Nitrate acts through the response regulator NarL to activate and repress anaerobic respiratory gene expression in Escherichia coli. The narX gene product encodes a cognate sensor (histidine protein kinase). However, previous work discovered that NarL-mediated nitrate regulation is essentially normal in DELTA-narX deletion mutants. In other two-component regulatory systems studied, the cognate sensor gene is essential for normal regulation. We suggested that NarX-mediated signal transduction reactions are also provided by a functionally redundant nitrate sensor, NarQ. We report here the identification and analysis of narQ insertion mutants. In narX+ strains, a narQ=Tn10 insertion had no perceptible effect on nitrate regulation. However, the same narQ=Tn10 insertion eliminated nitrate regulation when present in DELTA-narX deletion strains. Thus, either narX+ or narQ+ was sufficient for essentially normal NarL-mediated nitrate regulation. The narQ gene mapped to 53 minutes on the E. coli genetic map, a location distinct from all known nitrate regulatory or target genes. The predicted NarQ sequence shares substantial similarity with NarX, particularly in the histidine protein kinase region and in a region of shared similarity with the methyl-accepting chemotaxis proteins. Both NarQ and NarX apparently have N-terminal periplasmic domains, but the primary structures of these regions are largely dissimilar in the two sequences. Analysis of narX* and narL missense alleles in narQ+ versus narQ=Tn10 backgrounds suggests that NarQ and NarX may have subtle functional differences.
