Seven new genes designated dsrLJOPNSR were identified immediately downstream of dsrABEFHCMK, completing the dsr gene cluster of the phototrophic sulfur bacterium Allochromatium vinosum D (DSM 180 T ). Interposon mutagenesis proved an essential role of the encoded proteins for the oxidation of intracellular sulfur, an obligate intermediate during the oxidation of sulfide and thiosulfate. While dsrR and dsrS encode cytoplasmic proteins of unknown function, the other genes encode a predicted NADPH:acceptor oxidoreductase (DsrL), a triheme c-type cytochrome (DsrJ), a periplasmic iron-sulfur protein (DsrO), and an integral membrane protein (DsrP). DsrN resembles cobyrinic acid a,c-diamide synthases and is probably involved in the biosynthesis of siro(heme)amide, the prosthetic group of the dsrAB-encoded sulfite reductase. The presence of most predicted Dsr proteins in A. vinosum was verified by Western blot analysis. With the exception of the constitutively present DsrC, the formation of Dsr gene products was greatly enhanced by sulfide. DsrEFH were purified from the soluble fraction and constitute a soluble ␣ 2 ␤ 2 ␥ 2 -structured 75-kDa holoprotein. DsrKJO were purified from membranes pointing at the presence of a transmembrane electron-transporting complex consisting of DsrKMJOP. In accordance with the suggestion that related complexes from dissimilatory sulfate reducers transfer electrons to sulfite reductase, the A. vinosum Dsr complex is copurified with sulfite reductase, DsrEFH, and DsrC. We therefore now have an ideal and unique possibility to study the interaction of sulfite reductase with other proteins and to clarify the long-standing problem of electron transport from and to sulfite reductase, not only in phototrophic bacteria but also in sulfate-reducing prokaryotes.Phototrophic purple and green sulfur-oxidizing bacteria use sulfur compounds as electron donors for reductive carbon dioxide fixation during photolithotrophic growth (7, 10). In these organisms, light energy is used to transfer electrons from sulfur compounds to the level of the more highly reducing electron carriers NAD(P) ϩ and ferredoxin. In our laboratory we seek to understand oxidative sulfur metabolism in anoxygenic phototrophic bacteria by using the genetically accessible ␥-proteobacterium Allochromatium vinosum (formerly Chromatium vinosum [34]) as our model organism. It is a purple sulfur bacterium belonging to the family Chromatiaceae. A. vinosum carries out the complete eight-electron oxidations of sulfide and thiosulfate to sulfate. Intracellularly stored sulfur globules are an obligate intermediate in the process (57). The sulfur in the globules is present in the molecular structure of sulfur chains (58) and is enclosed by a protein envelope, a feature shared by most if not all of the chemotrophic sulfur-oxidizing bacteria that form intracellular sulfur globules (9, 14, 51). Topologically, the sulfur globules of A. vinosum and probably of other members of the Chromatiaceae are located extracytoplasmically, in the periplasm (51).O...

The dsr genes and the hydSL operon are present as separate entities in phototrophic sulfur oxidizers of the genera Allochromatium, Marichromatium, Thiocapsa and Thiocystis and are organized similarly as in Allochromatium vinosum and Thiocapsa roseopersicina, respectively. The dsrA gene, encoding the alpha subunit of 'reverse' siroheme sulfite reductase, is also present in two species of green sulfur bacteria pointing to an important and universal role of this enzyme and probably other proteins encoded in the dsr locus in the oxidation of stored sulfur by phototrophic bacteria. The hupSL genes are uniformly present in the members of the Chromatiaceae family tested. The two genes between hydS and hydL encode a membrane-bound b-type cytochrome and a soluble iron-sulfur protein, respectively, resembling subunits of heterodisulfide reductase from methanogenic archaea. These genes are similar but not identical to dsrM and dsrK, indicating that the derived proteins have distinct functions, the former in hydrogen metabolism and the latter in oxidative sulfur metabolism.