virA and virG control the plant-induced activation of the T-DNA transfer process of A. tumefaciens.

The Ti plasmid vir loci of Agrobacterium tumefaciens are transcriptionally activated in response to signal molecules produced by plant cells to initiate the T-DNA transfer process. We show that the pTiA6 vir loci are organized as a single regulon whose induction by plants is controlled by virA and virG. Mutations in virA result in attenuated induction. This locus is constitutively transcribed and noninducible. Mutations in virG eliminate vir induction. This locus is constitutively transcribed, plant-inducible, and self-regulated in a complex fashion, and it produces two distinct and differentially regulated transcripts. virA is proposed to encode a transport protein for the plant signal molecule, and virG a positive regulatory protein that together with the plant molecule activates vir expression.