The Escherichia coli NarX, NarQ, NarL and NarP proteins comprise a two-component regulatory system that controls the expression of many anaerobic electron-transport and fermentation-related genes in response to nitrate and nitrite. Either of the two sensor-transmitter proteins, NarX and NarQ, can activate the response-regulator proteins, NarL and NarP, which in turn are able to bind at their respective DNA regulatory sites to modulate gene expression. NarX contains a conserved 17 amino acid sequence, designated the 'P-box' element, that is essential for nitrate sensing. In this study we characterize narQ mutants that also confer altered nitrate control of NarL-dependent nitrate reductase (narGHJI) and fumarate reductase (frdABCD) gene expression. While some narQ mutations cause the constitutive activation or repression of reporter-gene expression even when the cells are grown in the absence of the nitrate signal (i.e. a 'locked-on' phenotype), other mutations abolish nitrate-dependent control (i.e. a 'locked-off' phenotype). Interestingly the narQ (A42-->T) and narQ (R50-->Q) mutations along with the analogous narX18 (A46-->T) and narX902 (R54-->E) mutations also confer a 'locked-on' or a 'locked-off' phenotype in response to nitrite, the second environmental signal detected by NarQ and NarX. Furthermore, these narQ and narX mutations also affect NarP-dependent gene regulation of nitrite reductase (nrfABCDEFG) and aeg-46.5 gene expression in response to nitrite. We therefore propose that the NarQ sensor-transmitter protein also detects nitrate and nitrite in the periplasmic space via its periplasmic domain. A signal transduction model, which we previously proposed for NarX, is now extended to NarQ, in which a nitrate- or nitrite-detection event in the periplasmic region of the cell is followed by a signal transduction event through the inner membrane to the cytoplasmic domain of NarQ and NarX proteins to modulate their protein kinase/phosphatase activities.