Effects of Hg, CH(3)-Hg, and Temperature on the Expression of Mercury Resistance Genes in Environmental Bacteria.

Twenty different bacterial isolates obtained from a mercury-contaminated site in Oak Ridge, Tenn., were grown on plate count agar amended with 25 mug of Hg or 3 mug 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 degrees C) with the presence of Hg (25 mug/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 mug of Hg per ml, but only 8 isolates showed increased production of merB transcripts. The MICs of Hg and R-Hg for 10 methyl mercury-resistant isolates ranged from 45 to 110 mug of Hg and 0.6 to 4.5 mug of R-Hg per ml. R-Hg was able to induce the expression of merB in 70% of methyl mercury-resistant strains.