Mercury and organomercurial resistances determined by plasmids in Pseudomonas

Mercury and organomercurial resistance determined by genes on ten Pseudomonas aeruginosa plasmids and one Pseudomonas putida plasmid have been studied with regard to the range of substrates and the range of inducers. The plasmidless strains were sensitive to growth inhibition by Hg(2+) and did not volatilize Hg(0) from Hg(2+). A strain with plasmid RP1 (which does not confer resistance to Hg(2+)) similarly did not volatilize mercury. All 10 plasmids determine mercury resistance by way of an inducible enzyme system. Hg(2+) was reduced to Hg(0), which is insoluble in water and rapidly volatilizes from the growth medium. Plasmids pMG1, pMG2, R26, R933, R93-1, and pVS1 in P. aeruginosa and MER in P. putida conferred resistance to and the ability to volatilize mercury from Hg(2+), but strains with these plasmids were sensitive to and could not volatilize mercury from the organomercurials methylmercury, ethylmercury, phenylmercury, and thimerosal. These plasmids, in addition, conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate, and fluorescein mercuric acetate. The other plasmids, FP2, R38, R3108, and pVS2, determined resistance to and decomposition of a range of organomercurials, including methylmercury, ethylmercury, phenylmercury, and thimerosal. These plasmids also conferred resistance to the organomercurials merbromin, p-hydroxymercuribenzoate, and fluorescein mercuric acetate by a mechanism not involving degradation. In all cases, organomercurial decomposition and mercury volatilization were induced by exposure to Hg(2+) or organomercurials. The plasmids differed in the relative efficacy of inducers. Hg(2+) resistance with strains that are organomercurial sensitive appeared to be induced preferentially by Hg(2+) and only poorly by organomercurials to which the cells are sensitive. However, the organomercurials p-hydroxymercuribenzoate, merbromin, and fluorescein mercuric acetate were strong gratuitous inducers but not substrates for the Hg(2+) volatilization system. With strains resistant to phenylmercury and thimerosal, these organomercurials were both inducers and substrates.