Summary: This paper reports the discovery and characterization of Tn, a novel-type transposon vehicle for dissemination of mercury resistance in natural bacterial populations. Tn (14876 bp), identified in a strain from a mercury mine, is a Tn3 family mercury resistance transposon far outside the Tn subgroup. As in other Tn3 family transposons, Tn duplicates 5 bp of the target sequence following insertion. Tn apparently acquired its mer operon as a single-ended relic of a transposon belonging to the classical mercury resistance transposons of the Tn subgroup. The putative transposase and the 47 bp terminal inverted repeats of Tn are closely related to those of the toluene degradative transposon Tn and fall into a distinct subgroup on the fringe of the Tn family. The amino acid sequence of the putative resolvase of Tn resembles site-specific recombinases of the integrase family. Besides the mer operon and putative transposition genes, Tn contains a 4 kb region that accommodates a number of apparently defective genes and mobile elements.


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