The gene encoding the Rhodopseudomonas viridis cytochrome c2 (cycA) has been introduced on a broad host range vector into Paracoccus denitrificans, leading to high-level expression of the holo-cytochrome with the heme moiety covalently attached to the apoprotein. The cytochrome was demonstrated to reside in the periplasmic space of the host cell. In contrast to R. viridis, aerobic rather than anaerobic growth conditions led to higher production levels of the holo-cytochrome in P. denitrificans. This heterologous expression system provides a suitable genetic background for the functional expression and mutagenesis of polypeptides involved in bacterial photosynthesis, offering the possibility of detailed structural and functional investigation.