Abstract
Mercury (Hg) resistance (
mer
) by the reduction of mercuric to elemental Hg is broadly distributed among the
Bacteria
and
Archaea
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 the
mer
system. MerA sequences in the phylum
Aquificae
form the deepest-branching lineage in Bayesian phylogenetic reconstructions of all known MerA homologs. We therefore hypothesized that the
merA
homologs in two thermophilic
Aquificae
,
Hydrogenobaculum
sp. strain Y04AAS1 (AAS1) and
Hydrogenivirga
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 the
mer
system paradigm, expression of the
Aquificae
mer
operons was not induced by exposure to Hg(II) as indicated by unity ratios of
merA
transcripts, normalized to
gyrA
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 of
mer
in the deepest-branching lineage of MerA from bacteria whose natural habitats are Hg-rich geothermal environments suggests that regulated expression of
mer
was a later innovation likely in environments where microorganisms were intermittently exposed to toxic concentrations of Hg.