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The molecular basis of the neoplasmic transformation of plant cells by the crown gall bacterium Agrobacterium tumefaciens is the transfer to and stable maintenance of T DNA - a well defined segment of the bacterial Ti plasmid - in plant cells1-4. Transformed cells express new biosynthetic capacities, such as the synthesis of opines5, of which octopine6 and nopaline7 are well known examples. Ti plasmids harbour genes enabling the bacteria to degrade opines and use them as carbon and nitrogen sources8. pTi-linked genes also determine the specificity of synthesis of opines in the plant tumour cells8. On this basis the Ti plasmid can be considered an unusual type of catabolic plasmid, which induces the synthesis of its substrate in transformed plant cells. This relationship, called genetic colonization2, has opened prospects for the use of Ti plasmids as vectors for genetic manipulation of plants. To evaluate this possibility we inserted a well defined DNA segment, the bacterial transposon Tn7 (ref. 9), into the Ti-plasmid DNA sequence that determines nopaline synthesis in A. tumefaciens strain T37 Nocc1 (ref. 10). As we report here, the inserted Tn7 DNA segment became part of the T DNA because the 9.6 × 106 molecular weight (MW) Tn7 DNA sequence was transferred to, and maintained in, the DNA of tumour tissue cultures induced by this mutant strain. |
