The right border region of pTiT37 T-DNA is intrinsically more active than the left border region in promoting T-DNA transformation.

Deletions of border regions of T-DNA on Agrobacterium Ti plasmid or mini-T plasmid have shown the right border region of pTiT37 T-DNA to be more active than the left border region in promoting T-DNA transformation. In this study we examine the possibility that the apparent difference in activity of left and right border regions may be due to position or orientation differences between the left and right borders with respect to the transferred onc genes. We have constructed eight similar single-border mini-T plasmids that contain a left border segment or a right border segment of pTiT37 T-DNA at various positions with either orientation with respect to the onc genes. We assayed the plant tumor-inducing activity of these mini-T plasmids in Agrobacterium tumefaciens LBA4404 containing the virulence helper plasmid pAL4404. Regardless of the position and orientation of the border-containing segment in the mini-T plasmid, mini-T plasmids with the right border segment were highly virulent, whereas those with the left border segment were only weakly so. These results indicate that the difference in transformational activity between the left and right border regions is intrinsic and not an effect of position or orientation with respect to the onc genes. The pattern of the mini-T plasmid sequences integrated into the plant genome suggests that T-DNA transformation involves the directional transfer of a linear intermediate bounded by the border repeats.