Short direct repeats flank the T-DNA on a nopaline Ti plasmid.

Crown gall disease results from the insertion of a segment of the Agrobacterium Ti plasmid, called T-DNA, into host plant nuclear DNA. We have subjected to sequence analysis the border regions of pTi T37 (ends of T-DNA) and one left T-DNA/plant DNA border fragment isolated from BT37 tobacco teratoma by molecular cloning. These sequence studies, taken together with published sequence of a right T-DNA/plant DNA border fragment, allowed us to identify the positions of left and right borders at the DNA sequence level. Comparison of left and right border regions of the Ti plasmid revealed a "core" direct repeat of 13 of 14 bases (12 contiguous) precisely at the borders of T-DNA. An extended repeat of 21 of 25 bases overlaps this core repeat. T-DNA on the Ti plasmid exhibits no longer direct or inverted repeats in the border regions, based on Southern hybridization studies. The physical structure of T-DNA differs from that of known prokaryotic and eukaryotic transposable elements but bears a structural resemblance to the prophage of bacteriophage lambda.