Arsenic\nMethylation\nby a Sulfate-Reducing Bacterium\nfrom Paddy Soil Harboring a Novel ArsSM Fusion Protein

Microbial\narsenic (As) methylation is an important process\nof As\nbiogeochemistry. Only a few As-methylating microorganisms have been\nisolated from paddy soil, hindering the mechanistic understanding\nof the process involved. We isolated 54 anaerobic and 32 aerobic bacteria\nfrom paddy soil with a high As methylation potential. Among the 86\nisolates, 14 anaerobes, including 7 sulfate-reducing bacteria (SRB),\nbut none of the aerobes were able to methylate arsenite [As(III)]\nor monomethylarsenite [MMA(III)] or both, suggesting that the As-methylating\nability is much more prevalent in anaerobes than in aerobes. We performed\na detailed investigation on As methylation by a SRB isolate, Solidesulfovibrio sp. TC1, and identified a novel\nbifunctional enzyme consisting of a fusion of As(III) <i>S</i>-adenosylmethionine (SAM) methyltransferase (ArsM) and a radical\nSAM protein. The enzyme (ArsSM) can catalyze As(III) methylation to\nMMA and DMA and subsequent adenosylation of DMA to form 5′-deoxy-5′-dimethylarsinoyl-adenosine\n(DDMAA), which is a key intermediate in the biosynthesis of arsenosugars.\nHigh concentrations of sulfide produced by SRB did not affect As(III)\nmethylation to MMA but inhibited MMA methylation to DMA. Genes encoding\nArsSM fusion proteins are widespread in anaerobes, particularly SRB,\nsuggesting that ArsSM-carrying anaerobes may play an important role\nin As methylation in an anoxic environment.