A Ti or tumour inducing plasmid is a circular plasmid that often, but not always, is a part of the genetic equipment that Agrobacterium tumefaciens and Agrobacterium rhizogenes use to transduce its genetic material to plants. The Ti plasmid is lost when Agrobacterium is grown above 28°C. Such cured bacteria do not induce crown galls, i.e. they become avirulent. pTi and pRi share little sequence homology but are functionally rather similar. The Ti plasmids are classified into different types based on the type of opine produced by their genes. The different opines specified by pTi are octopine, nopaline, succinamopine and leucinopine.
The plasmid has 196 genes that code for 195 proteins. There is one structural RNA. The plasmid is 206,479 nucleotides long, the GC content is 56% and 81% of the material is coding genes. There are no pseudogenes.
- virA codes for a receptor which reacts to the presence of phenolic compounds such as acetosyringone, syringealdehyde or acetovanillone which leak out of damaged plant tissues.
- virB encodes proteins which produce a pore/pilus-like structure.
- virC binds the overdrive sequence.
- virD1 and virD2 produce endonucleases which target the direct repeat borders of the T-DNA segment; virD4 is the coupling protein.
- virE binds to T-strand protecting it from nuclease attack, and intercalates with lipids to form channels in the plant membranes through which the T-complex passes, beginning with the right border.
- virG activates vir-gene expression after binding to a consensus sequence, once it has been phosphorylated by virA.
- Agrobacterium is called the natural genetic engineer.
- Size of the plasmid: ~250 kbp.
- Contains one or more T-DNA region.
- Contains a region enabling conjugative transfer.
- Contains regions for opine synthesis and catabolism.
- Responsible for crown gall disease in plants.
- Schell J, Van Montagu M., The Ti-plasmid of Agrobacterium tumefaciens, a natural vector for the introduction of nif genes in plants?, Basic Life Sci. 1977;9:159-79.
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