Jiangsu Key Laboratory for Organic Waste Utilization, Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China.
Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China.
Environ Sci Technol. 2024 Oct 29;58(43):19266-19276. doi: 10.1021/acs.est.4c04730. Epub 2024 Oct 15.
Microbial arsenic (As) methylation is an important process of As biogeochemistry. Only a few As-methylating microorganisms have been isolated from paddy soil, hindering the mechanistic understanding of the process involved. We isolated 54 anaerobic and 32 aerobic bacteria from paddy soil with a high As methylation potential. Among the 86 isolates, 14 anaerobes, including 7 sulfate-reducing bacteria (SRB), but none of the aerobes were able to methylate arsenite [As(III)] or monomethylarsenite [MMA(III)] or both, suggesting that the As-methylating ability is much more prevalent in anaerobes than in aerobes. We performed a detailed investigation on As methylation by a SRB isolate, sp. TC1, and identified a novel bifunctional enzyme consisting of a fusion of As(III) -adenosylmethionine (SAM) methyltransferase (ArsM) and a radical SAM protein. The enzyme (ArsSM) can catalyze As(III) methylation to MMA and DMA and subsequent adenosylation of DMA to form 5'-deoxy-5'-dimethylarsinoyl-adenosine (DDMAA), which is a key intermediate in the biosynthesis of arsenosugars. High concentrations of sulfide produced by SRB did not affect As(III) methylation to MMA but inhibited MMA methylation to DMA. Genes encoding ArsSM fusion proteins are widespread in anaerobes, particularly SRB, suggesting that ArsSM-carrying anaerobes may play an important role in As methylation in an anoxic environment.
微生物砷（As）甲基化是砷生物地球化学的一个重要过程。尽管从稻田土壤中分离到了少数能够进行砷甲基化的微生物，但这阻碍了对相关过程的机制理解。我们从具有高砷甲基化潜力的稻田土壤中分离到 54 株厌氧菌和 32 株需氧菌。在 86 株分离菌中，有 14 株厌氧菌（包括 7 株硫酸盐还原菌 [SRB]）能够将亚砷酸盐 [As(III)]或一甲基砷酸盐 [MMA(III)]或两者同时甲基化，但没有一株需氧菌能够做到，这表明砷甲基化能力在厌氧菌中比在需氧菌中更为普遍。我们对一株 SRB 分离菌， sp. TC1 的砷甲基化作用进行了详细研究，并鉴定出一种新型双功能酶，它由砷（III）-腺苷甲硫氨酸（SAM）甲基转移酶（ArsM）和一种自由基 SAM 蛋白融合而成。该酶（ArsSM）可以催化 As(III)甲基化为 MMA 和 DMA，随后将 DMA 腺苷酰化形成 5'-去氧-5'-二甲基砷酰基-腺苷（DDMAA），这是砷糖生物合成中的一个关键中间产物。SRB 产生的高浓度的硫化物不会影响 As(III)甲基化为 MMA，但会抑制 MMA 甲基化为 DMA。编码 ArsSM 融合蛋白的基因在厌氧菌中广泛存在，特别是在 SRB 中，这表明携带 ArsSM 的厌氧菌可能在缺氧环境中的砷甲基化过程中发挥重要作用。
