Thiosulfate and sulfur oxidation in purple sulfur bacteria

In chemotrophic and phototrophic sulfur oxidizers that do not form sulfur deposits a periplasmic thiosulfate-oxidizing multienzyme complex (Sox complex) has been described to be responsible for formation of sulfate from thiosulfate. In the anoxygenic phototrophic sulfur bacterium Allochromatium vinosum intracellular sulfur globules are an obligate intermediate during the oxidation of thiosulfate to sulfate. Despite this fundamental difference A. vinosum possesses five sox genes in two independent loci (soxBXA and soxYZ) encoding proteins related to components of the Sox complex from Paracoccus pantotrophus. Three sox-encoded proteins were purified from A. vinosum: the heterodimeric c-type cytochrome SoxXA, the monomeric SoxB and the heterodimeric thiosulfate-binding protein SoxYZ. Gene inactivation and complementation studies proved that these proteins are essential for thiosulfate oxidation to sulfate. The intermediary formation of sulfur globules in A. vinosum appears to be related to the lack of soxCD genes, the products of which are proposed to oxidize SoxY-bound sulfane sulfur. In their absence the latter is instead transferred to growing sulfur globules. The oxidation of the stored sulfur is completely dependent on the proteins encoded in the dsr operon. The dissimilatory sulfite reductase (DsrAB) interacts with membrane-bound as well as soluble Dsr proteins. From membranes the protein is copurified with the transmembrane electron-transporting complex DsrMKJOP. Furthermore, the soluble cytoplasmic proteins DsrC and DsrEFH are found in the same fraction, indicating an interaction of DsrC and DsrEFH with the reverse sulfite reductase. From the soluble fraction DsrAB is copurified with DsrL, a homodimeric iron-sulfur flavoprotein with NADH:acceptor oxidoreductase activity. The observed interactions of Dsr proteins serve as a basis for an improved model of sulfur oxidation in purple sulfur bacteria.