FIG. 5.

Model for sulfate reduction by D. vulgaris derived from the effects of nitrite on gene expression in the wild type. Transmembrane electron transport complexes qmoABC and dsrMKJOP donate electrons to the ApsBA- and DsrAB-catalyzed reactions, which consume two and six protons, respectively (not indicated). Periplasmic protons resulting from hydrogen cycling or other proton translocation mechanisms reenter the cell through ATP synthase, leading to additional ATP synthesis. The 2²/₃ ATP indicated is the theoretically maximal value. When nitrite is present, periplasmic protons and electrons are used by NrfHA to reduce nitrite to ammonia. The model suggests that this lowers the transmembrane proton gradient driving ATP synthase as well as transmembrane electron transport through qmoABC and dsrMKJOP. Nitrite inhibits the reduction of sulfite to sulfide by DsrAB, which slowly reduces nitrite to ammonia. Enzyme abbreviations: Sat, sulfate adenylyltransferase; PpaC, pyrophosphatase; ApsBA, adenosine phosphosulfate reductase; DsrAB, dissimilatory sulfite reductase; QmoABC, quinone-interacting membrane-bound oxidoreductase; DsrMKJOP, triheme cytochrome c-containing membrane-bound oxidoreductase; Ldh, lactate dehydrogenase; Por, pyruvate:ferredoxin oxidoreductase; Pta, phosphate transacetylase; Ack, acetate kinase; c₃ network, network of periplasmic, tetrahemic c-type cytochromes (8). APS, adenosine phosphosulfate.