Arsenic Methylation by a Sulfate-Reducing Bacterium from Paddy Soil Harboring a Novel ArsSM Fusion Protein

-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.