The role of the genesnrf EFG andccmFH in cytochromec biosynthesis inEscherichia coli

It has been suggested that two groups ofEscherichia coli genes, theccm genes located in the 47-min region and thenrfEFG genes in the 92-min region of the chromosome, are involved in cytochromec biosynthesis during anaerobic growth. The involvement of the products of these genes in cytochromec synthesis, assembly and secretion has now been investigated. Despite their similarity to other bacterial cytochromec assembly proteins, NrfE, F and G were found not to be required for the biosynthesis of any of thec-type cytochromes inE. coli. Furthermore, these proteins were not required for the secretion of the periplasmic cytochromes, cytochromec ₅₅₀ and cytochromec ₅₅₂, or for the correct targeting of the NapC and NrfB cytochromes to the cytoplasmic membrane. NrfE and NrfG are required for formate-dependent nitrite reduction (the Nrf pathway), which involves at least twoc-type cytochromes, cytochromec ₅₅₂ and NrfB, but NrfF is not essential for this pathway. Genes similar tonrfE, nrfF andnrfG are present in theE. coli nap-ccm locus at minute 47. CcmF is similar to NrfE, the N-terminal region of CcmH is similar to NrfF and the C-terminal portion of CcmH is similar to NrfG. In contrast to NrfF, the N-terminal, NrfF-like portion of CcmH is essential for the synthesis of allc-type cytochromes. Conversely, the NrfG-like C-terminal region of CcmH is not essential for cytochromec biosynthesis. The data are consistent with proposals from this and other laboratories that CcmF and CcmH form part of a haem lyase complex required to attach haemc to C-X-X-C-H haem-binding domains. In contrast, NrfE and NrfG are proposed to fulfill a more specialised role in the assembly of the formate-dependent nitrite reductase.