Genes coding for polyproteins that are cleaved posttranslationally into two or more functional proteins are rarely found in prokaryotes. One example concerns the biogenesis of the Bradyrhizobium japonicum cytochromes b and c1, two of the three constituent subunits of ubiquinol-cytochrome-c reductase (ubiquinol:ferricytochrome-c oxidoreductase, EC 1.10.2.2); the respective apoproteins for these subunits are encoded by the 5' and 3' halves of a single gene, fbcH. These two halves are linked by an extra piece of DNA encoding a characteristic signal peptide for protein translocation across the cytoplasmic membrane. Processing of the fbcH gene product is shown to occur at a typical signal peptidase recognition site. This reaction is reminiscent of that catalyzed by the regular bacterial signal peptidase that normally cleaves off presequences from the N termini of translocated proteins. Mutational alteration of the signal peptidase recognition site within FbcH results in the appearance of an uncleaved bc1 fusion protein in the membrane. Additionally, a functional heme-binding site in the apocytochrome c1 section of FbcH is shown to be a necessary prerequisite for the formation of the bc1 complex.