The Rieske/cytochrome b complex of Heliobacteria.

Heliobacteria have a Rieske/cytochrome b complex composed of a Rieske protein, a cytochrome b(6,) a subunit IV and a di-heme cytochrome c. The overall structure of the complex seems close to the b(6)f complex from cyanobacteria and chloroplasts to the exception of the di-heme cytochrome. We show here by biochemical and biophysical studies that a heme c(i) is covalently attached to the Rieske/cytochrome b complex from Heliobacteria. We studied the EPR signature of this heme in two different species, Heliobacterium modesticaldum and Heliobacillus mobilis. In contrast to the case of b(6)f complex, a strong axial ligand to the heme is present, most probably a protonatable amino acid residue.