脱氮草螺菌HC18对砷的甲基化及生化与分子机制研究

砷是自然界中分布广泛的有毒元素，也是当今世界首要控制的环境污染物之一。砷的甲基化主要是指在生物体内发生的无机砷转化为单甲基砷、二甲基砷和三甲基砷等衍生物的过程。该过程能够引起砷的形态、毒性和迁移性等变化，是砷元素生物地球化学循环的重要组成部分。在本项目中我们针对水稻土中分离的砷甲基化细菌脱氮草螺菌HC18进行详细研究，菌株HC18能够快速地甲基化三价的单甲基砷到二甲基砷酸盐和三甲基砷氧化物，但甲基化三价砷的速度非常的缓慢。目前已从HC18的基因组中发现了砷甲基转移酶基因簇，其编码典型的砷甲基转移酶（ArsM），仅含有三个正常保守结构域中的两个。该项目的目的是阐明砷甲基化基因簇的功能和表达调控机制，以及ArsM的酶学特性，结构和功能，揭示ArsM催化活性的特性及其在菌株HC18砷甲基过程中的生理功能，为进一步研究研究ArsM在砷甲基化途径中的结构和催化机制提供理论依据。

Arsenic is the most widespread toxic element in the environment, and one of the most serious contaminants in the world. Arsenic methylation mainly occurs in microbes, which transform inorganic arsenic into mono/di/trimethylarsenic species. This process directly influence the species, toxicity and mobility of arsenic. Methylation plays an important role in arsenic biogeochemistry and biogeocycling. We previously isolated an arsenic methylating microbe, Herbaspirillum denitrificans HC18, that is able to rapidly methylate methylarsenite (MMAs(III)) to dimethylarsenate (DMAs(V)) and trimethylarsenic oxide (TMAs(V)), but methylates inorganic As(III) very slowly. An arsenic methylation gene cluster was found in the genome of strain HC18 that encodes a typical As(III)-S-adenosylmethionine methyltransferase (ArsM) with only two of the three normally conserved domains. The goal of this project is to elucidate the function and regulation of the arsenic methylation gene cluster, as well as the enzymatic characteristics, structure and function of ArsM. This project will provide a theoretical basis for further studies of the structure and catalytic mechanism of ArsM in the arsenic methylation pathway.