Cloning and comparison of mercury- and organomercurial-resistance determinants from a Pseudomonas stutzeri plasmid

Plasmid pPB confers broad-spectrum mercury resistance (HgR) to a Pseudomonas stutzeri strain. Two pPB regions, separated by 25-30 kb and sharing homology with Tn501 mer (Hg detoxification) genes, were cloned separately and each was shown to carry a cluster of functional and independently regulated mer genes. One of the two gene clusters conferred resistance only to inorganic mercury, and had a structure identical to the classical model of narrow-spectrum mer operons. In the other cluster a novel merB gene, not homologous to the other known merB, but with the same function, was mapped upstream from merA, interposed between an organomercurial-responsive regulatory element and transport genes. Evidence suggests that merB and the other structural mer genes might be transcribed from two distinct promoters. The presence of two inverted repeat-like elements, identical to those of Tn5053, upstream from merR suggests that the pPB broad-spectrum-gene cluster could be part of a transposon-like element.