Wednesday, September 03, 2008


Ti plasmid
From Wikipedia, the free encyclopedia

The structure of the Ti plasmid
Ti 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 plasmid has 196 genes that code for 195 proteins. There is no one structural RNA. The plasmid is 206,479 nucleotides long, the GC content is 56% and 81 per cent of the material is coding genes. There are no pseudogenes.
The modification of this plasmid is very important in the creation of transgenic plants, but only in dicotyledon plants.

Agrobacterium tumefaciens
From Wikipedia, the free encyclopedia

Bacterium tumefaciens Smith and Townsend 1907Pseudomonas tumefaciens (Smith and Townsend 1907) Duggar 1909Phytomonas tumefaciens (Smith and Townsend 1907) Bergey et al. 1923Polymonas tumefaciens (Smith and Townsend 1907) Lieske 1928
Agrobacterium tumefaciens is the causal agent of Crown Gall disease (the formation of tumours) in over 140 species of dicot. It is a rod shaped, Gram negative soil bacterium (Smith et al., 1907). Symptoms are caused by the insertion of a small segment of DNA (known as the T-DNA, for 'transfer DNA') into the plant cell, which is incorporated at a semi-random location into the plant genome.
Agrobacterium tumefaciens (or A. tumefaciens) is an alphaproteobacterium of the family Rhizobiaceae, which includes the nitrogen fixing legume symbionts. Unlike the nitrogen fixing symbionts, tumor producing Agrobacterium are parasitic and do not benefit the plant. The wide variety of plants affected by Agrobacterium makes it of great concern to the agriculture industry (Moore et al., 1997).
Economically, A. tumefaciens is a serious pathogen of grape vines, stone fruits, nut trees, sugar beets, horse radish and rhubarb.

Conjugation
In order to be virulent, the bacterium must contain a tumour-inducing plasmid (Ti plasmid or pTi), of 180 kb, which encodes the T-DNA and all the genes necessary to transfer it to the plant cell. Many strains of A. tumefaciens do not contain a pTi.
Since the Ti plasmid is essential to cause disease, pre-penetration events in the rhizosphere occur to promote bacterial conjugation - exchange of plasmids amongst bacteria. In the presence of opines, A. tumefaciens produces a diffusible conjugation signal called 30C8HSL or the Agrobacterium autoinducer. This activates the transcription factor TraR, positively regulating the transcription of genes required for conjugation.

Method of infection
The Agrobacterium tumefecian infects the plant through its Ti plasmid. The Ti plasmid integrates a segment of its DNA, known as T DNA, into the chromosomal DNA of its host plant cells.
Transfer of T-DNA into plant cell
The T-DNA must be cut out of the circular plasmid. A VirD1/D2 complex nicks the DNA at the left and right border sequences. (ref to picture of Ti plasmid. This is the rolling circle mechanism that allows rplasmid to be transformed into many plant cells.) The VirD2 protein is covalently attached to the 5' end. VirD2 contains a motif that leads to the nucleoprotein complex being targeted to the type IV secretion system (T4SS).


http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120078/bio40.swf::The%20Ti%20Plasmid

A really damn good video talking abt agrobacterium tumefaciens that probably will help in Prelims. really! Go watch! NOW!

Word of the Day; 3:42 PM