Sulfate reduction via a trisulfide: Microorganisms can respire sulfur compounds in the absence of oxygen, eventually leading to the production of hydrogen sulfide. This ancient metabolism is common in modern anoxic environments, but the enzymatic pathways aren't yet fully resolved. Through in vivo and in vitro experiments, Santos et al. clarify the enzymology of the sulfate reduction pathway in both bacteria and archaea (see the Perspective by Fritz and Kroneck). Reduction of the sulfite intermediate results in the linkage of two cysteine residues to a third sulfur atom from sulfite, forming a trisulfide product. Because the reduction of sulfite conveys a strong isotopic signature on sulfur in the environment, isotope fractionation models should account for this additional step. Science , this issue p. 1541 ; see also p. 1476

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 α ₂ β ₂ γ 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.

The sequence of the dsr gene region of the phototrophic sulfur bacterium Chromatium vinosum D (DSMZ 180^T) was determined to clarify the in vivo role of ‘reverse’ sirohaem sulfite reductase. The dsrAB genes encoding dissimilatory sulfite reductase are part of a gene cluster, dsrABEFHCMK, that encodes four small, soluble proteins (DsrE, DsrF, DsrH and DsrC), a transmembrane protein (DsrM) with similarity to haem-b-binding polypeptides and a soluble protein (DsrK) resembling [4Fe---4S]-cluster-containing heterodisulfide reductase from methanogenic archaea. Northern hybridizations showed that expression of the dsr genes is increased by the presence of reduced sulfur compounds. The dsr genes are not only transcribed from a putative promoter upstream of dsrA but primary transcripts originating from (a) transcription start site(s) downstream of dsrB are also formed. Polar insertion mutations immediately upstream of dsrA, and in dsrB, dsrH and dsrM, led to an inability of the cells to oxidize intracellularly stored sulfur. The capability of the mutants to oxidize sulfide, thiosulfate and sulfite under photolithoautotrophic conditions was unaltered. Photoorganoheterotrophic growth was also unaffected. ‘Reverse’ sulfite reductase and DsrEFHCMK are, therefore, not essential for oxidation of sulfide or thiosulfate, but are obligatory for sulfur oxidation. These results, together with the finding that the sulfur globules of C. vinosum are located in the extracytoplasmic space whilst the dsr gene products appear to be either cytoplasmic or membrane-bound led to the proposal of new models for the pathway of sulfur oxidation in this phototrophic sulfur bacterium.