Anaerobic nitric oxide reductase; uses NADH to detoxify nitric oxide (NO), protecting several 4Fe-4S NO-sensitive enzymes. Has at least 2 reductase partners, only one of which (NorW, flavorubredoxin reductase) has been identified. NO probably binds to the di-iron center; electrons enter from the reductase at rubredoxin and are transferred sequentially to the FMN center and the di-iron center. Also able to function as an aerobic oxygen reductase (By similarity). Binds 3 Fe cations per monomer. Binds 1 FMN per monomer. Nitrogen metabolism; nitric oxide reduction. Homotetramer. Transcription is negatively regulated by FNR and does not require NarL or NarP (By similarity). Anaerobically activated by nitroprusside in both rich and minimal media. In anaerobic minimal medium, activated by nitrate. In rich medium aerobic cultures activated by nitroprusside. NorR required for activation. In the N-terminal section; belongs to the zinc metallo-hydrolase group 3 family.