Jiangsu Provincial Key Laboratory for Organic Solid Waste Utilization, Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing, 210095, China.
Department of Cellular Biology and Pharmacology, Herbert Wertheim College of Medicine, Florida International University, Miami, FL, USA.
Mol Microbiol. 2018 Jan;107(2):265-276. doi: 10.1111/mmi.13882. Epub 2017 Nov 23.
Arsenic (As) biomethylation is an important component of the As biogeochemical cycle that can influence As toxicity and mobility in the environment. Biomethylation of As is catalyzed by the enzyme arsenite (As[III]) S-adenosylmethionine methyltransferase (ArsM). To date, all identified ArsM orthologs with As(III) methylation activities have four conserved cysteine residues, which are thought to be essential for As(III) methylation. Here, we isolated an As(III)-methylating bacterium, Bacillus sp. CX-1, and identified a gene encoding a S-adenosylmethionine methyltranserase termed BlArsM with low sequence similarities (≤ 39%) to other ArsMs. BlArsM has six cysteine residues (Cys10, Cys11, Cys145, Cys193, Cys195 and Cys268), three of which (Cys10, Cys145 and Cys195) align with conserved cysteine residues found in most ArsMs. BlarsM is constitutively expressed in Bacillus sp. CX-1. Heterologous expression of BlarsM conferred As(III) resistance. Purified BlArsM methylated both As(III) and methylarsenite (MAs[III]), with a final product of dimethylarsenate (DMAs[V]). When all six cysteines were individually altered to serine residues, only C145S and C195S derivatives lost the ability to methylate As(III) and MAs(III). The derivative C10S/C11S/C193S/C268S was still active. These results suggest that BlArsM is a novel As(III) S-adenosylmethionine methyltransferase requiring only two conserved cysteine residues. A model of As(III) methylation by BlArsM is proposed.
砷（As）的生物甲基化是砷生物地球化学循环的一个重要组成部分，它可以影响砷在环境中的毒性和迁移性。砷的生物甲基化是由亚砷酸盐（As[III]）S-腺苷甲硫氨酸甲基转移酶（ArsM）催化的。迄今为止，所有具有 As(III)甲基化活性的已鉴定的 ArsM 同源物都有四个保守的半胱氨酸残基，这些残基被认为是 As(III)甲基化所必需的。在这里，我们分离出一株能进行 As(III)甲基化的细菌，芽孢杆菌 sp. CX-1，并鉴定出一个编码 S-腺苷甲硫氨酸甲基转移酶的基因，称为 BlArsM，它与其他 ArsM 的序列相似性（≤39%）较低。BlArsM 有六个半胱氨酸残基（Cys10、Cys11、Cys145、Cys193、Cys195 和 Cys268），其中三个（Cys10、Cys145 和 Cys195）与大多数 ArsM 中发现的保守半胱氨酸残基对齐。BlarsM 在芽孢杆菌 sp. CX-1 中组成型表达。BlarsM 的异源表达赋予了 As(III)抗性。纯化的 BlArsM 甲基化了 As(III)和甲基砷酸盐（MAs[III]），最终产物为二甲砷酸（DMAs[V]）。当所有六个半胱氨酸残基分别突变为丝氨酸残基时，只有 C145S 和 C195S 衍生物失去了甲基化 As(III)和 MAs(III)的能力。C10S/C11S/C193S/C268S 衍生物仍然具有活性。这些结果表明，BlArsM 是一种新型的 As(III) S-腺苷甲硫氨酸甲基转移酶，仅需要两个保守的半胱氨酸残基。提出了 BlArsM 进行 As(III)甲基化的模型。
