page_title: Ti plasmid
section_title: Virulence operon
hierarchical_section_title: Ti plasmid / Features / Virulence operon
caption_attribution_description: English: The composition of the Vir region of Ti plasmids
context_page_description: A tumour inducing plasmid is a plasmid found in pathogenic species of Agrobacterium, including A. tumefaciens, A. rhizogenes, A. rubi and A. vitis.
  Evolutionarily, the Ti plasmid is part of a family of plasmids carried by many species of Alphaproteobacteria. Members of this plasmid family are defined by the presence of a conserved DNA region known as the repABC gene cassette, which mediates the replication of the plasmid, the partitioning of the plasmid into daughter cells during cell division as well as the maintenance of the plasmid at low copy numbers in a cell. The Ti plasmids themselves are sorted into different categories based on the type of molecule, or opine, they allow the bacteria to break down as an energy source.
  The presence of this Ti plasmid is essential for the bacteria to cause crown gall disease in plants. This is facilitated via certain crucial regions in the Ti plasmid, including the vir region, which encodes for virulence genes, and the transfer DNA region, which is a section of the Ti plasmid that is transferred via conjugation into host plant cells after an injury site is sensed by the bacteria.
context_section_description: 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 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 the recipient cell. Within the recipient cell, VirD2 will also work together with VirE2 to direct the transferred DNA to the recipient cell's nucleus. There are suggestions that VirD2 may be phosphorylated and dephosphorylated by different proteins, affecting its ability to deliver DNA. Conversely, little is known about VirD3, and mutational analyses have not provided any support for its role in the virulence of Agrobacterium. Finally, VirD4 is a crucial part of the conjugation process, serving as a coupling factor that recognizes and transfers the T-strand to the transport channel. 
  The virE operon encodes for 2 proteins: VirE1 and VirE2. VirE2 is an effector protein translocated together with the T-strand into host plant cells. There, it binds to the T-strand to direct its delivery to the nucleus of the host plant cell. Part of this activity involves the presence of nuclear localization sequences within the protein, which marks the protein and the associated DNA for entry into the nucleus. It also protects the T-strand from nuclease attack. There is some speculation regarding the role of VirE2 as a protein channel, allowing DNA to move through the plant cytoplasmic
caption: The composition of the vir region of octopine-type Ti plasmids
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