The number of sequenced genomes of sulfate reducing organisms (SRO) has increased significantly in the recent years, providing an opportunity for a broader perspective into their energy metabolism. In this work we carried out a comparative survey of energy metabolism genes found in 25 available genomes of SRO. This analysis revealed a higher diversity of possible energy conserving pathways than classically considered to be present in these organisms, and permitted the identification of new proteins not known to be present in this group. The Deltaproteobacteria (and Thermodesulfovibrio yellowstonii) are characterized by a large number of cytochromes c and cytochrome c-associated membrane redox complexes, indicating that periplasmic electron transfer pathways are important in these bacteria. The Archaea and Clostridia groups contain practically no cytochromes c or associated membrane complexes. However, despite the absence of a periplasmic space, a few extracytoplasmic membrane redox proteins were detected in the Gram-positive bacteria. Several ion-translocating complexes were detected in SRO including H+-pyrophosphatases, complex I homologs, Rnf, and Ech/Coo hydrogenases. Furthermore, we found evidence that cytoplasmic electron bifurcating mechanisms, recently described for other anaerobes, are also likely to play an important role in energy metabolism of SRO. A number of cytoplasmic [NiFe] and [FeFe] hydrogenases, formate dehydrogenases, and heterodisulfide reductase-related proteins are likely candidates to be involved in energy coupling through electron bifurcation, from diverse electron donors such as H2, formate, pyruvate, NAD(P)H, β-oxidation, and others. In conclusion, this analysis indicates that energy metabolism of SRO is far more versatile than previously considered, and that both chemiosmotic and flavin-based electron bifurcating mechanisms provide alternative strategies for energy conservation.

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Microbial sulfate reduction has governed Earth's biogeochemical sulfur cycle for at least 2.5 billion years. However, the enzymatic mechanisms behind this pathway are incompletely understood, particularly for the reduction of sulfite-a key intermediate in the pathway. This critical reaction is performed by DsrAB, a widespread enzyme also involved in other dissimilatory sulfur metabolisms. Using in vitro assays with an archaeal DsrAB, supported with genetic experiments in a bacterial system, we show that the product of sulfite reduction by DsrAB is a protein-based trisulfide, in which a sulfite-derived sulfur is bridging two conserved cysteines of DsrC. Physiological studies also reveal that sulfate reduction rates are determined by cellular levels of DsrC. Dissimilatory sulfate reduction couples the four-electron reduction of the DsrC trisulfide to energy conservation.

Sulfate reduction is one of the earliest types of energy metabolism used by ancestral organisms to sustain life. Despite extensive studies, many questions remain about the way respiratory sulfate reduction is associated with energy conservation. A crucial enzyme in this process is the dissimilatory sulfite reductase (dSiR), which contains a unique siroheme-[4Fe4S] coupled cofactor. Here, we report the structure of desulfoviridin from Desulfovibrio vulgaris, in which the dSiR DsrAB (sulfite reductase) subunits are bound to the DsrC protein. with its conserved C-terminal cysteine reaching the distal side of the siroheme. We propose a novel mechanism for the process of sulfite reduction involving DsrAB, DsrC, and the DsrMKJOP membrane complex (a membrane complex with putative disulfide/thiol reductase activity), in which two of the six electrons for reduction of sulfite derive from the membrane quinone pool. These results show that DsrC is involved in sulfite reduction, which changes the mechanism of sulfate respiration. This has important implications for models used to date ancient sulfur metabolism based on sulfur isotope fractionations.The dissimilatory reduction of sulfur compounds is one of the earliest energy metabolisms detected on earth, at ϳ3.5 billion years ago (1, 2). At the end of the Archean (ϳ2.7 billion years ago), the advent of oxygenic photosynthesis led to a gradual increase in the levels of atmospheric oxygen, which in turn caused an increasing flux of sulfate to the oceans from weathering of sulfide minerals on land (3). As a consequence of this process, reduction of sulfate became a dominant biological process in the oceans, resulting in sulfidic anoxic conditions from about 2.5 to 0.6 billion years ago (3, 4). During this extended period, sulfate-reducing prokaryotes were main players in marine habitats where most evolutionary processes were taking place. Today, these organisms are still major contributors to the biological carbon and sulfur cycles, and their activities have important environmental and economic consequences.A key enzyme in sulfur-based energy metabolism is the dissimilatory sulfite reductase (dSiR), 3 which is present in organisms that reduce sulfate, sulfite, and other sulfur compounds. This enzyme is also found in some phototrophic and chemotrophic sulfur oxidizers, where it is proposed to operate in the reverse direction (reverse sulfite reductase, rSiR). The dSiR is minimally composed of two subunits, DsrA and DsrB, in an ϳ200-kDa ␣ 2 ␤ 2 arrangement. The dsrA and dsrB genes are paralogous and most likely arose from a very early gene duplication event that preceded the separation of the archaea and bacteria domains (5-8), in agreement with a very early onset of biological sulfite reduction. The dSiR belongs to a family of proteins that also include the assimilatory sulfite (aSiR) and nitrite (aNiR) reductases, the monomeric low molecular mass aSiRs, and other dSiRs like asrC and Fsr (9 -11). This family has in common a characteristic cofactor assembly that includes an iro...

DsrC is a small protein present in organisms that dissimilate sulfur compounds, working as a physiological partner of the DsrAB sulfite reductase. DsrC contains two redox active cysteines in a flexible carboxy-terminal arm that are involved in the process of sulfite reduction or sulfur(1) compound oxidation in sulfur-reducing(2) or sulfur-oxidizing(3) organisms, respectively. In both processes, a disulfide formed between the two cysteines is believed to serve as the substrate of several proteins present in these organisms that are related to heterodisulfide reductases of methanogens. Here, we review the information on DsrC and its possible physiological partners, and discuss the idea that this protein may serve as a redox hub linking oxidation of several substrates to dissimilative sulfur metabolism. In addition, we analyze the distribution of proteins of the DsrC superfamily, including TusE that only requires the last Cys of the C-terminus for its role in the biosynthesis of 2-thiouridine, and a new protein that we name RspA (for regulatory sulfur-related protein) that is possibly involved in the regulation of gene expression and does not need the conserved Cys for its function. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.