated that Tn5053 displays striking insertional preference for the res regions of these transposons: more than 70% of Tn5053 insertion events occur in clusters inside the target res regions, while most remaining insertion events occur no further than 200 base pairs away from both sides of the res regions. We demonstrate that Tn5053 insertions (both into and outside a res region of the target plasmid) require the presence of a functional cognate resolvase gene either in cis or in trans. To our knowledge, this is the ®rst case when a site-speci®c recombination system outside a transposon has been shown to be involved in transposition.

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SummaryWe demonstrate that horizontal spread of mer operons similar to worldwide spread of antibiotic-resistance genes in medically important bacteria occurred in bacteria found in ores, soils and waters. The spread was mediated by different transposons and plasmids. Some of the spreading transposons were damaged in different ways but this did not prevent their further spread. Certain transposons are mosaics composed of segments belonging to distinct sequence types. These mosaics arose as a result of homologous and site-specific recombination. Our data suggest that the mercury-resistance operons of Gram-negative environmental bacteria can be considered as a worldwide population composed of a relatively small number of distinct recombining clones shared, at least partially, by environmental and clinical bacteria.

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