THE BIOSYNTHESIS OF BACTERIAL AND PLASTIDIC C-TYPE CYTOCHROMES

The biosynthesis of bacterial and plastidic c-type cytochromes include s several steps that occur post-translationally. In the case of bacter ial cytochromes, the cytosolically synthesized pre-proteins are transl ocated across the cytoplasmic membrane, the pre-proteins are cleaved t o their mature forms and heme is ligated to the processed apoprotein. Although heme attachment has not been studied extensively at the bioch emical level, molecular genetic approaches suggest that the reaction g enerally occurs after translocation of the apoprotein to the periplasm . Recent studies with Bradyrhizobium japonicum and Rhodobacter capsula tus indicate that the process of heme attachment requires the function of a large number of genes. Mutation of these genes generates a pleio tropic deficiency in all c-type cytochromes, suggesting that the gene products participate in processes required for the biosynthesis of all c-type cytochromes. In eukaryotic cells, the biosynthesis of photosyn thetic c-type cytochromes is somewhat more complex owing to the additi onal level of compartmentation. Nevertheless, the basic features of th e pathway appear to be conserved. For instance, as is the case in bact eria, translocation and processing of the pre-proteins is not dependen t on heme attachment. Genetic analysis suggests that the nuclear as we ll as the plastid genomes encode functions required for heme attachmen t, and that these genes function in the biosynthesis of the membrane-a ssociated as well as the soluble c-type cytochrome of chloroplasts. A feature of cytochromes c biogenesis that appears to be conserved betwe en chloroplasts and mitochondria is the sub-cellular location of the h eme attachment reaction (p-side of the energy transducing membrane). C ontinued investigation of all three experimental systems (bacteria, ch loroplasts, mitochondria) is likely to lead to a greater understanding of the biochemistry of cytochrome maturation as well as the more gene ral problem of cofactor-protein association during the assembly of an energy transducing membrane.