NARK IS A NITRITE-EXTRUSION SYSTEM INVOLVED IN ANAEROBIC NITRATE RESPIRATION BY ESCHERICHIA-COLI

Escherichia coli can use nitrate as a terminal electron acceptor for a naerobic respiration. A polytopic membrane protein, termed NarK, has b een implicated in nitrate uptake and nitrite excretion and is thought to function as a nitrate/nitrite antiporter. The longest-lived radioac tive isotope of nitrogen, N-13-nitrate (half-life = 9.96 min) and the nitrite-sensitive fluorophore N-(ethoxycarbonylmethyl)-6-methoxyquinol inium bromide have now been used to define the function of NarK. At lo w 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 furt her detoxification in the periplasm through the action of nitrite redu ctase.