Physical and genetic map of the organomercury resistance (Omr) and inorganic mercury resistance (Hgr) loci of the IncM plasmid R831b.

Tn7 insertion mutagenesis has been used to facilitate the generation of a physical (restriction endonuclease) and genetic map of the IncM plasmid, R831b. The only selectable phenotypes carried by this 90-kb conjugative plasmid are resistances to inorganic mercury [Hg(II)] and to organomercury compounds. Mutants in the Hgr locus of R831b complemented previously described mutants in the mer operon of the IncFII plasmid R100, indicating functional homology of the locus in each of these different plasmids. However, the R831b Hgr locus is not notably similar in restriction site pattern to either the mer operon of R100 or the mercury resistance transposon, Tn501. Although the enzymes they encode are co-ordinately regulated, the Omr locus of R831b maps approx. 13.5 kb away from the Hgr locus. Three insertions which affect neither phenotype lie between the Hgr and Omr loci; thus, the loci are separated both physically and genetically. One mutant was obtained which tentatively identifies the position of the Tra locus of R831b as adjacent to the Hgr locus.