Molecular Basis\nof Thioredoxin-Dependent Arsenic Transformation\nin Methanogenic Archaea

Methanogenic archaea are known to play a crucial role\nin the biogeochemical\ncycling of arsenic (As); however, the molecular basis of As transformation\nmediated by methanogenic archaea remains poorly understood. Herein,\nthe characterization of the redox transformation and methylation of\nAs by Methanosarcina acetivorans, a\nmodel methanogenic archaeon, is reported. M. acetivorans was demonstrated to mediate As(V) reduction via a cytoplasmic As\nreductase (ArsC) in the exponential phase of methanogenic growth and\nto methylate As(III) via a cytoplasmic As(III) methyltransferase (ArsM)\nin the stationary phase. Characterization of the ArsC-catalyzed As(V)\nreduction and the ArsM-catalyzed As(III) methylation showed that a\nthioredoxin (Trx) encoded by MA4683 was preferentially utilized as\na physiological electron donor for ArsC and ArsM, providing a redox\nlink between methanogenesis and As transformation. The structures\nof ArsC and ArsM complexed with Trx were modeled using AlphaFold-Multimer.\nSite-directed mutagenesis of key cysteine residues at the interaction\nsites of the complexes indicated that the archaeal ArsC and ArsM employ\nevolutionarily distinct disulfide bonds for interacting with Trx compared\nto those used by bacterial ArsC or eukaryotic ArsM. The findings of\nthis study present a major advance in our current understanding of\nthe physiological roles and underlying mechanism of As transformation\nin methanogenic archaea.