Class II broad-spectrum mercury resistance transposons in Gram-positive bacteria from natural environments

We have studied the mechanisms of the horizontal dissemination of a broad-s pectrum mercury resistance determinant among Bacillus and related species. This mer determinant was first described in Bacillus cereus RC607 from Bost on Harbor, USA, and was then found in various Bacillus and related species in Japan, Russia and England. We have shown that the mer determinant can ei ther be located at the chromosome, or on a plasmid in the Bacillus species, and is carried by class II mercury resistance transposons: Tn5084 from B. cereus RC607 and B. cereus VKM684 (ATCC10702) and Tn5085 from Exiguobacteri um sp. TC38-2b. Tn5085 is identical in nucleotide sequence to TnMER11, the only other known met transposon from Bacillus species, but it does not cont ain an intron like TnMER11. Tn5085 is functionally active in Escherichia co li. Tn5083, which we have isolated from B. megaterium MK64-1, contains an R C607-like mer determinant, that has lost some mercury resistance genes and possesses a merA gene which is a novel sequence variant that has not been p reviously described. Tn5083 and Tn5084 ase recombinants, and are comprised of fragments from several transposons including Tn5085, and a relative of a putative transposon from B. firmus (which contains similar genes to the ca dmium resistance operon of Staphylococcus aureus), as well as others. The s equence data showed evidence for recombination both between transposition g enes and between met determinants. (C) 2001 Editions scientifiques et medic ales Elsevier SAS.