, 1999), formaldehyde dehydrogenase (1 mM in Pseudomonas putida C-83; Ando et al., 1979) and formate dehydrogenase (in Methylosinus trichosporium OB3bT; Jollie & Lipscomb, 1991). Detoxification of the mercuric ion in Bacteria proceeds via a FAD-containing, NAD(P)H-dependent

mercuric reductase (EC 1.16.1.1), catalysing the reaction: The elemental mercury formed is volatile and nonenzymatically Verteporfin molecular weight removed from the cell. Mercuric reductase has been characterized in a number of organisms and is encoded by merA, found in an operon with other genes of mercuric ion detoxification (Ravel et al., 2000). Purified cytochrome c oxidase (aa3 type, EC 1.9.3.1) from Acidithiobacillus ferrooxidans MON-1 catalyses reduced check details cytochrome c-dependent reduction of the mercuric ion (Sugio et al., 2010): The merA gene is predicted in the M. capsulatus (Bath) genome (AAU92601), with a predicted mass 59.3 kDa, similar to that of MerA from A. ferrooxidans, P. putida and Escherichia coli (Booth & Williams, 1984, Sahlman et al., 1984; Rinderle et al., 1983). Other genes of the

mer mercury detoxification system are also predicted. Subunits I–III of an aa3-type cytochrome c oxidase are predicted in the M. capsulatus (Bath) genome (AAU92994, AAU92995, AAU92991, respectively) with 78% identity at protein level of subunit I (AAU92994) to that in A. ferrooxidans ATCC 23270T (ACK79083). Although the biochemistry and genetics of mercury (II) detoxification are well studied, the physiological processes that fuel the process in vivo are not. Here we present strong evidence for the reduction of the mercuric ion to by M. capsulatus (Bath) and the physiological changes in methane oxidation in response to mercury (II). Methylococcus capsulatus (Bath) was obtained from the University of Warwick Culture Collection and maintained as previously described on nitrate mineral salts (NMS) medium (Whittenbury et al., 1970) solidified with 1.5% Oxoid No. 1 agar with methane as sole source of carbon and energy. [14C]-methane (specific

activity 54 mCi mmol−1) was obtained from Amersham Radiochemicals and was diluted in [99% 12C, 1% 13C]-methane (Air Liquid Ltd) in 38-mL serum tubes (Bellco) sealed with blue butyl rubber vaccine stoppers to give working stocks, which were displaced with mercury into gas-tight syringes for SPTLC1 use. All reagents were analytical grade from Sigma-Aldrich. Formaldehyde solutions were prepared by thermal depolymerization of paraformaldehyde (Boden et al., 2010). Nicotinamides were washed with ether before use (Boden et al., 2010). Liquid scintillation cocktails were from Perkin-Elmer. Mercuric chloride was 99.999% pure and obtained from Sigma-Aldrich. Methane of 99.5% chemical purity from Air Liquid Ltd was used throughout. Biomass was determined as previously described (Boden et al., 2010), with calibration curves constructed using cell suspensions of known optical density at 440 nm (OD440 nm) dried to constant weight.