Expression of VirB2 protein-containing structures on Agrobacterium in mating cultures

Exocellular structures containing VirB2 proteins were, for the first time, localized on the surface of Agrobacterium by transmission electron microsco py. Using colloidal gold (CG)-labeled VirB2-specific antibodies, it was sho wn that VirB2 proteins enter into the composition of short surface pili, wh ich emerge at the poles of acetosyringone (AS)-induced Agrobacterium cells. However, cells of the Ti plasmidless A. tumefaciens strain UBAPF-2 and cel ls not induced with AS were incapable of pilus synthesis. In suspension, ma ting Agrobacterium cells were connected together by short thick bridges. It was found that these bridges did not include as part of their structure CG -labeled VirB1 and VirB2 proteins. We did not find the tetracycline-resista nt transconjugants after mating of A. tumefaciens donor cells harboring bin ary systems with plasmid-free A. tumefaciens GM-I 9023 in vir-induced and v ir-uninduced conditions. However, the same strains can transfer pSUP106 pla smid via a vir-dependent way. We found that activated vir genes slightly st imulate pTd33 plasmid transfer via a tra-dependent pathway to plasmid-free strain UBAPF-2. It seems, that vir-induced T-DNA/plasmid DNA transfer machi nery is not essential for the conjugation process between agrobacterial cel ls but may participate in this process.