The Molecular Basis for Selenate Reduction in Citrobacter freundii

Abstract

The redox cycling of selenium in aquatic environments is strongly affected by microbial activity. Citrobacter freudii is a facultative anaerobic bacterium found in marine and freshwater settings that is known to reduce selenate. In this study, we conducted selenate reduction experiments with C. freundii to investigate the mechanisms involved the selenate reduction process. The results indicate that C. freundii catalyzes the reduction of selenate to elemental selenium in the absence of oxygen. We detected the functional selenate reductase gene ynfE, which is predicted to encode for a molybdenum-binding Tat-secreted protein. A FNR binding site was located immediately upstream of the ynfE gene suggesting the expression of the selenate reductase occurs under anaerobic conditions and is regulated by oxygen-sensing transcription factors. To gain a more complete understanding of the selenate reduction process, we sequenced the genome of C. freundii. The genome analysis revealed the complete selenate reductase operon ynfEGHdmsD, showing high sequence identity to selenate reductases found in related gammaproteobacteria. Genes for molybdate uptake, molybdopterin guanine dinucleotide biosynthesis, twin arginine translocation, and FNR regulation were also identified in the genome sequence. Based on the experimental results and genome sequence, we developed a model to describe anaerobic selenate reduction in C. freundii. The results of this work provide new insights into the molecular basis of selenate reduction activity in a geochemically relevant facultative anaerobic bacterium.