An initial characterization of the mercury resistance (mer) system of the thermophilic bacterium Thermus thermophilus HB27

The evolutionary origin of the broadly distributed mer system, which plays an important role in mercury detoxification and biogeochemistry, is presently unknown. The phylum Deinococcus/Thermus was found to be one of the deepest-branching bacterial lineage to have a homolog of merA, which specifies reduction of ionic to elemental mercury, and the mercuric reductase (MerA) of Thermus thermophilus HB27 was found to be basal to all bacterial MerA when this protein's phylogeny was constructed. A merA mutant of HB27 was fourfolds more sensitive to mercury toxicity than the wild type (wt), and lost detectable MerA-specific activities. The merA gene in HB27 was transcribed on a polycistronic message downstream from ORF encoding for homologs of O-acetyl-l-homoserine/O-acetyl-serine (OAH/OAS) sulfhydrylase and MerR, the mer operon transcription regulator, from a promoter located 69 nucleotides upstream of the sulfhydrylase translation start codon. The transcription of the putative mer operon in HB27 was induced 66.8±15.8-fold by exposure to 1 μM HgCl₂. The optimal temperature for MerA-specific activity corresponded to this strain's optimal growth temperature, 70 °C. Thus, T. thermophilus is the earliest mercury-resistant bacterium identified to date, a finding consistent with the hypothesis that the mer system originated among thermophilic microorganisms from geothermal environments.