We have isolated and characterized a dimer derivative of the extensively studied Escherichia coli insertion sequence IS2. The dimer structure - called (IS2)2 - consists of two IS2 elements arranged as a direct repeat, separated by 1 bp. The junction between the (IS2)2 dimer and target sequences is located at various positions in indepent isolates; however, one position was preferred. The transposition of (IS2)2 into a target plasmid resulted in cointegrate-type structures. The transposition frequency of the (IS2)2 dimer itself was significantly higher than that of the isogenic monomer IS2 insertion. The poor stability and high activity of (IS2)2 indicates that this is an active transposition intermediate. The mode of transposition of (IS2)2 is analogous to the joined dimer model described in the case of (IS21)2 and (IS30)2.
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