The system discriminates between nitrate and nitrite. In the presence of nitrate both sensors phosphorylate NarL and NarP to the same extent. In the presence of nitrite, autophosphorylation of NarX is low and NarX acts primarily as a phosphatase for NarL-P [29, 30, 31]. In contrast, NarQ phosphorylates NarL and NarP in response to both nitrate and nitrite. Therefore, with nitrite, NarP-P is the major regulator, and with nitrate both NarL and NarP are functional [31, 32]. NarP consists of the N-terminal receiver domain and a C-terminal DNA-binding domain. The receiver domain of NarL and NarP could function as a stabilizer of the carboxyl-terminal domain (CTD); it enhances DNA binding and/or enables repression per se in vivo. In addition, the receiver may make protein-protein contacts necessary for transcription activation [33]. NarP consists of the N-terminal receiver domain and a C-terminal DNA-binding domain. NarP is 44% homologous to NarL, the structure of which has been solved by X-ray crystallography [34, 35]. NarL and NarP work together in the regulation of gene expression. NarP binds to two inverted heptamers separated by 2 bp, termed 7-2-7 sites [3, 15]. The binding sites for NarP overlap with the binding sites for NarL. However, for NarL the presence of one heptamer is sufficient for binding [10, 36]. The location and spacing of the binding sites vary considerably in different promoters, and most promoters are also codependent on additional regulators, e.g., the oxygen sensor FNR, IHF, or Fis [37, 38]. The Nar sites are classified based on the location of the activator binding site (ref). Nar class I promoters have a single Nar-binding site immediately adjacent to the promoter [18, 39, 40], and Nar class II promoters have two Nar-binding sites upstream of the promoter [33]and reference therein. NarP acts as a repressor or activator of gene expression, or it interferes with other regulatory proteins bound near the promoter. For example, expression of nrf is activated by FNR and repressed by IHF and Fis. NarL and NarP act as antirepressors. In the presence of nitrate or nitrite NarL or NarP binds to a target site upstream of the binding sites for FNR, IHF, and Fis, counteracting the effect of IHF but not that of Fis. Similarly, activation of nir by FNR is suppressed by IHF, Fis, and H-NS. NarL or NarP counteracts repression by Fis and IHF, but not repression by H-NS [13, 37, 41, 41]. Expression of the napF operon is induced by NarP in the presence of FNR. NarL can displace NarP but is unable to activate transcription from this location [6, 42]. The ydh operon is activated by FNR and suppressed by NarP and NarL. It has been suggested that NarL is responsible for nitrate-mediated repression and NarP is responsible for nitrite-mediated repression of ydh, and both compete with RNA polymerase for binding at the promoter [17]. The accumulation of the protein NarP is decreased during iron-limiting conditions [43]. A review of the regulation of molybdenum cofactor (Moco) biosynthesis and molybdoenzyme gene expression via molybdenum and iron by FNR, NarXL/QP, Fur, and ArcA has been published [44]