The promoter region and transcriptional regulation of the nuoA-N gene locus encoding the proton-translocating NADH:quinone oxidoreductase was analysed. A 560 bp intergenic region upstream of the nuo locus was followed by a gene (designated lrhA for LysR homologue A) coding for a gene regulator similar to those of the LysR family. Disruption of lrhA did not affect growth (respiratory or non-respiratory) or expression of nuo significantly. Transcriptional regulation of nuo by electron acceptors, electron donors and the transcriptional regulators ArcA, FNR, NarL and NarP, and by IHF (integration host factor) was studied with protein and operon fusions containing the promoter region up to base pair -277 ('nuo277') or up to base pair -89 ('nuo899'). The expression of the nuo277-lacZ fusions was subject to ArcA-mediated anaerobic repression and NarL(+ nitrate)-mediated anaerobic activation. FNR and IHF acted as weak repressors under anaerobic conditions. Expression of nuo899-lacZ was stimulated during anaerobic fumarate respiration and aerobically by C4 dicarboxylates. Therefore, expression of nuo is regulated by O2 and nitrate via ArcA, NarL, FNR and IHF at sites within the -277 region, and by other factors including C4 dicarboxylates at a site between -277 and -899. A physiological role for the transcriptional stimulation by O2 and nitrate is suggested.