Ti plasmid

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nucleic acid
Ti plasmid
General
Surname Ti plasmid
Identifiers
GenBank

CP011249.1

RefSeq DNA

NZ_CP011249.1

properties
Taxon

Agrobacterium tumefaciens

Tumor-inducing plasmids ( english T umor i nducing plasmid ) are plasmids that often but not always, the genetic makeup of the bacteria Agrobacterium tumefaciens and rhizogenes Agrobacterium include.
The genes of the Ti plasmid enable agrobacteria to transfer DNA into plant cells and to modify them genetically. They trigger tumor-like growths and thus plant diseases.

Genes of the Ti plasmids

The Ti plasmid is about 200 kb in size. The following groups of important genes are found on the plasmid:

  • Transfer or tra gene
  • Virulence or vir genes (both necessary for DNA transfer into the plant cell)
  • Genes for opine catabolism (necessary for the breakdown of opines by the bacterium)

as well as the transfer or t-DNA which can be transferred into the plant cell

  • Tumorgenesis or onc genes (necessary for tumor induction in the plant cell, for example tms1 , tms2 and tmr ) and
  • Opine synthesis or ops genes (necessary for the synthesis of opines by the plant cell, e.g. nos for nopaline synthesis, ocs for octopine synthesis, etc.).

The t-DNA is flanked by imperfect tandem repeats of 25 bases : " left border " ( LB ) and " right border " ( RB ) signal the beginning and end of the t-DNA to the vir proteins. These cis elements are essential for the transmission of t-DNA.

The plasmid also has an origin of replication ( ori or origin of replication ) and other regions. The genes for the recognition of the plant cells and the attachment to them, however, are located in the bacterial genome .

Mechanism of gene activation and transfer

Acetosyringone and other phenolic compounds released from wounded plant parts activate the product of the vir A gene, a sensor kinase located in the outer bacterial cell membrane . This phosphorylates and activates the VirG protein. VirG is a response regulator , namely an activator of the other vir genes, the transcription rate of which thus increases many times over. The genes now transcribed include vir D, vir E and vir B. VirD is an endonuclease . The protein causes a single-strand break on the right repeat that flanks the t-DNA. Here begins the synthesis of a complementary strand; the single strand is displaced. This mechanism is similar to the rolling circle replication of plasmids, except that here a straight single strand with a predetermined end is created. VirE binds and stabilizes the single-stranded t-DNA and protects it from degradation. VirB is located in the bacterial cell membrane and allows t-DNA to be transferred through direct contact with the plant cell. In addition, the adhesion proteins chvA, chvB and pscA are involved in t-DNA transfer. The mechanism of DNA transfer is similar to that of bacterial conjugation . The VirD protein remains bound to the t-DNA and enters the plant cell with it. The transferred t-DNA is about 20 kb in size.

Integration of the t-DNA

The VirE protein has a nuclear localization sequence and after it has been introduced into the plant cell, it is transported along the cytoskeleton into the plant cell nucleus together with the encased t-DNA . The integration into the plant genome is rather unspecific, but is favored by certain tandem repetitions.

Effect of t-DNA

The onc genes contain, among other things, the genes for tryptophan monooxygenase (gene iaaM , where iaa stands for indole-3-acetic acid , the biologically active plant hormone auxin), indole acetamide hydrolase ( iaaH ) and isopentenyltransferase ( itpZ ). These enzymes disrupt the plant's hormonal balance by inducing an additional synthesis of the phytohormones auxin and cytokinin . The cells begin to divide; the growths called "galls" arise. The tumors induced by Ti plasmids usually consist of little or no differentiated callus mass . The root growths triggered by Ri plasmids of Agrobacterium rhizogenes , on the other hand, represent at least partially differentiated root tissue. The ops genes cause the plant cell to produce nitrogen-rich opines. Since the genes for the catabolism (breakdown) of the opines are not transferred, the bacteria have an exclusive source of energy and nitrogen at their disposal. It is interesting that the genes of the t-DNA from the bacterium have a typical eukaryotic structure and are active in the plant cell.

Genetic engineering

The genetic modification of plants is comparatively difficult. For this reason, the natural system of gene transfer in plant cells has been intensively researched and applied. When transforming plants, Ti plasmids are used, in which the genes for tumor formation have been replaced by the desired genes. The addition of nematodes causes damage to the plants at the cellular level and paves the way for infection by agrobacteria.

The system of plant transformation with the help of agrobacteria is successfully used to research plant genetics. It is also used to produce commercial genetically modified plants. examples are

and other.

Binary vectors

The system of binary vectors is used to keep the amount of foreign genes transferred to the plant as low as possible. Binary vectors are the most widely used system in A. tumefaciens- mediated transfer of genes to plants. In this system there are two plasmids in the bacterium. A disarmed Ti plasmid carrying the vir region but having its T-DNA region removed; called 'helper Ti plasmid'. In addition, a plasmid that carries the gene of interest in a T-DNA region that is to be transferred. When plants are infected with A. tumefaciens , which carries the two plasmids, the helper Ti plasmid triggers the association of the bacterium with the plant cell. The T-DNA of the second plasmid is then transferred from the bacterium to the plant genome.

Individual evidence

  1. ↑ Yadav, NS, et al . ( 1982 ). Short direct repeats flank the T-DNA on a nopaline Ti plasmid. Proceedings of the National Academy of Sciences -Biological Sciences 79 (20): 6322-6326, PMID 16593241 ; PDF (free full text access)
  2. ↑ Komori, T. et al . ( 2007 ). Current status of binary vectors and superbinary vectors. Plant Physiology 145 (4): 1155-1160; PMID 18056865 ; PDF (free full text access)

literature

  • Frank Kempken and Renate Kempken: Genetic engineering in plants: opportunities and risks . Jumper; 3rd, revised. u. updated edition (2006); ISBN 978-3540336617
  • David Clark, Nanette Pazdernik, Andreas Held (translator): Molecular Biotechnology: Fundamentals and Applications . Spectrum Academic Publishing 2009; ISBN 978-3827421289
  • PC Trivedi: Plant Tissue Culture and Biotechnology Jaipur 2006. ISBN 978-8171324477