phoR

PhoR is a membrane-bound sensor histidine kinase that forms part of the PhoR-PhoB two-component regulatory system in many bacteria, including Escherichia coli and Bacillus subtilis. It detects environmental inorganic phosphate (Pi) availability and, via phosphorylation of the response regulator PhoB, controls the Pho regulon—genes involved in phosphate acquisition, metabolism, and homeostasis.

Mechanism and function: PhoR autophosphorylates on a histidine residue in its cytoplasmic kinase domain and transfers the phosphate to PhoB. Under phosphate-limited conditions, PhoR activity increases, activating PhoB and inducing transcription of Pho regulon genes. When Pi is abundant, PhoR is inactive and PhoB is dephosphorylated, repressing regulon expression. This regulatory circuit coordinates phosphate uptake and utilization with cellular demand.

Structure and signaling: PhoR is a dimeric, membrane-associated kinase with two transmembrane segments, a periplasmic (or sensor) domain in many Gram-negative bacteria, and cytoplasmic HisKA and HATPase_c catalytic domains. Signal transmission from the sensor region to the kinase domain allows Pi sensing to modulate autokinase activity and downstream phosphorylation.

Genetic organization and regulation: The phoR gene is commonly adjacent to phoB in an operon, enabling coordinated expression. The PstSCAB phosphate transporter and regulatory factors such as PhoU influence signaling by modulating phosphate flux and sensor responsiveness.

Variants and significance: In Gram-positive bacteria, PhoR-PhoP systems perform similar roles with organism-specific variations in sensor architecture. The PhoR-PhoB axis is important for adaptation to phosphate limitation and can impact virulence-related traits in certain pathogens.