Key Lab of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences , Xiamen 361021, People's Republic of China.
University of Chinese Academy of Sciences , Beijing 100049, People's Republic of China.
Environ Sci Technol. 2015 Dec 15;49(24):14350-8. doi: 10.1021/acs.est.5b03357. Epub 2015 Nov 18.
Arsenic, a ubiquitous toxic substance, exists mainly as inorganic forms in the environment. It is perceived that organoarsenicals can be demethylated and degraded into inorganic arsenic by microorganisms. Few studies have focused on the mechanism of arsenic demethylation in bacteria. Here, we investigated arsenic demethylation in a typical freshwater cyanobacterium Nostoc sp. PCC 7120. This bacterium was able to demethylate monomethylarsenite [MAs(III)] rapidly to arsenite [As(III)] and also had the ability to demethylate monomethylarsenate [MAs(V)] to As(III). The NsarsI encoding a C·As lyase responsible for MAs(III) demethylation was cloned from Nostoc sp. PCC 7120 and heterologously expressed in an As-hypersensitive strain Escherichia coli AW3110 (ΔarsRBC). Expression of NsarsI was shown to confer MAs(III) resistance through arsenic demethylation. The purified NsArsI was further identified and functionally characterized in vitro. NsArsI existed mainly as the trimeric state, and the kinetic data were well-fit to the Hill equation with K0.5 = 7.55 ± 0.33 μM for MAs(III), Vmax = 0.79 ± 0.02 μM min(-1), and h = 2.7. Both of the NsArsI truncated derivatives lacking the C-terminal 10 residues (ArsI10) or 23 residues (ArsI23) had a reduced ability of MAs(III) demethylation. These results provide new insights for understanding the important role of cyanobacteria in arsenic biogeochemical cycling in the environment.
砷是一种普遍存在的有毒物质，主要以无机形式存在于环境中。人们认为有机胂可以被微生物脱甲基并降解为无机砷。很少有研究关注细菌中砷脱甲基的机制。在这里，我们研究了淡水蓝藻 Nostoc sp. PCC 7120 中的砷脱甲基。该细菌能够迅速将一甲基砷酸盐 [MAs(III)] 脱甲基化为亚砷酸盐 [As(III)]，并且还具有将一甲基砷酸盐 [MAs(V)] 脱甲基化为 As(III)的能力。从 Nostoc sp. PCC 7120 中克隆了编码 C·As 裂合酶的 NsarsI，该酶负责 MAs(III)脱甲基，并在砷超敏感菌株 Escherichia coli AW3110 (ΔarsRBC) 中异源表达。结果表明，NsarsI 的表达通过砷脱甲基赋予 MAs(III)抗性。进一步在体外对纯化的 NsArsI 进行了鉴定和功能表征。NsArsI 主要以三聚体形式存在，动力学数据与 Hill 方程拟合良好，Km0.5 为 7.55 ± 0.33 μM 对于 MAs(III)，Vmax = 0.79 ± 0.02 μM min(-1)，h = 2.7。缺失 C 端 10 个残基 (ArsI10) 或 23 个残基 (ArsI23) 的 NsArsI 截断衍生物的脱甲基能力均降低。这些结果为理解蓝藻在环境中砷生物地球化学循环中的重要作用提供了新的见解。
