Novel genes coding for lithotrophic sulfur oxidation of Paracoccus pantotrophus GB17

The gene region coding for lithotrophic sulfur oxidation of Paracoccus pant otrophus GB17 is located on a 13-kb insert of plasmid pEG12. Upstream of th e previously described six open reading frames (ORFs) soxABCDEF with a part ial sequence of soxA and soxF (C, Wodara, F. Bardischewsky, and C. G. Fried rich, J. Bacteriol. 179:5014-5023, 1997), 4,350 bp were sequenced. The sequ ence completed soxA, and uncovered six new ORFs upstream of soxA, designate d ORF1, ORF2, and ORF3, and soxXYZ. ORF1 could encode a 275-amino-acid poly peptide of 29,332 Da with a 61 to 63% similarity to LysR transcriptional re gulators. ORF2 could encode a 245-amino-acid polypeptide of 26,022 Da with the potential to form six transmembrane helices and with a 48 to 51% simila rity to proteins involved in redox transport in cytochrome c biogenesis. OR F3 could encode a periplasmic polypeptide of 186 amino acids of 20,638 Da w ith a similarity to thioredoxin-like proteins and with a putative signal pe ptide of 21 amino acids. Purified SoxXA, SoxYZ, and SoxB are essential for thiosulfate or sulfite-dependent cytochrome c reduction in vitro. N-termina l and internal amino acid sequences identified SoxX, SoxY, SoxZ, and SoxA t o be coded by the respective genes. The molecular masses of the mature prot eins determined by electrospray ionization spectroscopy (SoxX, 14,834 Da; S oxY, 11,094 Da; SoxZ, 11,717 Da; and SoxA, 30,452 Da) were identical or clo se to those deduced from the nucleotide sequence with differences for the c ovalent heme moieties, SoxXA represents a novel type of periplasmic c-type cytochromes, with SoxX as a monoheme and SoxA as a hybrid diheme cytochrome c. SoxYZ is an as-yet-unprecedented soluble protein. SoxY has a putative s ignal peptide with a twin arginine motif and possibly cotransports SoxZ to the periplasm. SoxYZ neither contains a metal nor a complex redox center, a s proposed for proteins likely to be transported via the Tat system.