In mitochondria and Purple bacteria quinol:cytochrome c oxidoreductase activity is performed by the bc 1 complex. The minimal functional unit of this complex is composed by a dihaemic cytochrome b , a Rieske protein and a cytochrome c (Hatefi 1985; Trumpower 1990) . This complex is proposed to translocate protons by a Q-cycle mechanism (Mitchell 1975; Trumpower 1990) . A very similar complex, the b 6 f , catalyses the quinol:plastocyanine/cytochrome c 6 oxidoreductase reaction in chloroplasts and Cyanobacteria (Cramer et al. 1994). Cytochrome b is larger than cytochrome b 6 , and cytochrome f differs from cytochrome c 1 in respect to its haem sixth ligand, which is the ? amino group of the N-terminus amino acid residue (Prince et al. 1995) . In the Bacillus genus a b 6 c 1 complex is responsible for the oxidation of menaquinol and reduction of cytochrome c (Yu et al. 1995) . A dihaemic cytochrome c is present in complex III from other Gram positive bacteria (Sone et al. 2003) . Copurification in the form of a supercomplex of complexes III and IV has been observed in several bacteria, such as Paracoccus denitrificans and mitochondria (Berry et al. 1985; Schagger et al. 2000).