Given that the Calvin–Benson–Bassham (CBB) cycle enzymes downstream of RuBisCO require reducing equivalents, it is an advantage that Hg2+ inhibits RuBisCO, shutting GKT137831 nmr down the CBB cycle, making reducing equivalents available to mercuric reductase. We anticipate that enzymes of the Quayle pathway were inhibited (given the lack of carbon assimilation), forcing oxidation

of formaldehyde and formate to CO2 to generate reducing equivalents to meet requirements of the detoxification. It should be noted that hexulose-3-phosphate synthase (EC 4.1.2.43) – a key enzyme in the Quayle pathway – in M. capsulatus (Bath) is inhibited by Hg2+ at 100 μM (Ferenci et al., 1974). Cytochrome c oxidase was unable to reduce Hg2+ under the assay conditions employed PFT�� cost – either with cytochrome c550 or with ferrocyanide as the cofactor

– the specific activities were zero in both cases. The specific activity of an apparent mercuric reductase (± SEM; n = 7) was 352 (±18) nmol NADH oxidized min−1 (mg protein)−1 or 16 (±2) nmol NADPH oxidized min−1 (mg protein)−1, suggesting that this enzyme may be present. In the literature, NADPH is the more usual cofactor; however, a number of species contain an NADH-dependent enzyme (Gachhui et al., 1997; Meissner & Falkinham, 1984). Blastp interrogation of the GenBank™ database shows that the closest matches to the M. capsulatus (Bath) MerA are those derived from genome sequences of Alicycliphilus denitrificans BC (YP_004126461), Acidovorax sp. JS42 (YP_985596) and Delftia acidovorans SPH-1 (YP_001561514) with 83%, 83% and 81% identity, respectively. It is interesting to note that these are members PLEKHM2 of the Betaproteobacteria, rather than the Gammaproteobacteria. The presence of apparent mercuric reductase activity in M. capsulatus Bath extracts not previously exposed to mercury (II)

indicates that the enzyme is constitutively expressed. RNA microarray data concerning M. capsulatus (Bath) demonstrates that merA and other predicted mercury detoxification genes are expressed during growth as performed here (A. Khalifa, personal communication). We conclude that it is likely that a constitutive, NADH-dependent mercuric reductase is active in M. capsulatus (Bath), with NADH provided at the expense of methane oxidation, although further experiments with inhibitors or knock-out mutants are required to determine whether the merA gene is required for mercury (II) reduction. In the ‘emergency situation’ of mercury (II) exposure, the cell ‘prioritises’ the oxidation of methane to CO2, halting carbon assimilation, presumably to make more NADH available to remove the ion as rapidly as possible by way of a fundamental survival mechanism. Although enzymes of the Quayle pathway and CBB cycle were inhibited – as demonstrated by the complete lack of 14C assimilation – the primary methane oxidation enzymes remained active for over 30 min.