INDUCTION OF BACTERIAL MERCURY-RESPONSIVE AND COPPER-RESPONSIVE PROMOTERS - FUNCTIONAL DIFFERENCES BETWEEN INDUCIBLE SYSTEMS AND IMPLICATIONS FOR THEIR USE IN GENE-FUSIONS FOR IN-VIVO METAL BIOSENSORS

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 the lacZ gene under the regulation of their normal trans-acting elements. The lux genes of Vibrio 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 beta-galactosidase activity as a measure of lacZ gene expression. In contrast, the induction of P-pcoE was hyposensitive, with an apparent Hill coefficient of 0.63 for induction of beta-galactosidase activity, and this map be related to the role of copper as an essential micronutrient. These response profiles suggest that transcriptional fusions of the merTPAD 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 the pcoE promoter are well suited to determination of the concentrations of copper salts. The comparison of induction profiles of P-merTPSAD using lacZ or lux 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.