Sulfate reducers have developed a multifaceted adaptative strategy to survive against oxidative stresses. Along with this oxidative stress response, we recently characterized an elegant reversible disulfide bond-dependent protective mechanism in the pyruvate:ferredoxin oxidoreductase (PFOR) of various Desulfovibrio species. Here, we searched for thiol redox systems involved in this mechanism. Using thiol fluorescent labeling, we show that glutathione is not the major thiol/disulfide balancecontrolling compound in four different Desulfovibrio species and that no other plentiful low molecular weight thiol can be detected. Enzymatic analyses of two thioredoxins (Trxs) and three thioredoxin reductases allow us to propose the existence of two independent Trx systems in Desulfovibrio vulgaris Hildenborough (DvH). The TR1/Trx1 system corresponds to the typical bacterial Trx system. We measured a TR1 apparent K m value for Trx1 of 8.9 M. Moreover, our results showed that activity of TR1 was NADPH-dependent. The second system named TR3/Trx3 corresponds to an unconventional Trx system as TR3 used preferentially NADH (K m for NADPH, 743 M; K m for NADH, 5.6 M), and Trx3 was unable to reduce insulin. The K m value of TR3 for Trx3 was 1.12 M. In vitro experiments demonstrated that the TR1/Trx1 system was the only one able to reactivate the oxygen-protected form of Desulfovibrio africanus PFOR. Moreover, ex vivo pulldown assays using the mutant Trx1 C33S as bait allowed us to capture PFOR from the DvH extract. Altogether, these data demonstrate that PFOR is a new target for Trx1, which is probably involved in the protective switch mechanism of the enzyme.Oxidative stress is a universal phenomenon experienced by both aerobic and anaerobic organisms from all three domains of life (1-3). To combat this problem, anaerobes have evolved multifaceted strategies to manage the deleterious effects of oxygen (O 2 ) exposure. In this regard, these organisms demonstrate varying degrees of tolerance to O 2 , ranging from the extremely sensitive methanogens, which typically are inhibited by only a few ppm of O 2 (4), to the much more aerotolerant Bacteroides (5) or sulfate-reducing Desulfovibrio species (6, 7). The extreme aerotolerance of these anaerobes can be related to their way of life. Abundance and metabolic activity of sulfate reducers in oxic zones of numerous biotopes (reviewed in Ref. 8) are frequently evaluated as higher than the ones found in neighboring anoxic zones. From the last decade, studies have uncovered original and complex adaptative strategies by which sulfate reducers seek to minimize the damage induced by oxidative conditions (8). One example is superoxide reductase, which is specific to anaerobes and scavenges superoxide ions by reduction. Although this enzyme is broadly distributed in sulfatereducing bacteria (9 -12), more species-specific mechanisms are also found as the disulfide bond-mediated protection of the pyruvate:ferredoxin oxidoreductase (PFOR) 3 (13). In Desulfovibrio africanus, this enzyme, which cont...

Although Desulfovibrio vulgaris Hildenborough (DvH) is a strictly anaerobic bacterium, it is able to consume oxygen in different cellular compartments, including extensive periplasmic O 2 reduction with hydrogen as electron donor. The genome of DvH revealed the presence of cydAB and cox genes, encoding a quinol oxidase bd and a cytochrome c oxidase, respectively. In the membranes of DvH, we detected both quinol oxygen reductase [inhibited by heptyl-hydroxyquinoline-N-oxide (HQNO)] and cytochrome c oxidase activities. Spectral and HPLC data for the membrane fraction revealed the presence of o-, b-and d-type haems, in addition to a majority of c-type haems, but no a-type haem, in agreement with carbon monoxide-binding analysis. The cytochrome c oxidase is thus of the cc(o/b)o 3 type, a type not previously described. The monohaem cytochrome c 553 is an electron donor to the cytochrome c oxidase; its encoding gene is located upstream of the cox operon and is 50-fold more transcribed than coxI encoding the cytochrome c oxidase subunit I. Even when DvH is grown under anaerobic conditions in lactate/sulfate medium, the two terminal oxidase-encoding genes are expressed. Furthermore, the quinol oxidase bd-encoding genes are more highly expressed than the cox genes. The cox operon exhibits an atypical genomic organization, with the gene coxII located downstream of coxIV. The occurrence of these membrane-bound oxygen reductases in other strictly anaerobic Deltaproteobacteria is discussed.