Arsenic biomethylation plays a critical role in modulating environmental arsenic toxicity yet remains understudied in the phylum Bacteroidetes. Here, we characterize HeArsM, a methyltransferase from the soil bacterium Hymenobacter edaphi, which effectively methylates arsenite [As(III)] into various species. We demonstrated that this activity is primarily supported by the thioredoxin (Trx)-thioredoxin reductase (TR)-NADPH system, which is significantly more effective than alternative reductants such as glutathione/glutaredoxin (GSH/Grx), cysteine, or tris(2-carboxyethyl)phosphine (TCEP). Site-directed mutagenesis identified Cys23, Cys48, and Cys143 as essential for catalysis, with Cys143 uniquely required for monomethylarsenite [MMAs(III)] methylation. Structural modeling using AlphaFold and energy minimization supports a thiol-disulfide exchange mechanism as the basis for arsenic methylation. These findings provide mechanistic insight into arsenic detoxification in Bacteroidetes and highlight H. edaphi as a model for understanding microbial arsenic cycling in terrestrial environments.
