Two previously uncharacterized potential broad-spectrum mercury (Hg) resistance operons (mer) are present on the chromosome of the soil Alphaproteobacteria Xanthobacter autotrophicus Py2. These operons, mer1 and mer2, contain two features which are commonly found in mer operons in the genomes of soil and marine Alphaproteobacteria, but are not present in previously characterized mer operons: a gene for the mercuric reductase (MerA) that encodes an alkylmercury lyase domain typical of those found on the MerB protein, and the presence of an additional gene, which we are calling merK, with homology to glutathione reductase. Here, we demonstrate that Py2 is resistant to 0.2 M inorganic mercury [Hg(II)] and 0.05 M methylmercury (MeHg). Py2 is capable of converting MeHg and Hg(II) to elemental mercury [Hg(0)], and reduction of Hg(II) is induced by incubation in sub toxic concentrations of Hg(II). Transcription of the merA genes increased with Hg(II) treatment, and in both operons merK resides on the same polycistronic mRNA as merA. We propose the use of Py2 as a model system for studying the contribution of mer to Hg mobility in soil and marine ecosystems. Bacterial mercury (Hg) resistance genes (mer) are important drivers in the biogeochemical cycle of Hg. These operons catalyze the conversion of inorganic mercury [Hg(II)] and sometimes methylmercury (MeHg) to elemental mercury [Hg(0)] (1). MeHg, which is more toxic than Hg(II) or Hg(0), is the form that biomagnifies in aquatic food webs (2, 3) and can cause toxicity to humans and animals that consume contaminated fish (4-6). Hg(II) is water soluble but sorbs strongly onto iron oxides and interacts with dissolved organic matter (7). Hg(0), the least toxic form, is both a liquid and a gas at room temperature and can evaporate from surface waters and soils (8, 9).All mer operons contain a gene encoding mercuric reductase, MerA, which converts Hg(II) to Hg(0), thereby conferring resistance (10). Some mer operons, called broad-spectrum mer operons, contain an additional gene encoding MerB, or alkylmercury lyase (AML), which degrades MeHg to Hg(II) and methane (11). Many mer operons contain genes for the transcriptional regulators MerR and MerD (12), as well the Hg transporters encoded by merT, merC, and merF, as well as additional Hg transfer proteins encoded by merP and merE (11, 13). Regulation of mer and the enzymatic activity of MerA is known in great detail for some operons and enzymes (10).It has been demonstrated that mer can influence the Hg cycle in lakes (14), but at present we know much less about the contribution of mer to the Hg cycle in soil and the terrestrial subsurface. It was long thought that Hg was relatively immobile in the subsurface due to sorption on sediment components such as iron oxides (1), but this assumption has come under scrutiny, since Hg has been unexpectedly found in groundwater at numerous sites, such as the Kirkwood-Cohansey aquifer in New Jersey (15, 16), the Waquoit Bay near Cape Cod, MA (17), and the coasts of California, northern ...