The Vir genes
The Vir region on the Ti/Ri plasmid is a collection of genes whose collective function is to excise the T-DNA region of the plasmid and promote its transfer and integration into the plant genome.
The system is induced by signals produced by plants following wounding. Phenolic compounds such as acetosyringone, activate the VirA gene, which is a constitutively expressed trans-membrane protein.
The activated VirA gene acts as a kinase, phosphorylating the VirG gene. In its phosphorylated form, VirG acts as an activator of transcription for the remaining Vir gene operons.
Vir D1 + D2 have endonuclease activity, and make single-stranded cuts within the left and right borders.
VirE acts as a ssDNA binding protein, protecting the single strand T-DNA region during the transport phase of the process. Once in the plant cell, the complementary strand of the T-DNA is synthesised.
These and the other Vir genes, are necessary for transfer of the T-DNA, but they function in trans, so none of these genes need to be included in the cloning vectors. Modified Agrobacterium strains provide all the necessary Vir functions on modified Ti plasmids.