FIG 8.

Model for PhoU function and regulation of the response to extracellular phosphate limitation in Caulobacter. The high-affinity transporter PstSCAB imports inorganic phosphate (red) into the cytoplasm. When flux through the transporter decreases, the histidine kinase PhoR is likely directly activated to autophosphorylate and then phosphotransfer to PhoB. Phosphorylated PhoB can bind to pho boxes (blue) in target genes, including the pst transporter. PhoU likely does not couple the Pst transporter to PhoR and instead is proposed to inhibit phosphate uptake by the Pst system or possibly regulate intracellular phosphate metabolism.