Translocation of transposition-deficient (TndPKLH2-like) transposons in the natural environment: mechanistic insights from the study of adjacent DNA sequences.

A family of plasmid-borne DNA fragments of different length, apparently inherited from an ancient plasmid, has been identified in the world population of environmental Acinetobacter strains. These fragments, named PPFs (parental plasmid DNA fragments), were >/=99.8 % identical to each other in the common regions, and contained in their central region a variant of an aberrant mercury-resistance transposon (Tn(d)PKLH2) that has lost its transposition genes. As a rule, recombinogenic elements were found at the breakpoints of identity between the different PPFs. Of these recombinogenic elements, a newly identified IS6 family element, a transposon, or a resolvase gene interrupted one end of the PPFs. At the opposite end, the breakpoint of some PPFs was mapped to the recombination point within, in each case, a different variant of a res site (RS2), whilst in other PPFs, this end was eroded by insertion of a newly identified IS6 family element. On the basis of DNA sequence data, possible mechanisms of translocation of defective Tn(d)PKLH2-like elements via recombination events implicating the nearby res (resolution) site and IS element are proposed.