The fumarate reductase operon of Wolinella succinogenes. Sequence and expression of the frdA and frdB genes.

The genes of the fumarate reductase of Wolinella succinogenes are organized in an operon. The three structural genes in the order frdC, frdA, frdB, are preceded by a common promoter (Körtner et al. 1990) and followed by a terminator of transcription. The proteins encoded by the genes are identical with the subunits present in the isolated enzyme. FrdA and FrdB are hydrophilic proteins consisting of 656 and 238 amino acids, respectively. The 12 cysteine residues present in FrdB form 3 ferredoxin-like clusters, whereas the 12 cysteines of FrdA are not clustered. Expression of FrdA and FrdB in Escherichia coli from a plasmid containing a DNA fragment with both genes in full length, gave rise to the EPR signals of the bi- and trinuclear iron-sulfur centers of the enzyme. Only the binuclear center was seen on the expression of FrdB together with a C-terminal fragment of FrdA (130 amino acid residues). Neither of the two centers was detected on the expression of FrdA together with a N-terminal fragment of FrdB including cysteine cluster I. Sequence comparison of FrdA and FrdB with the corresponding subunits of the fumarate reductases of E. coli or Proteus vulgaris or to those of the succinate dehydrogenases of E. coli or Bacillus subtilis revealed strong homologies (28-36% identical amino acid residues). Part of the homologous peptide stretches could be assigned to domains that are involved in the binding of the substrate of the FAD prosthetic group of the enzyme.