Chapter 14 : Dual Sensors and Dual Response Regulators Interact to Control Nitrate- and Nitrite-Responsive Gene Expression in Escherichia coli

Model for nitrate- and nitrite-regulated gene expression. Activation and repression of target operon expression are indicated by + and —, respectively; question marks denote situations that have not yet been tested. Strong interactions (10-fold or greater regulation) are boxed, whereas weaker interactions (less than 10-fold regulation) are circled. The NarL and NarP proteins are represented by circled L and circled P, respectively; the phosphorylated forms are represented by the same letters boxed. The NarX and NarQ proteins are represented by the letters X and Q, respectively. In the presence of nitrite, a relatively small proportion of NarL molecules is phosphorylated. Target operon designations are listed in Table 1 . The aeg-46.5 operon is denoted as “nap-cyc” in this figure. Modified from Rabin and Stewart(1993 ).

Linker regions of MCPs (Tsr, serine chemoreceptor; Tar, aspartate chemoreceptor; Tap, peptide chemoreceptor; Trg, ribose chemoreceptor) and sensors (EnvZ, osmolarity sensor; NarX and NarQ, nitrate/nitrite sensors). The segments immediately following the second transmembrane region are shown. Residues that are identical in all seven sequences are boxed. Mutational changes in tsr ( Ames and Parkinson, 1988 ) and narX ( Kalman and Gunsalus, 1990 ; Collins et al., 1992 ) are indicated.

Equilibrium model of NarX and NarQ protein functions in the presence of (A) nitrate or (B) nitrite. See text for details. The NarX and NarQ sensor proteins are shown in equilibria between their negative (rounded) and positive (rectangular) forms. The relative sizes of the equilibrium arrows indicate qualitatively how much of a given sensor population is found in each of the two forms. Positive interactions with the NarL response regulator are designated + (i.e., NarL kinase), and negative interactions are designated − (i.e., phospho-NarL phosphatase). A solid or dashed line (at 45° angle to the sensor protein) emphasizes whether a large or small proportion respectively, of a given sensor population is responsible for a specific influence on the NarL protein.

Promoter-regulatory regions for nitrate-regulated operons. Scale is in base pairs. Thin arrows denote transcription initiation sites; the dashed arrow for the narK control region denotes a minor constitutive transcript. Fnr protein binding sites are shown as dark hatched inverted arrows. NarL heptamer sequences are indicated by their position with respect to the transcription initiation site. Heptamers denoted by black arrows have been identified by both mutational analysis and by DNase I footprinting. Heptamers denoted by gray arrows have been identified by DNase I footprinting only. Heptamers denoted by light hatched arrows have been identified by mutational analysis only. Heptamers denoted by white arrows have been identified by sequence inspection only. The IHF protein binding sites in the narK and the narG control regions are denoted by striped boxes. The aeg-46.5 operon is denoted as “nap-cyc” in this figure. Modified from Stewart (1993 ) and Li et al. (1994 ).