The minimum internal and external sequence requirements for transposition of the eukaryotic transformation vector piggyBac.

The piggyBac element from Trichoplusia ni is recognized as a useful vector for transgenesis of a wide variety of species. This transposable element is 2472 bp in length, and has a complex repeat configuration consisting of an internal repeat (IR), spacer, and terminal repeat (TR) at both ends, and a single ORF encoding the transposase. Excision assays performed in microinjected T. ni embryos using plasmids deleted for progressively larger portions of the piggyBac internal sequence reveal that the 5' and 3' IR, spacer, and TR configuration is sufficient for precise excision of piggyBac when transposase is provided in trans. Interplasmid transposition assays using plasmids carrying varying lengths of intervening sequence between the piggyBac termini in T. ni demonstrate that a minimum of 55 bp of intervening sequence is required for optimal transposition, while lengths less than 40 bp result in a dramatic decrease in transposition frequency. These results suggest that the piggyBac transposase may bind both termini simultaneously before cleavage can occur, and/or that the formation of a transposition complex requires DNA bending between the two termini. Based on these results we constructed a 702-bp cartridge with minimal piggyBac 5' and 3' terminal regions separated by an intervening sequence of optimal length. Interplasmid transposition assays demonstrate that the minimal terminal configuration is sufficient to mediate transposition, and also verify that simply inserting this cartridge into an existing plasmid converts that plasmid into a non-autonomous piggyBac transposon. We also constructed a minimal piggyBac vector, pXL-Bac, that contains an internal multiple cloning site sequence between the minimal terminal regions. These vectors should greatly facilitate the utilization of the piggyBac transposon in a wide range of hosts.