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Horizontal spread of mer operons among Gram-positive bacteria in natural environments

¹ Institute of Molecular Genetics, Russian Academy of Sciences, Moscow 123182, Russia
² Center of Microbial Ecology, Michigan State University, East Lansing, MI, USA
³ School of Biological Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

ABSTRACT

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 (mer) operon variants, which occurred amongst diverse Bacillus and related species over wide geographical areas, is reported. One mer 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 mer operon types are sensitive to organomercurials, and are most common in the terrestrial mercury-resistant Bacillus populations studied in this work. The strains containing the latter operon types are resistant to organomercurials, and dominate in a Minamata Bay mercury-resistant Bacillus population, previously described in the literature. At least three distinct transposons (related to a class II vancomycin-resistance transposon, Tn 1546, from a clinical Enterococcus strain) and conjugative plasmids are implicated as mediators of the spread of these mer operons.

Author for correspondence: E. S. Bogdanova. Tel: +7 95 1960208. Fax: +7 95 1960221. e-mail: bogdes@img.ras.ru

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