a physical bridge along which DNA can be pulled to opposite poles of a dividing cell. Meanwhile, the RepB protein can bind specifically to the parS sequence, forming a complex with DNA that can be recognized by RepA. This system is particularly important for the proper partitioning of the Ti plasmid, as the plasmid is only present in few copy numbers in the bacterial cell. Maintenance of the Ti plasmid The Ti plasmid is maintained at low copy numbers within a bacterial cell. This is partly achieved by influencing the expression of the replication initiator RepC. When bound to ADP, RepA is activated to work with RepB, acting as a negative regulator of the repABC cassette. The levels of RepC is therefore kept low within a cell, preventing too many rounds of replication from occurring during each cell division cycle. Furthermore, there is a small RNA known as RepE encoded between repB and repC that lowers the expression of repC. RepE is complementary to RepC and will bind with the repC mRNA to form a double-stranded molecule. This can then block the translational production of the RepC protein. Separately, the expression of the repABC cassette and hence the copy number of the Ti plasmid is also influenced via a quorum sensing system in Agrobacterium. Quorum sensing systems respond to bacterial population densities by sensing a molecule, known as an autoinducer, that is produced by the bacterial cells at low levels and would build up to a threshold level when there is a high density of bacteria present. In this case, the autoinducer is the N-3-oxooctanoyl-L-homoserine lactone (3-O-C8-AHL) molecule, which is sensed by a regulator known as TraR. When activated, TraR will bind to regions known as tra boxes in the repABC gene cassette's promoter regions to drive expression. Therefore, a high level of population density increases the number of plasmids present within each bacterial cell, likely to support pathogenesis in the plant host. Features Virulence operon The expression of the vir region is usually repressed under normal conditions, and only becomes activated when the bacteria senses plant-derived signals from wound sites. This activation is necessary for the production of Vir proteins and the transfer of DNA and proteins into host plant cells. VirA and VirG form a two-component regulatory system within Agrobacterium. This is a type of sensing and signalling system found commonly in bacteria; in this case, they act