Agrobacterium tumefaciens strains containing tumour-inducing (Ti) plasmids1-3 incite cancerous growths called crown galls when inoculated into wounded dicotyledonous plants. Tumour tissue can be cultured axenically in vitro, and exhibits a transformed phenotype in the absence of the inciting bacterium. Transformed cells grow autonomously, are auxin and cytokinin autotrophic in vitro4 and synthesize opines5-8, novel amino acid derivatives dictated by Ti plasmid genetic information9-11. A small segment of the Ti plasmid, termed T-DNA is maintained in axenic tumour cells12-19. Mitochondrial and chloroplast DNA from a crown gall teratoma are free from T-DNA, whereas nuclear DNA contains T-DNA in amounts similar to that in total tumour cell DNA20,21. T-DNA appears to be attached to what is presumably plant DNA in the crown gall tumour cell: Southern blot analysis22 of tumour DNA digested with restriction endonucleases reveals T-DNA fragments that are not fully homologous to Ti plasmid DNA14,17,21. We report here the isolation by molecular cloning of a 'border fragment' T-DNA and flanking plant DNA from the crown gall teratoma BT37 and show that T-DNA is covalently joined to a repeated DNA element of the tobacco nuclear genome.
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