WOLINELLA-SUCCINOGENES FUMARATE REDUCTASE CONTAINS A DIHEME CYTOCHROME-B

The fumarate reductase operon of Wolinella succinogenes is made up of three structural genes (frd‐CAB). The frdC gene was located next to the promoter region and identified as the cytochrome b structural gene encoding 256 amino acid residues. The N‐terminal amino acid sequences of seven fragments derived from the cytochrome b moiety of the enzyme all mapped within the frdC gene. This suggested that the enzyme contained only one species of cytochrome b. Re‐evaluation of earlier measurements of subunit composition, haem B content and molecular weight led to the conclusion that the enzyme contained one molecule of cytochrome b with two haem B groups. The hydropathy plot of the amino acid sequence predicted five membrane‐spanning hydrophobic segments, the first four of which contained a single histidine residue each. These residues could form the axial ligands to the two haem B groups. FrdC was found to be homologous with the cytochrome b (SdhC) of the Bacillus subtilis succinate dehydrogenase, but not with the hydrophobic subunits of the fumarate reductase or succinate dehydrogenase of Escherichia coli. Copyright © 1990, Wiley Blackwell. All rights reserved