Induction of bacterial mercury- and copper-responsive promoters: Functional differences between inducible systems and implications for their use in gene-fusions for in vivo metal biosensors

Summary

We have compared the induction by the cognate metal salts of two promoters responsible for metal-resistance gene expression in bacteria. The mercuric ion resistance promoter, P_merTPAD, of transposon Tn501 and the copper resistance promoter, P_pcoE, from plasmid pRJ1004 were separately cloned to express thelacZ gene under the regulation of their normaltrans-acting elements. Thelux genes ofVibrio fischeri were also expressed from P_merTPAD. The induction of P_merTPAD gave a hypersensitive profile, as reported previously: the apparent Hill coefficient was 2.6 when using β-galactosidase activity as a measure oflacZ gene expression. In contrast, the induction of P_pcoE was hyposensitive, with an apparent Hill coefficient of 0.63 for induction of β-galactosidase activity, and this may be related to the role of copper as an essential micronutrient. These response profiles suggest that transcriptional fusions of themerTPAD promoter allow the construction of strains that are suitable for detecting threshold levels of mercuric ions, but not for accurate determinations of mercuric ion concentrations across a wide range. In contrast, transcriptional fusions to thepcoE promoter are well suited to determination of the concentrations of copper salts. The comparison of induction profiles of P_merTPAD usinglacZ orlux reporter genes, show different stimulus-response curves, probably due to differing instrument sensitivities. These results have practical implications in the construction of whole cell gene-fusion biosensors for the detection and quantitation of heavy metals.