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Appl. Environ. Microbiol. doi:10.1128/AEM.02542-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Se(VI) reduction and the precipitation of Se(0) by the facultative bacterium Enterobacter cloacae SLD1a-1 is regulated by FNR

Department of Environmental Sciences, Rutgers, The State University of New Jersey; Department of Plant Biology & Pathology, Rutgers, The State University of New Jersey; Department of Civil and Environmental Engineering, New Jersey Institute of Technology

^* To whom correspondence should be addressed. Email: nyee{at}envsci.rutgers.edu.

	Abstract

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 Enterobacter 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 similar rates as E. cloacae, with first order reaction constants of k = 2.07 x 10^-2 hour^-1 and k = 3.36 x 10^-2 hour^-1 respectively, and produced elemental selenium particles with identical morphologies and short range atomic order. 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 suggests that Se(VI) reduction and the precipitation of Se(0) by facultative anaerobes is regulated by oxygen sensing transcription factors, and occurs under suboxic conditions.

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