Ti plasmid
[1] 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.
[1] A key feature of Ti plasmids is their ability to drive the production of opines, which are derivatives of various amino acids or sugar phosphates, in host plant cells.
However, these tumour cells did possess the ability to produce opines metabolized by the infecting bacterial strain.
[8] The addition of deoxyribonucleases (DNases) to degrade DNA also failed to prevent the formation and growth of the plant tumors.
Studies into the T-DNA region determined their process of transfer and identified genes allowing the synthesis of plant hormones and opines.
[15] Separately, early work aimed to determine the functions of the genes encoded in the 'vir' region - these were broadly categorized into those that allowed bacterial-host interactions and those that enabled T-DNA delivery.
[3] Laboratory experiments have shown that the RepC protein binds to this region, suggesting its role as the origin of replication.
[4] The ability of RepA to form filaments allows it to create 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.
[1] When bound to ADP, RepA is activated to work with RepB, acting as a negative regulator of the repABC cassette.
[18] 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.
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.
[4] 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.
During the sensing, VirA, a histidine sensor kinase, will become phosphorylated before passing on this phosphate group to the response regulator VirG.
[1][19] 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.
These proteins influence the pathogenesis of the Agrobacterium towards different plant hosts, and mutations can reduce but not remove the virulence of the bacteria.
[26][27] 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.
[29] 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).
[34] 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.
[43][44] The ability of A. tumefaciens to induce crown gall tumours in certain plant species but not others has been attributed to the presence or absence of this virF gene.
[45] The T-DNA of Agrobacterium is approximately 15-20 kbp in length and will become integrated into the host plant genome upon its transfer via a process known as recombination.
Within the host plant cell's genome, the T-DNA of Agrobacterium is expressed to produce two main groups of proteins.
Together, these proteins will recognize and bind to a region known as the origin of transfer (oriT) in the Ti plasmid to form the relaxosome complex.
[28] This receptor is an essential component of T4SSs, and is thought to energize and mediate the transfer of the DNA into the translocation channel between two cells.
[58] There, the border sequences will be recognized by the transfer apparatus of A. tumefaciens and delivered in a standard manner into the target plant cell.
