Department of Environmental Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York, USA.
Department of Microbiology and Cell Biology, Montana State Universitygrid.41891.35, Bozeman, Montana, USA.
Appl Environ Microbiol. 2022 Mar 22;88(6):e0001022. doi: 10.1128/aem.00010-22. Epub 2022 Feb 9.
The 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) that can escape from the cell, conferring resistance to MeHg and Hg(II). Most identified 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). 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] 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 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 operon evolution.
操纵子编码的酶能够转化和解毒甲基汞（MeHg）和/或无机汞[Hg（II）]。有机汞裂解酶（MerB）和汞还原酶（MerA）可以依次作用，将 MeHg 去甲基化为 Hg（II），并将 Hg（II）还原为可挥发的元素汞（Hg），从而从细胞中逸出，从而对 MeHg 和 Hg（II）产生抗性。大多数已鉴定的操纵子编码 MerA 和 MerB 串联或 MerA 单独存在;然而，最近鉴定出的微生物基因组仅编码 MerB。然而，通过 MerB 将 MeHg 去甲基化产生细胞内 Hg（II）的影响，而不考虑进一步解毒或隔离金属的机制，尚不清楚。在这里，我们研究了在表达 MerA 和 MerB 的大肠杆菌菌株中 MeHg 的生物转化，这些菌株要么一起表达，要么单独表达，并在有 MeHg 存在的情况下培养这些菌株时，对细胞活力和 Hg 解毒动力学进行了表征。只表达 MerB 的菌株能够将 MeHg 去甲基化为 Hg（II）。与同时表达 MerA 和 MerB 的菌株相比，只表达 MerB 的菌株在暴露于 MeHg 时具有较低的 MIC，这与从细胞相关部分向培养基中转移 Hg 相一致，这与细胞裂解的发生相一致。数据支持这样一种模型，即在没有还原或其他形式的脱毒作用的情况下，细胞内产生的 Hg（II）会导致比母体 MeHg 化合物更大的细胞毒性。总的来说，这些结果表明，在 MeHg 解毒的情况下，MerB 必须伴随有另外的机制来减少、隔离或重新分配生成的 Hg（II）。汞是一种在全球范围内分布的污染物，对野生动物和人类健康构成威胁。汞的毒性在很大程度上受到其有机（甲基汞）和无机[Hg（II）和 Hg]形式之间微生物介导的生物转化的影响。在这里，我们在相关的细胞环境中表明，有机汞裂解酶（MerB）酶能够在没有随后的汞还原酶（MerA）介导的 Hg（II）还原的情况下将 MeHg 去甲基化。没有随后的 Hg（II）还原的 MeHg 去甲基化导致更大的细胞毒性和增加的细胞裂解。最近已经鉴定出携带 MerB 但不携带 MerA 的微生物，但尚未进行表征。我们的结果表明，在 MeHg 暴露的情况下，编码 MerB 但不编码 MerA 的操纵子使细胞处于不利地位，除非存在随后的还原或 Hg（II）隔离机制。这些发现可能有助于揭示除了揭示操纵子进化的驱动因素之外，还可能揭示 Hg（II）解毒的替代机制的存在。
