Enterobacteria use nitrate and nitrite both as electron acceptors and as sources of nitrogen for biosynthesis. Nitrate is reduced through nitrite to ammonium in both cases. The enzymes and structural genes for nitrate/nitrite respiration and assimilation are distinct, and are subject to different patterns of regulation. Respiratory enzyme synthesis is indifferent to the availability of ammonium, and is induced by anaerobiosis via the FNR protein. Respiratory enzyme synthesis is further induced by nitrate or nitrite via the NARL and NARP proteins, which are response regulators of two-component regulatory systems. The cognate sensor proteins NARX and NARQ monitor the availability of nitrate and nitrite, and control the activity of the NARL and NARP DNA-binding proteins accordingly. Additionally, nitrate represses the synthesis of respiratory nitrite reductase, and this control is mediated by the NARL protein. Assimilatory enzyme synthesis is indifferent to the availability of oxygen, and is induced by ammonium limitation via the NTRC protein. Assimilatory enzyme synthesis is further induced by nitrate or nitrite via the NASR protein, which may act as a transcription antiterminator. Even though the respiratory and assimilatory enzyme systems are genetically distinct and subject to different forms of regulation, the structural and regulatory genes are closely linked on the Klebsiella pneumoniae chromosome.