Characterisation of the 11 Kb DNA region adjacent to the gene encodingDesulfovibrio gigasflavoredoxin

Structural and biochemical characterization of flavoredoxin from the archaeon Methanosarcina acetivorans

Flavoredoxin is a FMN-containing electron transfer protein that functions in the energy-yielding metabolism of Desulfovibrio gigas of the Bacteria domain. Although characterization of this flavoredoxin is the only one reported, a database search revealed homologues widely distributed in both the Bacteria and Archaea domains that define a novel family. To improve our understanding of this family, a flavoredoxin from Methanosarcina acetivorans of the Archaea domain was produced in Escherichia coli and biochemically characterized, and a high-resolution crystal structure was determined. The protein was shown to be a homodimer with a subunit molecular mass of 21 kDa containing one noncovalently bound FMN per monomer. Redox titration showed an E(m) of -271 mV with two electrons, consistent with no semiquinone observed in the potential range studied, a result suggesting the flavoredoxin functions as a two-electron carrier. However, neither of the obligate two-electron carriers, NAD(P)H and coenzyme F420H2, was a competent electron donor, whereas 2[4Fe-4S] ferredoxin reduced the flavoredoxin. The X-ray crystal structure determined at 2.05 A resolution revealed a homodimer containing one FMN per monomer, consistent with the biochemical characterization. The isoalloxazine ring of FMN was shown buried within a narrow groove approximately 10 A from the positively charged protein surface that possibly facilitates interaction with the negatively charged ferredoxin. The structure provides a basis for predicting the mechanism by which electrons are transferred between ferredoxin and FMN. The FMN is bound with hydrogen bonds to the isoalloxazine ring and electrostatic interactions with the phosphate moiety that, together with sequence analyses of homologues, indicate a novel FMN binding motif for the flavoredoxin family.

Deletion of flavoredoxin gene inDesulfovibrio gigasreveals its participation in thiosulfate reduction

The gene encoding Desulfovibrio gigas flavoredoxin was deleted to elucidate its physiological role in the sulfate metabolism. Disruption of flr gene strongly inhibited the reduction of thiosulfate and exhibited a reduced growth in the presence of sulfite with lactate as electron donor. The growth with sulfate was not however affected by the lack of this protein. Additionally, flr mutant cells revealed a decrease of about 50% in the H2 consumption rate using thiosulfate as electron acceptor. Altogether, our results show in vivo that during sulfite respiration, trithionate and thiosulfate are produced and that flavoredoxin is specific for thiosulfate reduction.