Organomercurial Lyase (MerB)-Mediated Demethylation Decreases Bacterial Methylmercury Resistance in the Absence of Mercuric Reductase (MerA)

The mer operon encodes enzymes that transform and detoxify methylmercury (MeHg) and/or inorganic mercury [Hg(II)]. Organomercurial lyase (MerB) and mercuric reductase (MerA) can act sequentially to demethylate MeHg to Hg(II) and reduce Hg (II) to volatile elemental mercury (Hg-0) that can escape from the cell, conferring resistance to MeHg and Hg(II). Most identified mer operons encode either MerA and MerB in tandem or MerA alone; however, microbial genomes were recently identified that encode only MerB. However, the effects of potentially producing intracellular Hg(II) via demethylation of MeHg by MerB, independent of a mechanism to further detoxify or sequester the metal, are not well understood. Here, we investigated MeHg biotransformation in Escherichia coli strains engineered to express MerA and MerB, together or separately, and characterized cell viability and Hg detoxification kinetics when these strains were grown in the presence of MeHg. Strains expressing only MerB are capable of demethylating MeHg to Hg(II). Compared to strains that express both MerA and MerB, strains expressing only MerB exhibit a lower MIC with MeHg exposure, which parallels a redistribution of Hg from the cell-associated fraction to the culture medium, consistent with cell lysis occurring. The data support a model whereby intracellular production of Hg(II), in the absence of reduction or other forms of demobilization, results in a greater cytotoxicity than the parent MeHg compound. Collectively, these results suggest that in the context of MeHg detoxification, MerB must be accompanied by an additional mechanism(s) to reduce, sequester, or redistribute generated Hg(II). IMPORTANCE Mercury is a globally distributed pollutant that poses a risk to wildlife and human health. The toxicity of mercury is influenced largely by microbially mediated biotransformation between its organic (methylmercury) and inorganic [Hg(II) and Hg-0] forms. Here, we show in a relevant cellular context that the organomercurial lyase (MerB) enzyme is capable of MeHg demethylation without subsequent mercuric reductase (MerA)-mediated reduction of Hg(II). Demethylation of MeHg without subsequent Hg(II) reduction results in a greater cytotoxicity and increased cell lysis. Microbes carrying MerB alone have recently been identified but have yet to be characterized. Our results demonstrate that mer operons encoding MerB but not MerA put the cell at a disadvantage in the context of MeHg exposure, unless subsequent mechanisms of reduction or Hg(II) sequestration exist. These findings may help uncover the existence of alternative mechanisms of Hg(II) detoxification in addition to revealing the drivers of mer operon evolution.