Not required for the biosynthesis of any of the c-type cytochromes. Possible subunit of a heme lyase. Triple nrfE-nrfF-nrfG deletion mutants no longer attach heme 1 (covalently attached to 'Cys-122', the CWSCK motif) of cytochrome c552 (AC P0ABK9). Belongs to the CcmH/CycL/Ccl2/NrfF family.