Expression of the virulence genes (vir) on the hairy-root-inducing plasmid pRiA4 is induced by plant signals in Agrobacterium cells through a two-component regulatory system, the VirA-VirG system. We constructed an in vitro transcription system that consisted of the purified VirG protein and the Agrobacterium RNA polymerase holoenzyme. Both versions of VirG, the non-phosphorylated form and the VirA-phosphorylated form, were active but showed different patterns of the pH-dependency for transcriptional activation.
