Mercury Resistance and Mercuric Reductase Activities and Expression among Chemotrophic Thermophilic Aquificae

Mercury (Hg) resistance (mer ) by the reduction of mercuric to elemental Hg is broadly distributed among theBacteria andArchaea and plays an important role in Hg detoxification and biogeochemical cycling. MerA is the protein subunit of the homodimeric mercuric reductase (MR) enzyme, the central function of themer system. MerA sequences in the phylumAquificae form the deepest-branching lineage in Bayesian phylogenetic reconstructions of all known MerA homologs. We therefore hypothesized that themerA homologs in two thermophilicAquificae ,Hydrogenobaculum sp. strain Y04AAS1 (AAS1) andHydrogenivirga sp. strain 128-5-R1-1 (R1-1), specified Hg resistance. Results supported this hypothesis, because strains AAS1 and R1-1 (i) were resistant to >10 μM Hg(II), (ii) transformed Hg(II) to Hg(0) during cellular growth, and (iii) possessed Hg-dependent NAD(P)H oxidation activities in crude cell extracts that were optimal at temperatures corresponding with the strains' optimal growth temperatures, 55°C for AAS1 and 70°C for R1-1. While these characteristics all conformed with themer system paradigm, expression of theAquificae mer operons was not induced by exposure to Hg(II) as indicated by unity ratios ofmerA transcripts, normalized togyrA transcripts for hydrogen-grown AAS1 cultures, and by similar MR specific activities in thiosulfate-grown cultures with and without Hg(II). The Hg(II)-independent expression ofmer in the deepest-branching lineage of MerA from bacteria whose natural habitats are Hg-rich geothermal environments suggests that regulated expression ofmer was a later innovation likely in environments where microorganisms were intermittently exposed to toxic concentrations of Hg.