Microbiology Monographs Volume 11, 2009, pp 133-147

Ti and Ri Plasmids

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Agrobacterium species harboring tumor-inducing (Ti) or hairy root-inducing (Ri) plasmids cause crown gall or hairy root diseases, respectively. These natural plasmids provide the basis for vectors to construct transgenic plants. The plasmids are approximately 200 kbp in size. Complete sequence analysis indicates that the pathogenic plasmids contain gene clusters for DNA replication, virulence, T-DNA, opine utilization, and conjugation. T-DNA genes have lower G + C content, which is presumably suitable for expression in host plant cells. Besides these genes, each plasmid contains a large number of unique genes. Even plasmids of the same opine type differ considerably in gene content and have highly chimeric structures. The plasmids seem to interact with each other and with plasmids of other members of the Rhizobiaceae and are likely to shuffle genes for infection between Ti and Ri plasmids. Plasmid stability genes are discussed, which are important for plasmid evolution and construction of useful strains.