ILLEGITIMATE RECOMBINATION IN PLANTS - A MODEL FOR T-DNA INTEGRATION

Agrobacterium tumefaciens is a soil bacterium capable of transferring DNA (the T-DNA) to the genome of higher plants, where it is then stably integrated. Six T-DNA inserts and their corresponding preinsertion sites were cloned from Arabidopsis thaliana and analyzed. Two T-DNA integration events from Nicotiana tabacum were included in the analysis. Nucleotide sequence comparison of plant target sites before and after T-DNA integration showed that the T-DNA usually causes only a small (13-28 bp) deletion in the plant DNA but larger target rearrangements can occur. Short homologies between the T-DNA ends and the target sites, as well as the presence of filler sequences at the junctions, indicate that T-DNA integration is mediated by illegitimate recombination and that these processes in plants are very analogous to events in mammalian cells. We propose a model for T-DNA integration on the basis of limited base-pairing for initial synapsis, followed by DNA repair at the junctions. Variations of the model can explain the formation of filler DNA at the junctions by polymerase slipping and template switching during DNA repair synthesis and the presence of larger plant target DNA rearrangements.