Agrobacterium tumefaciens cells carrying a tumour inducing plasmid (Ti-plasmid) can transfer a defined region of transfer DNA (T-DNA) to plant cells as well as to yeast. This process of Agrobacterium-mediated transformation (AMT) eventually results in the incorporation of the T-DNA in the genomic DNA of the recipient cells. All available evidence indicates that T-strand transfer closely resembles conjugal DNA transfer as found between Gram-negative bacteria. However, where conjugal plasmid DNA transfer starts via relaxase-mediated processing of a single origin of transfer (oriT), the T-DNA is flanked by two imperfect direct border repeats which are both substrates for the Ti-plasmid encoded relaxase VirD2. Yeast was used as a model system to investigate the requirements of the recipient cell for the formation of T-DNA circles after AMT. It was found that, despite the absence of self-homology on the T-DNA, the homologous repair proteins Rad52 and Rad51 are involved in T-DNA circle formation. A model is presented involving the formation of T-DNA concatemers derived from T-strands by a process of strand-transfer catalysed by VirD2. These concatemers are then resolved into T-DNA circles by homologous recombination in the recipient cell.
© 2014 John Wiley & Sons Ltd.