The gram-negative, strictly anaerobic epsilonproteobacterium Sulfurospirillum multivorans is able to gain energy from dehalorespiration with tetrachloroethene (perchloroethylene [PCE]) as a terminal electron acceptor. The organism can also utilize fumarate as an electron acceptor. Prolonged subcultivation of S. multivorans in the absence of PCE with pyruvate as an electron donor and fumarate as an electron acceptor resulted in a decrease of PCE dehalogenase (PceA) activity. Concomitantly, the pceA transcript level equally decreased as shown by reverse transcriptase PCR. (19). The organism utilizes pyruvate, hydrogen, or formate as an electron donor. When grown with hydrogen or formate as an electron donor and PCE as an electron acceptor, ATP synthesis is coupled to reductive dechlorination via electron transport phosphorylation (dehalorespiration) (8). The key enzyme of PCE utilization that converts PCE via TCE to cis-1,2-dichloroethene (cDCE) is the PCE reductive dehalogenase (PceA). Mature PceA is a monomeric enzyme harboring one norpseudovitamin B 12 and two Fe/S clusters as cofactors (12,15). The cytoplasmic precursor of the PCE dehalogenase (pre-PceA) bears an N-terminal signal peptide of 37 amino acids, including the motif SRRXFXK, which identifies pre-PceA as a substrate of the Tat (twin arginine translocation) protein export pathway (2). Mature PceA is localized in the periplasm, as shown by freeze fracture immunogold labeling techniques (9).The pce operon comprises pceA, the PCE dehalogenase gene, and pceB, encoding a putative membrane integral protein (16). Genes for transcriptional regulation of the pceAB genes are not found in a 6-kb DNA fragment including the pce operon (GenBank accession number AF022812).The ability of reductive dechlorination by anaerobic bacteria was found in phylogenetically distant species (8). Transcriptional regulation of dehalogenase gene expression was described for the 3-chloro-4-hydroxy-phenylacetate (Cl-OHPA) dehalogenase (CprA) of the gram-positive Desulfitobacterium dehalogenans. In this organism, the enzyme was strictly regulated by the absence or presence of the substrate. When the cultures were grown with alternative electron acceptors, such as fumarate or nitrate, CprA transcription ceased within the first subcultivation and was induced within less than 30 min (21). A CRP/FNR-like transcriptional activator, CprK, encoded within the cpr operon, was shown to interact with the DNA upstream of the cpr promoter region only when Cl-OHPA was bound to the protein (18). A direct role of CprK in the induction of dehalogenase gene expression in D. dehalogenans was assumed, and a model for transcriptional regulation of cprA expression was developed (13).While detailed information is available on the control of dehalogenase gene expression in D. dehalogenans grown in the presence of chlorinated aromatic compounds, such as Cl-OHPA, a transcriptional regulation of the PCE dehalogenase genes of Desulfitobacterium species has not been described so far (23). Tsukagoshi and coworker...

Reductive dehalogenation of organohalides is carried out by organohalide-respiring bacteria (OHRB) in anoxic environments. The tetrachloroethene (PCE)-respiring Epsilonproteobacterium Sulfurospirillum multivorans is one of few OHRB able to respire oxygen. Therefore, we investigated the organism's capacity to dehalogenate PCE in the presence of oxygen, which would broaden the applicability to use S. multivorans, unlike other commonly oxygen-sensitive OHRB, for bioremediation, e.g. at oxic/anoxic interphases. Additionally, this has an impact on our understanding of the global halogen cycle. Sulfurospirillum multivorans performs dehalogenation of PCE to cis-1,2-dichloroethene at oxygen concentrations below 0.19 mg/L. The redox potential of the medium electrochemically adjusted up to +400 mV had no influence on reductive dehalogenation by S. multivorans in our experiments, suggesting that higher levels of oxygen impair PCE dechlorination by inhibiting or inactivating involved enzymes. The PCE reductive dehalogenase remained active in cell extracts of S. multivorans exposed to 0.37 mg/L oxygen for more than 96 h. Analysis of the proteome revealed that superoxide reductase and cytochrome peroxidase amounts increased with 5% oxygen in the gas phase, while the response to atmospheric oxygen concentrations involved catalase and hydrogen peroxide reductase. Taken together, our results demonstrate that reductive dehalogenation by OHRB is not limited to anoxic conditions.