From 2007.igem.org

• Methods Enzymol. 2007;422:352-60.

  • The EnvZ/OmpR two-component regulatory system is best known for regulating the porin genes ompF and ompC in response to changes in the osmolarity of the growth medium.
  • In response to an unknown signal, EnvZ is autophosphorylated by ATP on a histidine residue. The phosphoryl group is subsequently transferred to a conserved aspartate residue on OmpR. Phosphorylation of OmpR increases its affinity for the regulatory regions of the porin genes, altering their expression. Phosphorylation also alters the interaction with EnvZ and OmpR.
  • the phospho-OmpR protein has a reduced affinity for the EnvZ kinase.

• J Mol Biol. 2005 Jul 29;350(5):843-56.

  • OmpR functions as a global regulator, by regulating the expression of many genes in addition to the porin genes.
  • OmpR consists of two domains; phosphorylation of the N-terminal receiver domain increases DNA binding affinity of the C-terminal domain and vice versa.
  • Many response regulators including PhoB and FixJ dimerize upon phosphorylation. Here, we demonstrate that OmpR dimerization is stimulated by phosphorylation or by DNA binding.
  • Our results are consistent with a model in which OmpR binds to DNA in a head-to-head orientation, in contrast to the previously proposed asymmetric head-to-tail model. They also raise the possibility that OmpR may be capable of adopting more than one orientation as it binds to a vast array of genes to activate or repress transcription.

• Osmoregulation by a Two-Component Regulatory System

  • The Function of Porin

    • Another way the cell controls its response to osmolarity changes is through the porin proteins. Porins are a relatively abundant class of proteins in the outer membranes of bacteria. Cells can have as many as 10⁵ porin molecules per cell; most of the porins are fairly non-specific, and the diffusion of important molecules such as glucose proceeds rather slowly through them.
    • Two of the most common E.coli porins are OmpF and OmpC. Considered together, OmpF and Omp C are expressed at all times at a constant level; however, the individual porins OmpF and OmpC are present in various relative abundances based on medium osmolarity. In other words, OmpF and OmpC together will make up about 2% of the total cellular protein at any time, but when there is a relatively large amount of OmpC, there is a relatively small amount of OmpF. Like several other porins, the OmpC and OmpF porins create channels that are considered to be nonspecific. Many small molecules can pass through these porin channels; they show a slight specificity for cations and will exclude very large, negatively charged, hydrophobic molecules.
    • OmpC and OmpF are regulated through the EnvZ/OmpR signal transduction pair, a sensitive two-component regulatory system that responds to subtle changes in the medium osmolarity. EnvZ is the inner membrane protein that senses and responds to the osmolarity change; it then signals OmpR, which acts at the regulatory sequences of the OmpF and OmpC genes. It is important to note that the amount of OmpF and OmpC are each controlled by a variety of mechanisms; for example, synthesis of OmpF is also repressed by high temperature. Likewise, the EnvZ/OmpR pair also help to regulate a tripeptide permease system that is completely free of osmotic control.

• J Mol Biol. 2002 Jan 25;315(4):497-511.

  • In E. coli, the two-component regulatory system that controls the expression of outer membrane porins in response to environmental osmolarity consists of the sensor kinase EnvZ and the response regulator OmpR.
  • Phosphorylated OmpR activates expression of the OmpF porin at low osmolarity, and at high osmolarity represses ompF transcription and activates expression of OmpC.
  • Our results indicate that all three of the OmpR binding sites at the ompC promoter must be filled in order to activate gene expression.
  • it appears that OmpR-phosphate must adopt different conformations when bound at ompF and ompC.

• Microbiology (2001), 147, 2981-2989.

  • Expression of ompF was more than 40-fold higher under glucose limitation than under nitrogen (ammonia) limitation in chemostat cultures at the same growth rate.
  • In contrast, ompC expression was higher under N limitation.
  • A major contributor to low ompF expression under N limitation was negative control by the RpoS sigma factor. RpoS levels were high under N limitation and loss of RpoS resulted in a 19-fold increase in ompF transcription, but little change was observed with ompC. Lack of RpoS under glucose limitation had a lesser stimulatory effect on ompF expression. Porin production was minimally dependent on EnvZ under N limitation due to OmpR phosphorylation by acetyl phosphate.
  • Another critical factor in regulation was the level of cAMP, as a cyaA mutant hardly expressed ompF under glucose limitation but boosted ompC.
  • multiple inputs and no single regulator were responsible for the high peak of ompF expression under glucose limitation.

• J Bacteriol. 2005 Mar;187(6):2038-49. 

  • Curli fibers could be described as a virulence factor able to confer adherence properties to both abiotic and eukaryotic surfaces.
  • OmpR was shown to activate csgD expression, resulting in curli production.
  • The CpxR regulator was shown to negatively affect curli gene expression when binding to its recognition site that overlaps the csgD OmpR-binding site.
  • Band-shift assays with purified CpxR proteins indicate that CpxR binds to the csgD promoter region at multiple sites that are ideally positioned to explain the csg repression activity of CpxR.
  • the Cpx pathway is activated at both transcriptional and posttranscriptional levels in response to a high osmolarity medium and that CpxR represses csgD expression in high-salt-content medium, resulting in low curli production. However, csgD repression in response to high sucrose content is not mediated by CpxR but by the global regulatory protein H-NS. Therefore, multiple systems (EnvZ/OmpR, Cpx, Rcs, and H-NS) appear to be involved in sensing environmental osmolarity, leading to sophisticated regulation of the curli genes.

• J Biochem (Tokyo). 2005 Jan;137(1):51-9. 

  • E. coli has two osmo-responsive two-component regulatory systems, the EnvZ-OmpR and KdpD-KdpE systems, each of which consists of a sensor histidine protein kinase and a response regulator.
  • The OmpR and KdpE response regulators belong to the same family of DNA-binding proteins, and act as positive transcriptional factors in response to the medium osmolarity. However, OmpR specifically activates the ompC gene encoding the OmpC outer membrane protein, whereas KdpE exclusively activates the kdpABC operon encoding the high-affinity Kdp potassium-transporter.
  • We propose that the promoter selectivity of OmpR is determined not only by its DNA-binding specificity, but also by the spatial configuration of the promoter on which OmpR must properly associate with RNA polymerase.

• A simulation model of Escherichia coli osmoregulatory switch using E-CELL system.
  BMC Microbiol. 2004 Nov 30;4(1):44.

• J Bacteriol. 2004 Oct;186(20):6689-97.

  • MicC, a second small-RNA regulator of Omp protein expression in E. coli.
  • We suggest that the MicF and MicC RNAs act in conjunction with the EnvZ-OmpR two-component system to control the OmpF/OmpC protein ratio in response to a variety of environmental stimuli.
  • MicC was also shown to inhibit ribosome binding to the ompC mRNA leader in vitro and to require the Hfq RNA chaperone for its function.