Mechanism of Agrobacterium-mediated genetic transformation of plant cell
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Agrobacterium-mediated genetic transformation is the only known example of horizontal gene transfer from bacteria to eucaryota including plants, fungi and animal cells. The knowledge of the basic mechanism of this process is the key to understand problems concerning methods of plant transformation and transgene expression. The main element of the system is Ti plasmid (tumor-inducing) containing T-DNA (transferred DNA) delimited by 25. nucleotide sequences (left and right borders). Any DNA located on Ti plasmid and flanked by these two borders might be recognised by Agrobacterium as a T-DNA and integrated into plant chromosome. The process is controlled by ten vir operons located on Ti plasmid. The most important products of the vir genes are VirA and VirG controlling the expression of all the other vir genes. VirD1 and VirD2 recognise 25 bp border sequences and take part in endonucleolitic cleavage. Additionally, VirD2 covalently attached to the 5?-end of single stranded (ss) T-DNA targets it into the nucleus of a plant cell. T-strand is coated by VirE2 molecules, each containing two sites of nuclear localisation signals (NLS). Eleven of VirB and VirD4 proteins are required to form a transmembrane channel and transfer T-strand to the plant cell. Some genes of the bacterial chromosome are responsible for bacterial attachment and colonisation of the plant cell.
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A. Nadolska-Orczyk, Pracownia Inzynierii Komorkowej i Transformacji, Instytut Hodowli i Aklimatyzacji Roslin, Radzikow, 05-870 Blonie, Poland