A novel dimethylsulfoxide reductase family of molybdenum enzyme, Idr, is involved in iodate respiration by Pseudomonas sp. SCT.

Pseudomonas sp. strain SCT is capable of using iodate (IO3-) as a terminal electron acceptor for anaerobic respiration. A possible key enzyme, periplasmic iodate reductase (Idr), was visualized by active staining on non-denaturing gel electrophoresis. Liquid chromatography-tandem mass spectrometry analysis revealed that at least four proteins, designated as IdrA, IdrB, IdrP(1), and IdrP(2), were involved in Idr. IdrA and IdrB were homologues of catalytic and electron transfer subunits of respiratory arsenite oxidase (Aio); however, IdrA defined a novel clade within the dimethylsulfoxide (DMSO) reductase family. IdrP(1) and IdrP(2) were closely related to each other and distantly related to cytochrome c peroxidase. The idr genes (idrABP( 1)P( 2)) formed an operon-like structure, and their transcription was upregulated under iodate-respiring conditions. Comparative proteomic analysis also revealed that Idr proteins and high affinity terminal oxidases (Cbb(3) and Cyd), various H2O2 scavengers, and chlorite (ClO2-) dismutase-like proteins were expressed specifically or abundantly under iodate-respiring conditions. These results suggest that Idr is a respiratory iodate reductase, and that both O-2 and H2O2 are formed as by-products of iodate respiration. We propose an electron transport chain model of strain SCT, in which iodate, H2O2, and O-2 are used as terminal electron acceptors.