S176: The arsenic biogeocycle: microbial pathways of methylation and demethylation

Environmental microbes affect biogeocycling of arsenic by uptake and efflux, oxidation and reduction, methylation and demethylation. Arsenic methylation is catalyzed by an enzyme (AS3MT or ArsM) that transfers methyl groups from S-adenosylmethionine to trivalent arsenite, forming MAs(III), DMAs(III) and the gaseous final product TMAs(III). ArsM has potential use for arsenic bioremediation, so we cloned and expressed an arsM gene from the soil bacterium Rhodopseudomonas palustris in an arsenic hypersensitive strain of Escherichia coli, where it methylated and volatilized arsenic, conferring resistance, demonstrating that ArsM is sufficient to detoxify arsenic. RparsM was expressed in Oryza sativa, which produced methylated arsenic species, including TMAs(III) gas. Biotransformation of arsenic in transgenic rice is a first step in creation of safer rice for the human food supply. The arsM gene was cloned from the alga Cyanidioschyzon merolae, which forms the major biomass in arsenic-rich hot springs in Yellowstone National Park. CmArsM produces volatile TMAs(III) at 50-70ºC, illustrating the importance of microbes to biogeochemical cycling of arsenic. The crystal structure of purified CmArsM sheds light on its catalytic cycle.

Annually 1.4M kg of MAs(V) is applied commercially in the USA as a pesticide and herbicide. This MAs(V) is demethylated to more toxic and carcinogenic As(III). We identified a Burkhoderia species from golf course soil that reduces MAs(V) to MAs(III) and a Streptomyces species that demethylates MAs(III) to As(III). In mixed culture the two bacteria demethylate MAs(V) to As(III), a novel pathway for demethylation of an organic arsenicals.  (Supported by NIH grant R37 GM55425).