Most of the essential cellular components, like
nucleic acids,
lipids and
sugars, are phosphorylated. The
phosphate equilibrium in Escherichia coli is regulated by the
phosphate (Pi) input from the surrounding medium. Some 90
proteins are synthesized at an increased rate during Pi
starvation and the global control of the cellular metabolism requires cross-talk with other regulatory mechanisms. Since the Pi concentration is normally low in E. coli's natural habitat, these cells have devised a mechanism for synthesis of about 15
proteins to accomplish two specific functions: transport of Pi and its intracellular regulation. The synthesis of these
proteins is controlled by two genes (the phoB-phoR operon), involving both negative and positive functions. PhoR
protein is a
histidine protein kinase, induced in Pi
starvation and is a transmembrane
protein. It phosphorylates the regulator
protein PhoB which is also Pi
starvation-induced. The PhoB phosphorylated form binds specifically to
a DNA sequence of 18
nucleotides (the pho Box), which is part of the promoters of the Pho genes. The genes controlled by phoB constitute the Pho regulon. The repression of phoA (the gene encoding
alkaline phosphatase) by high Pi concentrations in the medium requires the presence of an intact Pst operon (pstS, pstC, pstA, pstB and phoU) and phoR. The products of pstA and pstC are membrane bound, whereas the product of pstS is periplasmic and PstB and PhoU
proteins are cytoplasmic. The function of the PhoU
protein may be regulated by cofactor
nucleotides and may be involved in signaling the activation of the regulon via PhoR.