4 GENES, 2 ENDS, AND A RES REGION ARE INVOLVED IN TRANSPOSITION OF TN5053 - A PARADIGM FOR A NOVEL FAMILY OF TRANSPOSONS CARRYING EITHER A MER OPERON OR AN INTEGRON

The complete nucleotide sequence of an 8447 bp-long mercury-resistance transposon (Tn5053) has been determined, Tn5053 is composed of two mo dules: (i) the mercury-resistance module and (ii) the transposition mo dule. The mercury-resistance module carries a mer operon, merRTPFAD, a nd appears to be a single-ended relic of a transposon closely related to the classical mercury-resistance transposons Tn21 and Tn501, The tr ansposition module of Tn5053 is bounded by 25 bp terminal inverted rep eats and contains four genes involved in transposition, i.e. tniA, tni S, tniQ, and tniR, Transposition of Tn5053 occurs via cointegrate form ation mediated by the products of the tniABQ genes, followed by site-s pecific cointegrate resolution, This is catalysed by the product of th e 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 inte rchangeable. Sequence analysis suggests that Tn5053 and Tn402 are repr esentatives of a new family of transposable elements, which fall into a recently recognized superfamily of transposons including retroviruse s, insertion sequences of the IS3 family, and transposons Tn552 and Tn 7, We suggest that the tni genes were involved in the dissemination of integrons.