The fate of selenium in the environment is controlled, in part, by microbial selenium oxyanion reduction and Se(0) precipitation. In this study, we identified a genetic regulator that controls selenate reductase activity in the Se-reducing bacterium Enterobacter cloacae SLD1a-1. Heterologous expression of the global anaerobic regulatory gene fnr (fumarate nitrate reduction regulator) from E. cloacae in the non-Se-reducing strain Escherichia coli S17-1 activated the ability to reduce Se(VI) and precipitate insoluble Se(0) particles. Se(VI) reduction by E. coli S17-1 containing the fnr gene occurred at rates similar to those for E. cloacae, with first-order reaction constants of k ؍ 2.07 ؋ 10 ؊2 h ؊1 and k ؍ 3.36 ؋ 10 ؊2 h ؊1 , respectively, and produced elemental selenium particles with identical morphologies and short-range atomic orders. Mutation of the fnr gene in E. cloacae SLD1a-1 resulted in derivative strains that were deficient in selenate reductase activity and unable to precipitate elemental selenium. Complementation by the wild-type fnr sequence restored the ability of mutant strains to reduce Se(VI). Our findings suggest that Se(VI) reduction and the precipitation of Se(0) by facultative anaerobes are regulated by oxygen-sensing transcription factors and occur under suboxic conditions.The chemical speciation of selenium controls its mobility in natural waters and biological effect on animal life (5, 27, 44). While selenium is an essential micronutrient (27), the higher valence states of Se(VI) and Se(IV) are toxic at elevated concentrations and can cause severe poisoning of fish and waterfowl in contaminated environments (11,22,32,40). The reduction of soluble selenate [Se(VI), SeO 4 2Ϫ ] and selenite [Se(IV), SeO 3 2Ϫ ] to the less toxic Se(0) converts selenium into an insoluble mineral form. In soils and sediments, this transformation is largely mediated by microorganisms (30,31,43,47), although abiotic reduction of selenium oxyanions can also occur in the presence of the Fe(II)-Fe(III) hydroxide mineral green rust (28). Anaerobic Se-respiring bacteria can use Se(VI) and Se(IV) as terminal electron acceptors and precipitate elemental selenium granules (26,29,30). Aerobic and phototrophic Se-resistant bacteria can also catalyze the reduction of selenium oxyanions to form insoluble Se(0) particles (1,15,35,38). However, despite the ubiquity of Se-reducing bacteria in diverse terrestrial and aquatic environments (P. Narasingarao and M. M. Haggblom, unpublished data), the mechanisms of Se(0) biomineralization are poorly understood.Microbial reduction of selenium oxyanions generates red elemental selenium with either crystalline or amorphous structures (24,25). Recently, Oremland et al. (29) demonstrated that the Se(0) particles formed by the Se-respiring bacteria Sulfurospirillum barnesii, Bacillus selenitireducens, and Selenihalanaerobacter shriftii are structurally unique compared to elemental selenium formed by chemical synthesis. Furthermore, the Se(0) particles precipitated by these three stra...
