PREFORMED DIMERIC STATE OF THE SENSOR PROTEIN VIRA IS INVOLVED IN PLANT-AGROBACTERIUM SIGNAL-TRANSDUCTION

Plant signal molecules such as acetosyringone and certain monosacchari des induce the expression of Agrobacterium tumefaciens virulence (vir) genes, which are required for the processing, transfer, and possibly integration of a piece of the bacterial plasmid DNA (T-DNA) into the p lant genome. Two of the vir genes, virA and virG, belonging to the bac terial two-component regulatory system family, control the induction o f vir genes by plant signals. virA encodes a membrane-bound sensor kin ase protein and virG encodes a cytoplasmic regulator protein. Although it is well established from in vitro studies that the signal transduc tion process involves VirA autophosphorylation and subsequent phosphat e transfer to VirG, the structural state of the VirA protein involved in signal transduction is not understood. In this communication, we de scribe an in vivo crosslinking approach which provides physical eviden ce that VirA exists as a homodimer in its native configuration. The di merization of VirA neither requires nor is stimulated by the plant sig nal molecule acetosyringone. We also present genetic data which suppor t the hypothesis that VirA exists as a homodimer which is the function al state transducing the plant signal in an intersubunit mechanism. To our knowledge, this report provides the first evidence that a bacteri al membrane-bound sensor kinase exists and functions as a homodimer in vivo.