A mutation blocking the formation of membrane or periplasmic endogenous and exogenous c-type cytochromes in Escherichia coli permits the cytoplasmic formation of Hydrogenobacter thermophilus holo cytochrome c552.

A mutant of Escherichia coli, JCB606, shown to be pleiotropically deficient in the formation of endogenous membrane and periplasmic c-type cytochromes, synthesised the apo form of the exogenous cytochrome c550 from Paracoccus denitrificans, but not the holo form. In contrast, a cytoplasmically located holo form of Hydrogenobacter thermophilus cytochrome c552 was found in E. coli JCB606. These findings support the proposition that the formation of the cytoplasmic H. thermophilus cytochrome c552 in E. coli does not involve the physiological pathway of c-type cytochrome biosynthesis in E. coli and that the haem insertion may be catalysed.