to sense plant-derived signals to drive the expression of the vir region. During the sensing, VirA, a histidine sensor kinase, will become phosphorylated before passing on this phosphate group to the response regulator VirG. The activated response regulator VirG can then bind to a region of DNA known as the vir box, located upstream of each vir promoter, to activate the expression of the vir region. One possible downstream functions of the sensing mediated by VirA and VirG is the directional movement, or chemotaxis, of the bacteria towards plant-derived signals; this allows the Agrobacterium to move towards the wound site in plants. Furthermore, with the induction of the vir region, the transfer of T-DNA can be mediated by the Vir proteins. The virB operon is the largest operon in the vir region, encoding for 11 VirB proteins involved in the transfer process of T-DNA and bacterial proteins into host plant cells (see transfer apparatus below). The virC operon encodes for two proteins: VirC1 and VirC2. These proteins influence the pathogenesis of the Agrobacterium towards different plant hosts, and mutations can reduce but not remove the virulence of the bacteria. Both the virC and virD operons can be repressed by a chromosomally encoded protein known as Ros. Ros binds to a region of DNA that overlaps with the binding site of the VirG regulator, and therefore competes with VirG to control their expression levels. Functionally, VirC1 and VirC2 promote the assembly of a relaxosome complex during the conjugative transfer of T-DNA from the bacteria to the host plant cell. This is an energy-dependent process mediated via their NTPase activity, and occurs as they bind to a region of DNA known as overdrive. As a result, they act to increase the amount of T-DNA strands produced. Following the production of the DNA strand to be transferred (transfer strand, T-strand), the VirC proteins can also help to direct the transfer strand to the transfer apparatus. The virD operon encodes for 4 proteins: VirD1-D4. VirD1 and VirD2 are involved in the processing of T-DNA during conjugation to produce the T-strand; this is the single-stranded DNA molecule that is transported to the host plant cell (see transfer apparatus below). During the processing, VirD1 will act as a topoisomerase to unwind the DNA strands. VirD2, a relaxase, will then nick one of the DNA strands and remain bound to the DNA as it is transferred to