Genetic studies for selenate and tellurate reduction processes in facultative bacteria

Abstract

The bioavailability and toxicity of selenium in the environment is strongly affected by redox transformations. In this study, we conducted spectroscopic, genetic, and macroscopic chemical measurements to investigate the ability of Citrobacter freundii to catalyze the reduction of soluble selenate oxyanions [Se(VI)] to poorly soluble elemental selenium [Se(0)]. The results indicate that C. freundii forms a red precipitate on selenate containing agar after 48 hours of incubation. In liquid culture, bulk chemical measurements show the removal of selenate oxyanions from solution only after oxygen is completely removed from the bacterial media. Selenate reduction by C. freundii is a substrate specific process and does not simply bind or absorb to cells, as it does not reduce arsenic under the same conditions. X-ray absorption near edge spectra analysis of cell pellets collected from selenate incubations after 7 days show the formation of solid-phase elemental selenium. PCR amplification and DNA sequencing revealed that C. freundii carries the ynfEGH operon, a gene cluster in the DMSO reductase family previously shown to be responsible for selenate reduction in other gamma proteobacteria. Sequence analysis of ynfEGH operon possess a FNR binding site (Fumarate nitrate reduction regulator), and twin-arginine translocation (Tat) signal sequence, The environmental implications of selenate reduction by C. freundii for bioremediation purposes are discussed. The genetic identity and co-factor composition of the bacterial tellurate reductase are currently unknown. In this study, we examined the requirement of molybdopterin biosynthesis and molybdate transporter genes for tellurate reduction by Escherichia coli. The results demonstrate that mutants carrying deletions of the moaA, moaB, moaE, or mog gene in molybdopterin biosynthesis pathway lost the ability to reduce tellurate. Deletion of the modB or modC genes in molybdate transport pathway also resulted in complete loss of tellurate reduction activity. Genetic complementation by the wild-type sequences restored tellurate reduction activity in the mutant strains. These findings provide genetic evidence that the tellurate reductase in E. coli is a molybdopterin-containing enzyme.