Required for formate-dependent nitrite reduction. Not required for the biosynthesis of any of the c-type cytochromes nor for the secretion of the periplasmic cytochromes. Triple nrfE-nrfF-nrfG deletion mutants no longer attach heme 1 (covalently attached to 'Cys-122', the CWSCK motif) of cytochrome c552 (AC P0ABK9).