Three different whole cell biosensor constructs were made by fusing the mercury inducible promoter, P(mer), and its regulatory gene, merR, from transposon Tn21 with reporter genes luxCDABE, lacZYA, or gfp. In Escherichia coli these biosensor constructs responded to low levels of mercury by producing light, beta-galactosidase or green fluorescent protein, respectively. Since the responses were quantitative, the constructs were used to quantify bioavailable mercury in different environments. The constructs were cloned into mini-Tn5 delivery vectors, thus enabling the transfer of the mer-lux, mer-lac or mer-gfp cassettes to a variety of Gram-negative bacteria. The mer-lux cassette was transferred to a Pseudomonas putida strain, which was used to quantify water-extractable mercury in contaminated soil.