Involvement of Rad52 in T‐DNA circle formation during Agrobacterium tumefaciens‐mediated transformation of Saccharomyces cerevisiae

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.