Horizontal dissemination of the genes responsible for resistance to toxic pollutants may play a key role in the adaptation of bacterial populations to environmental contaminants. However, the frequency and extent of gene dissemination in natural environments is not known. A natural horizontal spread of two distinct mercury resistance operon variants, which occurred amongst diverse and related species over wide geographical areas, is reported. One variant encodes a mercuric reductase with a single N-terminal domain, whilst the other encodes a reductase with a duplicated N-terminal domain. The strains containing the former operon types are sensitive to organomercurials, and are most common in the terrestrial mercury-resistant populations studied in this work. The strains containing the latter operon types are resistant to organomercurials, and dominate in a Minamata Bay mercury-resistant population, previously described in the literature. At least three distinct transposons (related to a class II vancomycin-resistance transposon, Tn , from a clinical strain) and conjugative plasmids are implicated as mediators of the spread of these operons.


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