c Cytochromes as Electron Carriers in Microbial Chlorate Respiration

Sammanfattning: Microbial respiration of oxochlorates is important for the biotreatment of effluents from industries where oxochlorates are produced or handled. Several bacterial species are capable to use perchlorate and/or chlorate as an alternative electron acceptor in absence of oxygen. The present study deals with the electron transport from the membrane-bound components to the periplasmic chlorate reductase, in the gram-negative bacterium Ideonella dechloratans. Both chlorate reductase and the terminal oxidase of I. dechloratans were found to utilize soluble c cytochromes as electron donors. For further investigation, two major heme-containing components were purified and characterized. The most abundant was a 9 kDa c-type cytochrome (class I), denoted cytochrome c-Id1. This protein was shown to serve as electron donor for both chlorate reductase, and for a terminal oxidase. The other major component was a 55 kDa homotetrameric cytochrome c', (class II). A function for this cytochrome could not be demonstrated but it does not appear to serve as electron donor to chlorate reductase. A gene predicted to encode a soluble c cytochrome was found in close proximity to the gene cluster for chlorate reduction. The predicted sequence did not match any of the cytochromes discussed above. The gene was cloned and expressed heterologously, and the resulting protein was investigated as a candidate electron donor for chlorate reductase. Electron transfer from this protein could not be demonstrated, suggesting that the gene product does not serve as immediate electron donor for chlorate reductase.