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Plant transformation via Agrobacterium frequently results in formation of multiple copy T-DNA arrays at one target site of the chromosome. The T-DNA copies are arranged in repeats, direct or inverted around one of the T-DNA borders. A Ti plasmid-derived transformation vector has been constructed enabling direct selection of transformants carrying at least two linked copies of T-DNA in the same orientation. The selection is based on expression of a promoterless neomycin phosphotransferase gene on one T-DNA copy from a promoter located on the other T-DNA copy. After co-cultivation of tobacco protoplasts with Agrobacterium, as many as 30% of regenerated transformed plants carried directly repeated T-DNA copies. The junction regions between two T-DNAs were amplified and 13 amplified fragments were cloned and sequenced. The involvement of T-DNA left and right border sequences in direct repeat junctions was determined. In some junctions, additional filler DNA was detected. The length of filler DNA varied from a few up to almost 300 bp. The longer filler DNAs from two clones were found to be T-DNA fragments in direct or reverse orientation. We discuss the recently suggested models for T-DNA integration and propose that the formation of direct repeats in genomes does not necessarily result from ligation of intermediates (i.e. T-strands), but more likely from the co-integration of several intermediates into one target site. |
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