Low-Molecular-Weight Thiols and Thioredoxins Are Important Players in Hg(II) Resistance in Thermus thermophilus HB27

Mercury (Hg), one of the most toxic and widely distributed heavy metals, has a high affinity for thiol groups. Thiol groups reduce and sequester Hg. Therefore, low-molecular-weight (LMW) and protein thiols may be important cell components used in Hg resistance. To date, the role of low-molecular-weight thiols in Hg detoxification remains understudied. The mercury resistance (mer ) operon ofThermus thermophilus suggests an evolutionary link between Hg(II) resistance and low-molecular-weight thiol metabolism. Themer operon encodes an enzyme involved in methionine biosynthesis, Oah. Challenge with Hg(II) resulted in increased expression of genes involved in the biosynthesis of multiple low-molecular-weight thiols (cysteine, homocysteine, and bacillithiol), as well as the thioredoxin system. Phenotypic analysis of gene replacement mutants indicated that Oah contributes to Hg resistance under sulfur-limiting conditions, and strains lacking bacillithiol and/or thioredoxins are more sensitive to Hg(II) than the wild type. Growth in the presence of either a thiol-oxidizing agent or a thiol-alkylating agent increased sensitivity to Hg(II). Furthermore, exposure to 3 μM Hg(II) consumed all intracellular reduced bacillithiol and cysteine. Database searches indicate thatoah2 is present in allThermus sp.mer operons. The presence of a thiol-related gene was also detected in some alphaproteobacterialmer operons, in which a glutathione reductase gene was present, supporting the role of thiols in Hg(II) detoxification. These results have led to a working model in which LMW thiols act as Hg(II)-buffering agents while Hg is reduced by MerA.IMPORTANCE The survival of microorganisms in the presence of toxic metals is central to life's sustainability. The affinity of thiol groups for toxic heavy metals drives microbe-metal interactions and modulates metal toxicity. Mercury detoxification (mer ) genes likely originated early in microbial evolution in geothermal environments. Little is known about howmer systems interact with cellular thiol systems.Thermus spp. possess a simplemer operon in which a low-molecular-weight thiol biosynthesis gene is present, along withmerR andmerA . In this study, we present experimental evidence for the role of thiol systems in mercury resistance. Our data suggest that, inT. thermophilus , thiolated compounds may function side by side withmer genes to detoxify mercury. Thus, thiol systems function in consort withmer -mediated resistance to mercury, suggesting exciting new questions for future research.