Effects of Hg 2+ , CH 3 -Hg + , and Temperature on the Expression of Mercury Resistance Genes in Environmental Bacteria

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Abstract
Twenty different bacterial isolates obtained from a mercury-contaminated site in Oak Ridge, Tenn., were grown on plate count agar amended with 25 μg of Hg 2+ or 3 μg of CH 3 -Hg + (R-Hg + ) per ml. The total cellular RNA was extracted from each isolate by an acid-guanidine-thiocyanate-phenol-chloroform method. The transcripts of merA and merB were detected and quantitated by Northern (RNA) hybridization. A qualitative assay of mercuric reductase was used to confirm the enzyme activity. Low temperature (4°C) with the presence of Hg 2+ (25 μg/ml) significantly increased the net merA transcripts of mid-log-phase cells of six environmental isolates. The net merA transcript production by 18 of the isolates increased when they were grown on 50% plate count broth with 15 μg of Hg 2+ per ml, but only 8 isolates showed increased production of merB transcripts. The MICs of Hg 2+ and R-Hg + for 10 methyl mercury-resistant isolates ranged from 45 to 110 μg of Hg 2+ and 0.6 to 4.5 μg of R-Hg + per ml. R-Hg + was able to induce the expression of merB in 70% of methyl mercury-resistant strains.