Association of As(III) Methyltransferases and Radical S-Adenosylmethionine Enzymes

Microbial arsenic methylation catalyzed by the enzyme arsenite (As(III)) S-adenosylmethionine methyltransferase (ArsM) plays a pivotal role in the arsenic biogeochemical cycle. This produces methylated species and, when coupled with radical SAM (rSAM) enzymes, produces more complex compounds such as arsenosugars and the antibiotic arsinothricin (AST). In this study, we identified an arsM from Chloracidobacterium thermophilum that encodes the enzyme CtArsM and is adjacent to a gene for an rSAM enzyme. Most ArsMs have three domains for SAM binding, As(III) binding, and a C-terminal domain of uncertain function. CtArsM has only the first two domains. CtArsM methylates methylarsenite (MAs(III)) but not As(III) and confers resistance to MAs(III) when expressed in Escherichia coli. At 45 °C, CtArsM rapidly converts 95% of MAs(III) into volatile trimethylarsine (TMAs(III)). Mutational analysis indicates that conserved cysteine residues Cys134 and Cys183 form the MAs(III) binding site, while Cys137 is not required but increases the amount of volatilized arsenic. Mutation of Cys137 shifts the product profile, primarily resulting in dimethylarsinic acid (DMAs(V)). These findings suggest that cysteine modification can modulate ArsM activity, providing a basis for potential applications in arsenic bioremediation. Moreover, ArsM enzymes adjacent to rSAM genes may act synergistically in the biosynthesis of novel organoarsenical antibiotics.