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

Lehrstuhl für Technische Mikrobiologie, Fachbereich Bio- und Chemieingenieurwesen, Universität Dortmund, D-44221 Dortmund, Germany Correspondence Cornelius G. Friedrich cornelius.friedrich{at}udo.edu The soxVW genes are located upstream of the sox gene cluster encoding the sulfur-oxidizing ability of Paracoccus pantotrophus . SoxV is highly homologous to CcdA, which is involved in cytochrome c maturation of P. pantotrophus . SoxV was shown to function in reduction of the periplasmic SoxW, which shows a CysXaaXaaCys motif characteristic for thioredoxins. From strain GB V, which carries an -kanamycin-resistance-encoding interposon in soxV , and complementation analysis it was evident that SoxV but not the periplasmic SoxW was essential for lithoautotrophic growth of P. pantotrophus with thiosulfate. However, the thiosulfate-oxidizing activities of cell extracts from the wild-type and from strain GB V were similar, demonstrating that the low thiosulfate-oxidizing activity of strain GB V in vivo was 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 that soxVW genes are exclusively present in sox operons harbouring the soxCD genes, encoding sulfur dehydrogenase, suggesting that SoxCD might be a redox partner of SoxV. No complementation of the ccdA mutant P. pantotrophus TP43 defective in cytochrome c maturation was achieved by expression of soxV in trans , demonstrating that the high identity of SoxV and CcdA does not correspond to functional homology. Abbreviations: AMS, 4-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid; Sox, sulfur oxidation; TCEP, tris(2-carboxyethyl)phosphine; TMPD, N , N , N ', N '-tetramethyl-1,4-benzenediamine