Genetic identification of the selenate reductase in Enterobacter cloacae SLD1a-1

Bacterial selenate reduction is a key microbial process that affects the speciation and mobility of selenium in the environment. In this study, we identified the selenate reductase gene in the facultative anaerobe Enterobacter cloacae SLD1a-1. Genome sequencing revealed a membrane-bound, twin-arginine translocation (TAT) exported molybdoenzyme operon designated as srnABCD, under the regulation of the fumarate and nitrate reductase regulator (FNR) transcription factor. The srnA gene encodes a molybdenum-containing subunit; srnB and srnC encode iron-sulfur and membrane anchor subunits, respectively; and srnD encodes a TAT chaperone. Targeted mutagenesis of the srnA gene resulted in a mutant defective in selenate reduction. Complementation with the wild-type srnA sequence restored the abolished phenotype. Heterologous expression of srnA in an Escherichia coli ΔynfEF mutant conferred selenate reduction activity, demonstrating cross-species functionality. Protein structure modeling of the selenate reductase using Boltz-1 showed a funnel-shaped active site involved in selenate binding and reduction. These findings provide new molecular insights into the genetics and mechanism of bacterial selenate reduction.

Selenium pollution poses risks to ecosystems and human health, largely due to the mobility and toxicity of selenate, a common form found in soil and water. Diverse bacterial species are able to convert soluble selenate into insoluble elemental selenium, but the genes and enzymes that underpin this process are not fully understood. In this study, we identified a gene in Enterobacter cloacae SLD1a-1 that enables the bacterium to catalyze selenate reduction. We showed that this gene produces a functional enzyme even when it is transferred into a different species, Escherichia coli. Protein structure modeling revealed features of the enzyme that help it recognize and reduce selenate. This information advances our understanding of how selenium is enzymatically cycled in the environment.