EnvZ/OmpR two-component system

Regulation of Porin genes

Osmoregulation is a complex process involving several actions in the cell. E. coli and several other prokaryotes have a two-component regulatory system that plays an important role in regulating the cellular response to different solute concentrations in their growth medium. The sensory kinase in this two-component system is EnvZ, and the response regulator is OmpR. These two components interact to bring about the differential regulation of two genes(ompF and ompC) which are responsible for the production of outer membrane porins OmpF and OmpC.

Overview

EnvZ is a sensor-transmitter that spans the inner cytoplasmic membrane and has two domains: a sensory domain and a transmitter domain. The sensory domain detects the changes caused in membrane surface tension due to changes in medium osmolarity. Variations in membrane surface tension trigger conformational changes in EnvZ which are transmitted to the response regulator OmpR via the transmitter domain. Upon phosphorylation, OmpR becomes an active dimer that exhibits enhanced DNA-binding ability specific for both the ompC and ompF genes. The signaling cascade can be elucidated as follows:

1. Activation of the inner membrane sensor histidine kinase EnvZ,
2. Autophosphorylation of EnvZ at His243,
3. Phosphate transfer to OmpR at Asp 55,
4. Binding of OmpR-P to upstream sites on the ompF and ompC porin promoters to differentially modulate their transcription.
5. Dephosphorylation of OmpR-P via the phosphatase function of EnvZ.

It is important to note that ompF and ompC are expressed at all times at constant levels; however, the individual porins OmpF and OmpC are present in various relative abundances based on medium osmolarity. Also, 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 is involved in tripeptide permease system that is completely free of osmotic control. However, the control of EnvZ-OmpR over OmpF and OmpC is a significant mechanism.

Low osmolarity response

File:Ompf-OmpC.png

Response to medium Osmolarity

When medium osmolarity is low, EnvZ exhibits comparatively low kinase activity (i.e., high phosphatase activity) towards OmpR. Hence the amount of the phosphorylated form of OmpR in cells is relatively small. In this particular situation, the OmpR-P complex binds cooperatively to F1, F2 and F3 activator sites of ompF gene and upregulates its transcription. These sites have high relative affinity for OmpR-P complex as compared to the C1,C2 and C3 activator sites of ompC.Therefore transcription levels of ompC is less as compared to ompF under low osmolarity condition. This phenomenon has been attributed to the fact that OmpF has a larger pore diameter (1.12 nm) than OmpC (1.08 nm) which results in a 10-fold faster diffusion rate that provides a selective advantage at low osmolarity to rapidly scavenge scarce nutrients.

High osmolarity response

In the event of high medium osmolarity, EnvZ actively undergoes auto-phosphorylation and then efficiently transfers its phosphoryl groups to the N-terminal receiver domain of OmpR through EnvZ-OmpR complex formation. As the number of phosphorylated OmpR protein molecules increases, two events occur: OmpR binds not only to the high affinity binding sites upstream of the ompF promoter but also to the one low affinity-binding site. Binding to this low affinity site results in repression of ompF gene. Further, OmpR binds to the three low affinity activator sites upstream of the OmpC promoters as a result ompC gene expression is stimulated and more OmpC porin protein is expressed on outer membrane of the cell. Also, transcription of micF antisense mRNA is initiated at high medium osmolarity. micF binds to complementary sequence of ompF mRNA to block its translation. These two mechanisms ensure that there is relatively higher expression of OmpC as compared to OmpF under the conditions of high medium osmolarity.

Two component Osmoregulatory Pathways in Eukaryotes

It was only around 1990’s that eukaryotic His-Asp phosphorelays began to be discovered. They have been identified in a number of eukaryotic systems prominent being S.cerevisiae, A.thaliana and Dictyostelium discoideum. Among the various HAP phoshorelays high osmolarity response pathway in S.cerevisiae is the most thoroughly characterized. It involves phosphorelay between Sln1p, Ypd1p and Ssk1p proteins. Sln1p is the sensory histidine kinase, Ypd1p mediates the multistep phosphorelay and Ssk1p is the response regulator. Interestingly, unlike bacterial response regulators phosphorylation of Ssk1p results in downregulation of the pathway.

References

• "A simulation model of Escherichia coli osmoregulatory switch using E-CELL system". BMC Microbiology. 4: 44. 2004. doi:10.1186/1471-2180-4-44. PMC 543474. PMID 15571621. {{cite journal}}: Unknown parameter |authors= ignored (help)CS1 maint: unflagged free DOI (link)
• "Signal transduction via the histidyl-aspartyl phosphorelay". Wiley Online Library. 2 (3): 167–84. 1997. PMID 9189755. {{cite journal}}: Unknown parameter |authors= ignored (help)
• "EnvZ-OmpR Interaction and Osmoregulation in Escherichia coli". Journal of Biological Chemistry. 277 (27): 24155–61. 2002. doi:10.1074/jbc.M110715200. PMID 11973328. {{cite journal}}: Unknown parameter |authors= ignored (help)CS1 maint: unflagged free DOI (link)

External links

• https://www.bio.cmu.edu/courses/03441/TermPapers/99TermPapers/TwoCom/osmolarity.html