The nap operon of Escherichia coli K-12, encoding a periplasmic nitrate reductase (Nap), encodes seven proteins. The catalytic complex in the periplasm, NapA-NapB, is assumed to receive electrons from the quinol pool via the membrane-bound cytochrome NapC. Like NapA, B and C, a fourth polypeptide, NapD, is also essential for Nap activity. However, none of the remaining three polypeptides, NapF, G and H, which are predicted to encode non-haem, iron-sulphur proteins, are essential for Nap activity, and their function is currently unknown. The relative rates of growth and electron transfer from physiological substrates to Nap have been investigated using strains defective in the two membrane-bound nitrate reductases, and also defective in either ubiquinone or menaquinone biosynthesis. The data reveal that Nap is coupled more effectively to menaquinol oxidation than to ubiquinol oxidation. Conversely, parallel experiments with a second set of mutants revealed that nitrate reductase A couples more effectively with ubiquinol than with menaquinol. Three further sets of strains were constructed with combinations of in frame deletions of ubiCA, menBC, napC, napF and napGH genes. NapF, NapG and NapH were shown to play no role in electron transfer from menaquinol to the NapAB complex but, in the Ubi+Men- background, deletion of napF, napGH or napFGH all resulted in total loss of nitrate-dependent growth. Electron transfer from ubiquinol to NapAB was totally dependent upon NapGH, but not on NapF. NapC was essential for electron transfer from both ubiquinol and menaquinol to NapAB. The results clearly established that NapG and H, but not NapF, are essential for electron transfer from ubiquinol to NapAB. The decreased yield of biomass resulting from loss of NapF in a Ubi+Men+ strain implicates NapF in an energy- conserving role coupled to the oxidation of ubiquinol. We propose that NapG and H form an energy- conserving quinol dehydrogenase functioning as either components of a proton pump or in a Q cycle, as electrons are transferred from ubiquinol to NapC.