TARGET SPECIFICITY OF INSERTION ELEMENT IS30

The Escherichia coli resident mobile element IS30 has pronounced targe t specificity. Upon transposition, the element frequently inserts exac tly into the same position of a preferred target sequence. Insertion s ites in phages, plasmids and in the genome of E. coli are characterize d by an exceptionally long palindromic consensus sequence that provide s strong specificity for IS30 insertions, despite a relatively high le vel of degeneracy. This 24-bp-long region alone determines the attract iveness of the target DNA and the exact position of IS30 insertion. Th e divergence of a target site from the consensus and the occurrence of 'non-permitted' bases in certain positions influence the target activ ity. Differences in attractiveness are emphasized if two targets are p resent in the same replicon, as was demonstrated by quantitative analy sis. In a system of competitive targets, the oligonucleotide sequence representing the consensus of genomic IS30 insertion sites proved to b e the most efficient target. Having compared the known insertion sites , we suppose that IS30-like target specificity, which may represent an alternative strategy in target selection among mobile elements, is ch aracteristic of the insertion sequences IS3, IS6 and IS21, too.