Intramolecular transposition by Tn10

Transposon Tn10 promotes the formation of a circular product containing only transposon sequences. We show that these circles result from an intramolecular transposition reaction in which all of the strand cleavage and ligation events have occurred but newly created transposon/target junctions have not undergone repair. The unligated strand termini at these junctions are those expected according to a simple model in which the target DNA is cleaved by a pair of staggered nicks 9 bp apart, transposon sequences are separated from flanking donor DNA by cleavage at the terminal nucleotides on both strands (at both ends) of the element, and 3' transposon strand ends are ligated to 5' target strand ends. The stability of the unligated junctions suggests that they are protected from cellular processing by transposase and/or host proteins. We propose that the nonreplicative nature of Tn10 transposition is determined by the efficiency with which the nontransferred transposon strand is separated from flanking donor DNA and by the nature of the protein-DNA complexes present at the strand transfer junctions.