Novel genes of the sox gene cluster, mutagenesis of the flavoprotein SoxF,and evidence for a general sulfur-oxidizing system in Paracoccus pantotrophus GB17

The novel genes soxFGH were identified, completing the sox gene cluster of Paracoccus pantotrophus coding for enzymes involved in lithotrophic sulfur oxidation, The periplasmic SoxF, SoxG, and SoxH proteins were induced by th iosulfate and purified to homogeneity from the soluble fraction, soxF: code d for a protein of 420 amino acids with a signal peptide containing a twin- arginine motif, SoxF was 37% identical to the flavoprotein FccB of flavocyt ochrome c sulfide dehydrogenase of Allochromatium vinosum. The mature SoxF (42,832 Dal contained 0.74 mot of flavin adenine dinucleotide per moi, soxG coded for a novel protein of 303 amino acids with a signal peptide contain ing a twin-arginine motif, The mature SoxG (29,657 Dal contained two zinc b inding motifs and 0.90 atom of zinc per subunit of the homodimer, soxH code d for a periplasmic protein of 317 amino acids with a double-arginine signa l peptide. The mature SoxH (32,317 Dal contained two metal binding motifs a nd 0.29 atom of zinc and 0.20 atom of copper per subunit of the homodimer, SoxXA, SoxYZ, SoxB, and SoxCD (C, G, Friedrich, A. Quentmeier, F, Bardische wsky, D, Pother, R, Kraft, S, Kostka, and H, Print, J, Bacteriol. 182:4476- 4487, 2000) reconstitute a system able to perform thiosulfate-, sulfite-, s ulfur-, and hydrogen sulfide-dependent cytochrome c reduction, and this sys tem is the first described for oxidizing different inorganic sulfur compoun ds. SoxF slightly inhibited the rate of hydrogen sulfide oxidation but not the rate of sulfite or thiosulfate oxidation, From use of a homogenote muta nt with an in-frame deletion in soxF and complementation analysis, it was e vident that the soxFGH gene products were not required for lithotrophic gro wth with thiosulfate.