Transcription of a virulence gene on the hairy-root-inducing plasmid A4, which is induced by plant factors in Agrobacterium tumefaciens, was also activated by phosphate limitation in both A. tumefaciens and Escherichia coli. The starting site of RNA synthesized under the two inducing conditions was the same, and an identical promoter was responsible for both inducible expressions. The response of the virulence gene to phosphate limitation did not require the positive regulator VirG for the virulence regulon, but depended entirely on the presence of PhoB protein, the positive regulator for the phosphate regulon. The DNA signal upstream of the virulence gene, which is targeted by the VirG protein, was recognized by the E. coli PhoB protein in vitro. These results indicate that cross-talk between the two regulons occurred during the recognition of a DNA signal by the regulatory protein.