The menaquinone:cytochrome c reductase, or bc complex, of Bacillus subtilis belongs to a third class of bc-type complex, distinct from the 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 both 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 bH and of an important role for Cys43 in binding of heme bH. It is proposed that heme is also covalently attached to the cytochrome b subunit of b6f complexes of chloroplasts and cyanobacteria.