SoxV transfers electrons to the periplasm of Paracoccus pantotrophus – an essential reaction for chemotrophic sulfur oxidation

Abstract

ThesoxVWgenes are located upstream of thesoxgene cluster encoding the sulfur-oxidizing ability ofParacoccus pantotrophus. SoxV is highly homologous to CcdA, which is involved in cytochromecmaturation ofP. pantotrophus. SoxV was shown to function in reduction of the periplasmic SoxW, which shows a CysXaaXaaCys motif characteristic for thioredoxins. From strain GBΩV, which carries an Ω-kanamycin-resistance-encoding interposon insoxV, and complementation analysis it was evident that SoxV but not the periplasmic SoxW was essential for lithoautotrophic growth ofP. pantotrophuswith thiosulfate. However, the thiosulfate-oxidizing activities of cell extracts from the wild-type and from strain GBΩV were similar, demonstrating that the low thiosulfate-oxidizing activity of strain GBΩVin vivowas not due to a defect in biosynthesis or maturation of proteins of the Sox system and suggesting that SoxV is part of a regulatory or catalytic system of the Sox system. Analysis of DNA sequences available from different organisms harbouring a Sox system revealed thatsoxVWgenes are exclusively present insoxoperons harbouring thesoxCDgenes, encoding sulfur dehydrogenase, suggesting that SoxCD might be a redox partner of SoxV. No complementation of theccdAmutantP. pantotrophusTP43 defective in cytochromecmaturation was achieved by expression ofsoxV in trans, demonstrating that the high identity of SoxV and CcdA does not correspond to functional homology.