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 GBVV, which carries an V-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 GBVV were similar, demonstrating that the low thiosulfate-oxidizing activity of strain GBVV 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. INTRODUCTIONThe reversible formation of protein-disulfide bonds plays an important role in transport of electrons, protein biogenesis, protein stability and enzyme catalysis (Akyama & Ito, 1993;Yamanaka et al., 1994;Okamoto et al., 1995;Missiakis & Raina, 1997; reviewed by Fabianek et al., 2000;Ritz & Beckwith, 2001). Different thiol : disulfide oxidoreductases have been identified which are involved in the formation, isomerization and reduction of disulfide bonds in different bacteria. All these oxidoreductases share a conserved CysXaaXaaCys motif and a common tertiary structure known as the thioredoxin-like fold (Martin, 1995). Members of the DsbD and CcdA families are involved in the transport of electrons from the cytoplasm to the periplasm for reduction of apocytochromes to enable addition of the haem moiety (reviewed by Thöny-Meyer, 1997). The electrons are transferred from a cytoplasmic thioredoxin first to the central transmembrane b-domain of DsbD then to the Cterminal thioredoxin-like c-domain and therefrom to the N-terminal a-domain, which then reduces either DsbC or CcmG (Rietsch et al., 1997;Stewart et al., 1999;Chung et al., 2000;Katzen & Beckwith, 2000).The Gram-negative, facultatively chemolithoautotrophic bacterium Paracoccus pantotrophus grows under aerobic conditions lithotrophically with molecular hydrogen and thiosulfate as energy source for autotrophic carbon dioxide fixation (Robertson & Kuenen, 1983;Rainey et al., 1999). In P. pantotrophus the soxVWXYZABCDEFGH genes constitute two transcriptional units, soxVW and soxX-H (Rother et al., ...