The operon encodes an essential and modular electron transfer pathway for extracellular iodate reduction by MR-1.

Extracellular iodate reduction by spp. contributes to iodide generation in the biogeochemical cycling of iodine. However, there is a disagreement on whether spp. use different extracellular electron transfer pathways with dependence on electron donors in iodate reduction. In this study, a series of gene deletion mutants of MR-1 were created to investigate the roles of , and operons in iodate reduction. The iodate-reducing activity of the mutants was tested with lactate, formate, and H as the sole electron donors, respectively. In the absence of single- gene, iodate reduction efficiency of the mutants was only 12.9%-84.0% with lactate at 24 hours, 22.1%-85.9% with formate at 20 hours, and 19.6%-57.7% with H at 42 hours in comparison to complete reduction by the wild type. Progressive inhibition of iodate reduction was observed when the homolog from the operon was deleted in the single- gene mutants. This result revealed complementation of by at the single-gene level, indicating modularity of the extracellular electron transfer pathway encoded by operon. Under the conditions of all electron donors, significant inhibition of iodate reduction and accumulation of HO were detected for Δ. Collectively, these results demonstrated that the operon encodes an essential and modular iodate-reducing pathway without electron donor dependence in MR-1. The operon was involved in HO elimination with all electron donors. The findings in this study improved the understanding of molecular mechanisms underlying extracellular iodate reduction.IMPORTANCEIodine is an essential trace element for human and animals. Recent studies revealed the contribution of microbial extracellular reduction of iodate in biogeochemical cycling of iodine. Multiple reduced substances can be utilized by microorganisms as energy source for iodate reduction. However, varied electron transfer pathways were proposed for iodate reduction with different electron donors in the model strain MR-1. Here, through a series of gene deletion and iodate reduction experiments, we discovered that the operon was essential for iodate reduction with at least three electron donors, including lactate, formate, and H. The operon was first demonstrated to be capable of complementing the function of at single-gene level.