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Agrobacterium tumefaciens is a species of bacteria that causes tumors (commonly known as 'galls' or 'crown galls') in dicots (Smith et al., 1907). This Gram-negative bacterium causes crown gall by inserting 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. The T-DNA contains genes for plant growth hormones, which result in tumor production. The T-DNA also contains genes encoding enzymes that cause the plant to create specialized amino acids which the bacteria can metabolize, called opines (Zupan et al., 2000). Opines are a class of chemicals that serve as a source of energy for A. tumefaciens, but not for most other organisms. The specific type of opine produced by A. tumefaciens C58 infected plants is nopaline (Escobar et al., 2003). Agrobacterium is an alpha proteobacterium 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).
Agrobacterium tumefaciens C58, the first fully sequenced pathovar , was first isolated from a cherry tree crown gall. The genome was simultaneously sequenced by Goodner et al., 2001 and Wood et al., 2001. The genome of A. tumefaciens C58 consists of a circular chromosome, two plasmids, and a linear chromosome. The presence of a covalently bonded circular chromosome is common to Bacteria, with few exceptions. However, the presence of both a single circular chromosome and single linear chromosome is unique to this genus. The two plasmids are pTiC58, responsible for the processes involved in virulence, and pAtC58, coined the “cryptic” plasmid (Goodner et al., 2001) (Wood et al., 2001). The pAtC58 plasmid has been shown to be involved in the metabolism of opines and to conjugate with other bacteria in the absence of the pTiC58 plasmid (Vaudequin-Dransart et al., 1998). If the pTi plasmid is removed the tumor growth that is the means of classifying this species of bacteria does not occur.
The DNA transmission capabilities of Agrobacterium have been extensively exploited in biotechnology as a means of inserting foreign genes into plants. The plasmid T-DNA that is transferred to the plant is an ideal vehicle for genetic engineering (Zambryski, 1983). This is done by cloning a desired gene sequence into the T-DNA that will be inserted into the host DNA. This process has been performed using firefly luciferase gene to produce glowing plants. This luminescence has been a useful device in the study of plant chloroplast function and as a reporter gene (Root, 1988). Under laboratory conditions the T-DNA has also been transferred to human cells, demonstrating the diversity of insertion application (Kunik et al., 2001).
The mechanism by which Agrobacterium inserts materials into the host cell by a type IV secretion system, is very similar to mechanisms used by pathogens to insert materials (usually proteins) into human cells by type III secretion. It also employes a type of signaling conserved in many Gram-negative bacteria called quorum sensing. This making Agrobacterium an important subject of medical research as well.
Goodner B, Hinkle G, Gattung S, Miller N, et al. 2001. Genome Sequence of the Plant Pathogen and Biotechnology Agent Agrobacterium tumefaciens C58. Science. 294:2323-2328.
Kunik T, Tzfira T, Kapulnik Y, Gafni Y, Dingwall C, Citovsky V. 2001. Genetic transformation of HeLa cells by Agrobacterium. Proc. Natl. Acad. Sci. 98:1871-1876.
Moore LW, Chilton WS, Canfield ML. 1997. Diversity of Opines and Opine-Catabolizing Bacteria Isolated from Naturally Occurring Crown Gall Tumors. App. Environ. Microbiol. 63:201-207
Root M. 1988. Glow in the dark biotechnology. Bioscience. 38:745-747.
Vaudequin-Dransart V, Petit A, Chilton WS , Dessaux Y. 1998. The cryptic plasmid of Agrobacterium tumefaciens cointegrates with the Ti plasmid and cooperates for opine degradation. Molec. Plant-microbe Interact. 11:583-591.
Wood DW, Setubal JC, Kaul R, Monks DE, et al. 2001. The Genome of the Natural Genetic Engineer Agrobacterium tumefaciens C58. Science. 294:2317-2323.
Zambryski P. et al. 1983. Ti plasmid vector for introduction of DNA into plant cells without alteration of their normal regeneration capacity. EMBO J. 2:2143-2150.
Zupan J, Muth TR, Draper O, Zambryski P. 2000. The transfer of DNA from Agrobacterium tumefaciens into plants: a feast of fundamental insights. Plant J. 23:11-28.
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