We report a novel pathway for arsenic detoxification in the legume symbiont
Sinorhizobium meliloti. Although a majority of arsoperons consist of three genes, arsR(transcriptional regulator), arsB[As(OH)3/H+ antiporter], and arsC(arsenate reductase), the S. meliloti arsoperon includes an aquaglyceroporin ( aqpS) in place of arsB. The presence of AqpS in an arsenic resistance operon is interesting, since aquaglyceroporin channels have previously been shown to adventitiously facilitate uptake of arsenite into cells, rendering them sensitive to arsenite. To understand the role of aqpSin arsenic resistance, S. meliloti aqpSand arsCwere disrupted individually. Disruption of aqpSresulted in increased tolerance to arsenite but not arsenate, while cells with an arsCdisruption showed selective sensitivity to arsenate. The results of transport experiments in intact cells suggest that AqpS is the only protein of the S. meliloti arsoperon that facilitates transport of arsenite. Coexpression of S. meliloti aqpSand arsCin a strain of E. colilacking the arsoperon complemented arsenate but not arsenite sensitivity. These results imply that, when S. melilotiis exposed to environmental arsenate, arsenate enters the cell through phosphate transport systems and is reduced to arsenite by ArsC. Internally generated arsenite flows out of the cell by downhill movement through AqpS. Thus, AqpS confers arsenate resistance together with ArsC-catalyzed reduction. This is the first report of an aquaglyceroporin with a physiological function in arsenic resistance.