Transfer DNA

The transfer DNA (abbreviated T-DNA) is the transferred DNA of the tumor-inducing (Ti) plasmid of some species of bacteria such as Agrobacterium tumefaciens and Agrobacterium rhizogenes. The T-DNA is transferred from bacterium into the host plant's nuclear DNA genome. The T-DNA is bordered by 25-base-pair repeats on each end. Transfer is initiated at the right border and terminated at the left border and requires the vir genes of the Ti plasmid.

The bacterial T-DNA is about 24,000 base pairs long and contains genes that code for enzymes synthesizing opines and phytohormones. By transferring the T-DNA into the plant genome, the bacterium essentially reprograms the plant cells to grow into a tumor and produce a unique food source for the bacteria. The synthesis of the plant hormones auxin and cytokinin enables the plant cell to grow uncontrollably, thus forming the crown gall tumors typically induced by Agrobacterium infection. The opines are amino acid derivatives used by the bacterium as a source of carbon and energy.

Agrobacterium-mediated T-DNA transfer is widely used as a tool in biotechnology. In genetic engineering, the tumor-promoting and opine-synthesis genes are removed from the T-DNA and replaced with a gene of interest and/or a selection marker, which is required to establish which plants have been successfully transformed. Examples of selection markers include neomycin phosphotransferase, hygromycin B phosphotransferase (which both phosphorylate antibiotics) and phosphinothricin acetyltransferase (which acetylates and deactivates phosphinothricin, a potent inhibitor of glutamine synthetase). Agrobacterium is then used as a vector to transfer the engineered T-DNA into the plant cells where it integrates into the plant genome. This method can be used to generate transgenic plants carrying a foreign gene.

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