'Locked-on' and 'locked-off' signal transduction mutations in the periplasmic domain of the Escherichia coli NarQ and NarX sensors affect nitrate- and nitrite-dependent regulation by NarL and NarP

Mol Microbiol. 1997 Jun;24(5):1049-60. doi: 10.1046/j.1365-2958.1997.4131779.x.


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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Alanine
  • Amino Acid Sequence
  • Arginine
  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism*
  • Binding Sites
  • Cytoplasm
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism*
  • Escherichia coli / genetics*
  • Escherichia coli / metabolism
  • Escherichia coli Proteins*
  • Gene Expression Regulation, Bacterial
  • Lac Operon
  • Membrane Proteins / genetics*
  • Molecular Sequence Data
  • Mutagenesis, Site-Directed
  • Nerve Tissue Proteins / genetics
  • Nitrate Reductase
  • Nitrate Reductases / genetics
  • Nitrates / metabolism
  • Nitrites / metabolism
  • Phosphoproteins / genetics*
  • Protein Kinases*
  • Signal Transduction*


  • Bacterial Proteins
  • DNA-Binding Proteins
  • Escherichia coli Proteins
  • Membrane Proteins
  • Nerve Tissue Proteins
  • Nitrates
  • Nitrites
  • Phosphoproteins
  • narP protein, E coli
  • narQ protein, E coli
  • NarL protein, E coli
  • Arginine
  • Nitrate Reductases
  • Nitrate Reductase
  • Protein Kinases
  • narX protein, E coli
  • Alanine