Studies of the cytochrome subunits of menaquinone: Cytochrome c reductase (bc complex) of Bacillus subtilis. Evidence for the covalent attachment of heme to the cytochrome b subunit

The menaquinone:cytochrome c reductase, or bc complex of Bacillus subtilis belongs to a third class of bc-type complex, distinct bc₁ and b₆f classes. Using a mutagenesis approach, we demonstrate that the cytochrome b (QcrB) and c (QcrC) subunits of the complex give rise to bands at 22 and 29 kDa, respectively, after denaturing electrophoresis; that both subunits are required for proper complex assembly and/or stability; and that subunits retain one heme molecule under denaturing conditions. This unusual property of a b-type cytochrome was investigated further. We present evidence for the existence of a covalent linkage between the polypeptide and heme b(H) and of an important role for Cys⁴³ in binding of heme b(H). It is proposed that heme is also covalently attached to the cytochrome b subunit of b₆f complexes of chloroplasts and cyanobacteria.