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‘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 (1997)

Chiang, Robin C. ; Cavicchioli, Ricardo ; Gunsalus, Robert P.

Oxford UK : Blackwell Science Ltd

Molecular microbiology 24 (1997), S. 0

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ISSN: 1365-2958

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Medicine

Notes: 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.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-2958.1997.4131779.x

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Role of the periplasmic domain of the Escherichia coli NarX sensor-transmitter protein in nitrate-dependent signal transduction and gene regulation (1996)

Cavicchioli, Ricardo ; Chiang, Robin C. ; Kalman, Lisa V. ; [et al.]

Oxford BSL : Blackwell Science Ltd

Molecular microbiology 21 (1996), S. 0

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ISSN: 1365-2958

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Medicine

Notes: The narX, narQ and narL genes of Escherichia coli encode a nitrate-responsive two-component regulatory system that controls the expression of many anaerobic electron-transport- and fermentation-related genes. When nitrate is present, the NarX and NarQ sensor-transmitter proteins function to activate the response-regulator protein, NarL, which in turn binds to its DNA-recognition sites to modulate gene expression. The sensor-transmitter proteins are anchored in the cytoplasmic membrane by two transmembrane domains that are separated by a periplasmic region of ≈115 amino acids. In this study we report the isolation and characterization of narX* (star) mutants that constitutively activate nitrate reductase (narGHJI) gene expression and repress fumarate reductase (frdABCD) gene expression when no nitrate is provided for the cell. An additional narX mutant was identified that has lost its ability to respond to environmental signals. Each narX defect was caused by a single amino acid substitution within a conserved 17 amino acid sequence, called the ‘P-box’, in the periplasmic exposed region of the NarX protein. As a result, DNA binding is then ‘locked-on’ or ‘locked-off’ to give the observed pattern of gene expression. Diploid analysis of these narX mutants showed that a NarX P-box mutant which confered a ‘locked-on’ phenotype was trans dominant over wild-type NarX. Both were also trans dominant over the NarX P-box mutant which conferred a ‘locked-off’ phenotype. Certain narX P-box mutations, when combined with a narX‘linker’ region mutation, were recessive to the NarX linker mutation. Finally, a truncated form of the NarX protein that lacked the periplasmic and membrane regions also showed a ‘locked-on’ phenotype in vivo. Thus, the periplasmic and membrane domains are essential for signal transduction to NarL. From these findings, we propose that nitrate is detected in the periplasmic space of the cell, and that a signal-transduction event through the cytoplasmic membrane into the interior of the cell modulates the NarX-dependent phosphorylation/dephosphorylation of NarL.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-2958.1996.491422.x

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