ORGANOMERCURIAL RESISTANCE DETERMINANTS IN PSEUDOMONAS-K-62 ARE PRESENT ON 2 PLASMIDS

Pseudomonas strain K-62 was found to contain six plasmids. A mutant de rivative cured of the 26-kb plasmid showed a higher sensitivity to mer curials; however, the strain was still able to volatilize them. Loss o f the 68-kb plasmid in addition to the 26-kb plasmid abolished the abi lity of mercury volatilization in this strain and led to a further dec rease in the level of mercurial resistance. These results are the firs t to demonstrate that the organomercurial resistance or Pseudomonas st rain K-62 is plasmid-based, and that both the 26- and 68-kb plasmids a re required for full expression of the mercurial resistance. Probes sp ecific for the mer genes merA, merB, and merR strongly hybridized with the 26-kb plasmid, but not with the 68-kb plasmid. Two fragments of t he 26-kb plasmid that hybridized with the mer genes were cloned and ex pressed in Escherichia coli. One recombinant plasmid (pMRA17) inducibl y encoded a typical broad-spectrum mercurial resistance, whereas the o ther recombinant plasmid (pMRB01) constitutively conferred hypersensit ivity to phenylmercury in the absence of mercuric reductase activity. The results suggest that the two organomercurial lyases in the cells a re transcribed from different operator-promoters.