This family is only found in bacteria. Family members include PhoR. A member of the two-component regulatory system PhoR/PhoB involved in the phosphate regulon genes expression. PhoR is the bifunctional histidine autokinase/phospho-PhoB phosphatase that donates a phosphoryl group to PhoB when environmental Pi is limiting and removes the phosphoryl group from phosphorylated PhoB (PhoB-P) when environmental Pi is abundant [1].

1: The PhoU protein from Escherichia coli interacts with PhoR, PstB, and metals to form a phosphate-signaling complex at the membrane. Gardner SG, Johns KD, Tanner R, McCleary WR; J Bacteriol. 2014;196:1741-1752. PMID:24563032