InterPro domain: IPR004737

This family represents nitrate/nitrite antiporters in bacteria, fungi, algae and marine diatoms. Nitrate transporters in Escherichia coli are involved in excretion of nitrite produced by the dissimilatory reduction of nitrate. NarK is polytopic membrane protein with 12 transmembrane domains which is involved in nitrate uptake and nitrite excretion and is thought to function as a nitrate/nitrite antiporter [ 1 , 2 ]. At low concentrations of nitrate, NarK mediates the electrogenic excretion of nitrite rather than nitrate/nitrite exchange. This process prevents intracellular accumulation of toxic levels of nitrite and allows further detoxification in the periplasm through the action of nitrite reductase [ 3 ].

In the opportunistic fungal pathogen Aspergillus fumigatus the nitrate transporter crnA clusters with nitrite reductase (niiA) and nitrate reductase (niaD). NiaD, niiA and crnA are induced by nitrate and repressed by ammonium at the transcriptional level [ 4 ]. In Hebeloma cylindrosporum the nitrate transporter polypeptide (NRT2) is characterised by 12 transmembrane domains and presents both a long putative intracellular loop and a short C-terminal tail, two structural features which distinguish fungal high-affinity transporters from their plant homologues. Transcription is repressed by ammonium and strongly stimulated by nitrate but also by organic nitrogen or nitrogen deficiency.

1. A single channel for nitrate uptake, nitrite export and nitrite uptake by Escherichia coli NarU and a role for NirC in nitrite export and uptake. Biochem. J. 417, 297-304
2. Involvement of NarK1 and NarK2 proteins in transport of nitrate and nitrite in the denitrifying bacterium Pseudomonas aeruginosa PAO1. Appl. Environ. Microbiol. 72, 695-701
3. NarK is a nitrite-extrusion system involved in anaerobic nitrate respiration by Escherichia coli. Mol. Microbiol. 12, 579-86
4. Mapping of the nitrate-assimilation gene cluster (crnA-niiA-niaD) and characterization of the nitrite reductase gene (niiA) in the opportunistic fungal pathogen Aspergillus fumigatus. Curr. Genet. 33, 206-15