Apo forms of cytochrome c550 and cytochrome cd1 are translocated to the periplasm of Paracoccus denitrificans in the absence of haem incorporation caused either mutation or inhibition of haem synthesis.

An apo form of cytochrome c550 can be detected by immunoblotting cell-free extracts of a mutant of Paracoccus denitrificans that is deficient in c-type cytochromes. This apoprotein is found predominantly in the periplasm, the location of the holocytochrome in the wild-type organism, indicating that translocation of the polypeptide occurs in the absence of haem attachment. The polypeptide molecular weight, as judged by sodium dodecyl sulphate/polyacrylamide gel electrophoresis, is indistinguishable from that of the holoprotein and the chemically prepared apoprotein; this suggests that the N-terminal signal sequence is removed in the mutant as in the wild-type organism. In the presence of levulinic acid, an inhibitor of haem biosynthesis, apocytochrome c550 and aponitrite reductase (cytochrome cd1) accumulated in the periplasm of wild-type cells. Synthesis of these apoproteins was blocked by chloramphenicol. Thus in P. denitrificans the synthesis of these polypeptides is neither autoregulated nor regulated by the availability of haem. That the apoproteins appear in the periplasm argues against the possibility of polypeptide/haem co-transport from cytoplasm to periplasm. These observations are related to, and contrasted with, the biosynthesis of c-type cytochromes in eukaryotic cells.