From one gene to two proteins: the biogenesis of cytochromes b and c1 in Bradyrhizobium japonicum.

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.