Right-hand border regions of octopine T-DNA are recognized by RNA polymerase of Agrobacterium as well as by VirD1 and VirD2 proteins.

The T-DNA of octopine Ti plasmid of Agrobacterium tumefaciens contains TL- and TR-DNA regions each bounded by 25 base-pair-repeats (designated A, B, C and D from left to right). Short DNA segments containing the borders B, C and D were found to function as promoter when placed in the rightward orientation upstream of promoter-less lacZ. Promoter consensus sequence of Agrobacterium were found within these border repeats and in their adjacent regions. The expression of lacZ was low when the segments contained the overdrive, a sequence known to enhance T-DNA transfer. Simultaneous overproduction of VirD1 and D2 proteins, endonuclease acting on the border repeats, interfered with the promoter functions of the border segments. In spite of their activity under these conditions, the border regions do not seem to be involved in the gene expression, because they are not followed by appropriate open reading frames. We propose that RNA polymerase of Agrobacterium competes with VirD products for T-DNA borders and thereby affects the transfer of T-DNA.