Organisation of Vir Region - The vir region contains 8 operons (designated as virA, vir B, vir C, vir D, vir E, vir F, vir G and vir H ), which together span about 40 kb of DNA and have 25 genes.
This region mediates the transfer of T-DNA into plant genomes, and hence is essential for virulence or production of crowngall/hairy root disease; therefore, it is called the virulence region or vir region.

The genes of vir region are not transferred themselves; they only induce the transfer of T-DNA. On the other hand, the genes present in T-DNA are not required for its transfer; only the 24 bp direct repeat left and right borders of T-DNA are essential for the transfer. Of the 8 vir operons, 4 operons, viz., virA, vir B, vir D and vir G, are essential for virulence, while the remaining 4 operons play an accessory role.

The operons virA and vir G are constitutive, encode one protein each, and are concerned with the regulation of all the vir operons. The other vir operons encode various proteins involved in T-DNA transfer.