ISOLATION, CHARACTERIZATION AND TRANSPOSITION OF AN (IS2)(2) INTERMEDIATE

We have isolated and characterized a dimer derivative of the extensive ly studied Escherichia coli insertion sequence IS2. The dimer structur e - called (IS2)(2) - consists of two IS2 elements arranged as a direc t 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) i nto a target plasmid resulted in cointegrate-type structures. The tran sposition frequency of the (IS2)(2) dimer itself was significantly hig her than that of the isogenic monomer IS2 insertion. The poor stabilit y and high activity of (IS2)(2) indicates that this is an active trans position intermediate. The mode of transposition of (IS2)(2) is analog ous to the joined dimer model described in the case of (IS21)(2) and ( IS30)(2).