The responses to phosphate starvation by PhoR-PhoB two-component regulatory system are also affected by other regulatory systems in Escherichia coli. The interactions among PhoB, PhoR, PhoU and CreC were investigated at transcript and protein levels using real-time PCR and fluorescence resonance energy transfer analyses. CreC showed an interaction with PhoB, before an interaction between PhoB and PhoR, suggesting that this acts as a constant sensor for the early response to phosphate starvation. PhoU was found to interact with neither PhoB nor PhoR, while this acted as a phosphate starvation-inducible protein. These results showed the detailed time- and phosphate concentration-dependent interactions among PhoB, PhoR, PhoU and CreC in response to phosphate starvation in E. coli.