A tumor- inducing (hence the acronym) plasmid found in the bacterium Agrobacterium tumefaciens (q.v.) that is responsible for crown gall disease of dicotyledonous plants. When Ti DNA is integrated into the DNA of the host plant, cytokinins and auxins (both of which see) are produced. It is these hormones that cause gall formation. Only a small part of the plasmid actually enters the plant; the rest stays in the bacterium, where it has other functions. The wild-type plasmid produces tumor cells, but it can be modified so that it can carry foreign genes into cells without making the recipient cells tumorous. During tumor induction, a specific segment of the Ti plasmid, called the T-DNA (transferred DNA), integrates into the host plant nuclear DNA. Ti-mediated tumorigenesis is the first case of a horizontal mobile element (q.v.) that transfers DNA between cells that belong to separate kingdoms. See Chronology, 1974, Zaenen et al.; 1981, Kemp and Hall; genetic engineering, promiscuous DNA, selfish DNA.
Subjects: Genetics and Genomics.