The mechanism of induction of the plant cancer crown gall by Agrobacterium tumefaciens has been briefly described. The salient points are as follows. 1. Large plasmids of molecular weight (100 to 150) S 10(6), called Ti-plasmids, are essential to the transformation process. 2. Ti-plasmids carry a DNA segment that can be transferred to, and maintained and expressed in, transformed plant cells. 3. This DNA segment has been identified both by direct hybridization experiments between Ti DNA fragments and crown gall DNA and by the study of a deletion mutant of a Ti-plasmid. 4. Indirect evidence suggests that genes involved in the synthesis of abnormal amino acids (such as octopine and nopaline) by crown gall cells, and known to be carried on the Ti-plasmids, are in fact located on the DNA segment that is transferred to the plant cells. 5. Ti-plasmids are efficient conjugative plasmids, since they can promote their own transfer by conjugation to various plasmid-free bacterial strains. Their conjugative properties may be involved also in the Ti DNA transfer from Agrobacterium to plant cells. 6. Preliminary evidence indicates that the transferable segment of the Ti-plasmid has the structure of a transposon, since it appears to be flanked by a sequence exhibiting most of the properties of the sequences that border the known bacterial drug-resistance gene transposons.
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