Robust growth in many bacteria is dependent upon proper regulation of the adaptive response to phosphate (P i ) limitation. This response enables cells to acquire P i with high affinity and utilize alternate phosphorous sources. The molecular mechanisms of P i signal transduction are not completely understood. PhoU, along with the high-affinity, P i -specific ATP-binding cassette transporter PstSCAB and the two-component proteins PhoR and PhoB, is absolutely required for P i signaling in Escherichia coli. Little is known about the role of PhoU and its function in regulation. We have demonstrated using bacterial two-hybrid analysis and confirmatory coelution experiments that PhoU interacts with PhoR through its PAS (Per-ARNT-Sim) domain and that it also interacts with PstB, the cytoplasmic component of the transporter. We have also shown that the soluble form of PhoU is a dimer that binds manganese and magnesium. Alteration of highly conserved residues in PhoU by site-directed mutagenesis shows that these sites play a role in binding metals. Analysis of these phoU mutants suggests that metal binding may be important for PhoU membrane interactions. Taken together, these results support the hypothesis that PhoU is involved in the formation of a signaling complex at the cytoplasmic membrane that responds to environmental P i levels.