Demethylation of the Antibiotic Methylarsenite is Coupled to Denitrification in Anoxic Paddy Soil.

Arsenic (As) biomethylation is an important component of the As biogeochemical cycle, which produces methylarsenite [MAs(III)] as an intermediate product. Its high toxicity is used by some microbes as an antibiotic to kill off other microbes and gain a competitive advantage. Some aerobic microbes have evolved a detoxification mechanism to demethylate MAs(III) via the dioxygenase C-As lyase ArsI. How MAs(III) is demethylated under anoxic conditions is unclear. We found that nitrate addition to a flooded paddy soil enhanced MAs(III) demethylation. A facultative anaerobe sp. CZDM1 isolated from the soil was able to demethylate MAs(III) under anoxic nitrate-reducing conditions. A putative C-As lyase gene () was identified in the genome of strain CZDM1. The expression of in the As-sensitive AW3110 conferred the bacterium the ability to demethylate MAs(III) under anoxic nitrate-reducing condition and enhanced its resistance to MAs(III). Both sp. CZDM1 and AW3110 harboring could not demethylate MAs(III) under fermentative conditions. Five conserved amino acid resides of cysteine, histidine, and glutamic acid are essential for MAs(III) demethylation under anoxic nitrate-reducing conditions. Putative genes are widely present in denitrifying bacteria, with 75% of the sequenced genomes containing , also possessing dissimilatory nitrate reductase genes or . These results reveal a novel mechanism in which MAs(III) is demethylated via ArsI by coupling to denitrification, and such a mechanism is likely to be common in an anoxic environment such as paddy soils and wetlands.