Catalyzes nitrate uptake, nitrite uptake and nitrite export across the cytoplasmic membrane (PubMed:2668029, PubMed:11967075, PubMed:15667293, PubMed:16804183, PubMed:23665960, PubMed:25959928). Functions as a nitrate/nitrite exchanger, and protons are unlikely to be co-transported (PubMed:2668029, PubMed:15667293, PubMed:23665960, PubMed:25959928). nitrate(in) + nitrite(out) = nitrate(out) + nitrite(in) Highly expressed during anaerobic growth in the presence of nitrate. Each molecule contains 12 transmembrane helices, forming the N bundle (TM1-6) and the C bundle (TM7-12) (PubMed:25959928). These N and C bundles are connected with a linker loop between TM6 and TM7 (PubMed:25959928). The substrate-binding pocket is located at the interface between the N and C bundles (PubMed:23665960, PubMed:25959928). Substrate recognition is coupled to the transport cycle by the concomitant movement of the transmembrane helices and the key tyrosine and arginine residues in the substrate-binding site (PubMed:25959928). Deletion mutant, under conditions for the induction of nitrate respiration, is unable to perform nitrate transport (PubMed:2668029). The narK-narU double mutant is defective in nitrate-dependent growth unless nitrate transport is facilitated by the nitrate ionophore, reduced benzyl viologen (BV) (PubMed:11967075). Deletion of both NarK and NirC also decreases nitrite uptake (PubMed:11967075). Belongs to the major facilitator superfamily. Nitrate/nitrite porter (TC 2.A.1.8) family.