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

Abstract

We have studied the mechanisms of the horizontal dissemination of a broad-spectrum mercury resistance determinant among Bacillus and related species. This mer determinant was first described in Bacillus cereus RC607 from Boston 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 either be located at the chromosome, or on a plasmid in the Bacillus species, and is carried by class II mercury resistance transposons: Tn 5084 from B. cereus RC607 and B. cereus VKM684 (ATCC10702) and Tn 5085 from Exiguobacterium sp. TC38-2b. Tn 5085 is identical in nucleotide sequence to Tn MERI1, the only other known mer transposon from Bacillus species, but it does not contain an intron like Tn MERI1. Tn 5085 is functionally active in Escherichia coli. Tn 5083, which we have isolated from B. megaterium MK64-1, contains an RC607-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 previously described. Tn 5083 and Tn 5084 are recombinants, and are comprised of fragments from several transposons including Tn 5085, and a relative of a putative transposon from B. firmus (which contains similar genes to the cadmium resistance operon of Staphylococcus aureus), as well as others. The sequence data showed evidence for recombination both between transposition genes and between mer determinants.