Homologous DNA sequences in different Ti-plasmids are essential for oncogenicity

LARGE plasmids, called Ti-plasmids1, are responsible for the transformation of dicotyledonous plants by Agrobacterium tumefaciens2–5. A comparison of Ti-plasmids derived from different Agrobacterium strains6–10revealed two major groups of homology. These correlate with the Ti-plasmid-determined properties of octopine or nopaline utilisation by Ti-plasmid-harbouring agrobacteria2,5,11–13, and octopine or nopaline synthesis by Ti-plasmid-transformed plant cells11,13. It was originally suggested that octopine or nopaline synthesis in crown-gall tissues might be mediated by bacterial genes, transferred to the transformed plant cells14; this can now be restated as being mediated by Ti-plasmid genes, and is supported both by genetic data15and by hybridisation studies16. The capacity to determine both the catabolism and the synthesis of one or the other of these or similar compounds seems to be the dominant evolutionary significance of the Ti-plasmids and one can therefore speak of ‘octopine’- and ‘nopaline’- Ti-plasmids. In view of the fact that the different types of Ti-plasmids only have a limited degree of DNA sequence homology6, but produce transformed plant cells with similar phenotypes, it was conceivable that at least part of the common DNA sequences would be directly and/or indirectly involved in oncogenicity. We report here the finding of a specific DNA segment which is highly conserved among several different Agrobacterium tumefaciens Ti-plasmids. We also give genetic evidence that this region is essential for oncogenicity and overlaps with the Ti-plasmid DNA found in transformed plant cells.