The right hand copy of the nopaline Ti-plasmid 25 bp repeat is required for tumour formation.

At either end of the nopaline Ti-plasmid T-region resides a copy of a 25 bp repeated element. The normal T-DNA endpoint is 1 bp internal of the right copy, with the transcription initiation site of the nopaline synthase (nos) gene being approximately 300 bp away in the same direction. Here we describe results which demonstrate that deletion of any combination of sequences between the nos initiation site and the right copy of the 25 bp repeat does not affect oncogenicity. Thus a mutant retaining the right copy and only 3 bp internal of it is indistinguishable from the wild type parent in its oncogenic properties. However deletion of a further 39 bp, including complete removal of the right copy abolishes crown gall tumour formation on Kalanchöe and tobacco. From these results we infer that unlike the left border, the right copy of the 25 bp repeat is required for T-DNA transfer and/or integration. This is the first conclusive demonstration of the involvement of a copy of the repeats in this process.