Abstract
BackgroundAchromobacter sp. AO22 (formerly Alcaligenes sp. AO22), a bacterial strain isolated from a lead-contaminated industrial site in Australia, was previously found to be resistant to moderate to high levels of mercury, copper and other heavy metals. However, the nature and location of the genetic basis for mercuric ion resistance in this strain, had not been previously identified.FindingsAchromobacter sp. AO22 contains a functional mer operon with all four essential genes (merRTPA) and shows >99% DNA sequence identity to that of Tn501. The mer operon was present on a transposon, designated TnAO22, captured by introducing a broad-host-range IncP plasmid into Achromobacter sp. AO22 and subsequently transferring it to E. coli recipients. The transposition frequency of TnAO22 was 10-2 to 10-3 per target plasmid transferred. Analysis of TnAO22 sequence revealed it belonged to the Tn21 subgroup of the Tn3 superfamily of transposons, with the transposition module having >99% identity with Tn5051 of a Pseudomonas putida strain isolated from a water sample in New York.ConclusionTnAO22 is thus a new variant of Tn5051 of the Tn3 superfamily and the transposon and its associated mercury resistance system are among the few such systems reported in a soil bacterium. Achromobacter sp. AO22 can thus be exploited for applications such as in situ mercury bioremediation of contaminated sites, or the mobile unit and mer operon could be mobilized to other bacteria for similar purposes.
Highlights
AO22 can be exploited for applications such as in situ mercury bioremediation of contaminated sites, or the mobile unit and mer operon could be mobilized to other bacteria for similar purposes
These are commonly located on mobile genetic elements such as plasmids, transposons or modules of recombinant structures, some reside chromosomally [6,7]. mer transposons frequently belong to the Tn3 family where the members are typically flanked by 38 bp inverted repeats (IRs) and contain two genes, tnpR and tnpA, encoding the enzymes resolvase and transposase, respectively, and a resolution site at which site-specific recombination occurs to resolve the cointegrates formed during transposition [8]
A number of variations on the genetic organisation of mer operons from Gram negative bacteria have been reported, but most contain the essential genes merRTPA with optional accessory genes and open reading frames (ORFs). merR encodes the transcriptional regulator of the operon, merT and merP encode a Hg (II) transport system across the cell membrane and merA encodes mercuric reductase that reduces the toxic Hg(II) to elemental Hg(0) in the cytoplasm which is released into the environment
Summary
TnAO22 is a new variant of Tn5051 of the Tn3 superfamily and the transposon and its associated mercury resistance system are among the few such systems reported in a soil bacterium. AO22 can be exploited for applications such as in situ mercury bioremediation of contaminated sites, or the mobile unit and mer operon could be mobilized to other bacteria for similar purposes
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