PhoP-PhoQ: Difference between revisions

'''PhoP-PhoQ''' is a two component regulatory system found in some gram-negative bacteria such as ''Escherichia Coli'', ''Salmonella Typhimurium'', and ''Yersinia Pestis''. In a classic two component regulatory system, there exists a sensor kinase and a response regulator. In the phoP-phoQ system, phoQ acts as the sensor kinase and phoP acts as the response regulator. The purpose of this signal transduction system in bacteria is to modify cellular output in response to environmental signals. In response to particular environmental stimuli, such as a low [Mg²⁺], the sensor kinase, phoQ autophosphorylates. Phosphorylated phoQ then transphosphorylates the response regulator, phoP, which in turn binds DNA and modulates transcription.

E. coli phoP is a 223 residue protein containing a 120-residue regulatory domain joined by a 5-residue linker to a 98 residue c-terminal DNA binding effector domain. At a conserved Asp residue, the regulatory domain can be modified by a phosphoryl group from the protein kinase function of phoQ. Phosphorylation of the regulatory domain modulates the activity of the effector domain to bind DNA and regulate transcription. Phosphorylated, or "activated" phoP binds to "phoP boxes" on bacterial DNA, which consist of two direct hexanucleotide repeats separated by a five nucleotide spacer located at the -35 position:

The activated phoP regulatory domain dimerizes with two-fold symmetry using the face formed by <scene name='Sandbox_Reserved_344/Dimerization_surface/1'>α-4 helix, β-5 sheet and α-5 helix face</scene>. The phosphoryl group donated from phoQ is used to stabilize the active conformation by inducing dimerization. Due to the absence of an intramolecular domain interface which precludes direct transmission of an activation signal, this function as a dimerization motif seems to be the phosphoryl group's only role.

In this structure, <scene name='Sandbox_Reserved_344/P-analog/1'>Beryllofluoride</scene> (BeF₃^-) is used as a phosphoryl analog to induce the active state conformation. There are numerous bonds that form as a result of BeF₃^- coordination. F₁ of BeF₃^- helps satisfy the octahedral coordination of a present active site <scene name='PhoP-PhoQ/Mg-f1/4'>Mg</scene>²⁺. The conserved active site lysine, <scene name='PhoP-PhoQ/Lys101-f3/2'>Lys101</scene>, forms important intramolecular and intermolecular salt bridges, one of which is with F₃ of BeF₃^-. A conserved "switch residue" <scene name='PhoP-PhoQ/Thr79-f2/2'>Thr79</scene>, involved in the activation of all response regulators, forms a H-bond with F₂ of BeF₃^-. F₂ of BeF₃^- also forms a H-bond with the backbone nitrogen atom of <scene name='PhoP-PhoQ/Gly53-f2/3'>Gly53</scene>.

Most known virulence factors have been isolated by designing laboratory conditions that presumably stimulate environmental signals present in host tissues. However, many pathogenic bacteria do not express their virulence factors until specific host signals are detected, signals which are near impossible to accurately reproduce in a laboratory. To overcome this, a new approach entitled IVET (in vivo expression technology) has been developed. IVET uses animal tissue as the selective medium to enrich bacterial virulence factors specifically induced during infection.<ref name=Angelichio>PMID: PMC132940</ref>

Pathogenic bacteria seldom express virulence genes constitutively, they instead need to be able to express the correct virulence genes in the correct environment. Not all virulence factors confer a selective advantage to the microbe at the same stage of infection. Thus, it is the job of the phoP-phoQ system to modulate virulence gene expression according to the cellular micro-environment.