IS903 transposase mutants that suppress defective inverted repeats

The inverted repeats (IRs) of the insertion element IS903 are composed of t wo functional regions. An inner region, consisting of basepairs 6-18, is th e transposase binding site. The outer region (positions 1-3) is not contact ed during initial transposase binding, but is essential for efficient trans position. We have examined the interaction of the in with the transposase b y isolating transposase suppressors of in mutations. These suppressors defi ne two patches within the N-terminus of the protein. One class of suppresso rs, which rescued the majority of outer in mutants tested, contained mutati ons in close proximity to an aspartate residue (D121) believed to form part of the catalytic DDE motif, suggesting that their suppressive effect is in the positioning of the catalytic site at the terminus of the transposon. T he hypertransposition phenotype of mutant VA119 is also consistent with thi s hypothesis. The second class was more allele specific and preferentially suppressed a mutation at position 3 of the in, Finally, we showed that muta tions at the termini of the in elevate the frequency of cointegrate formati on by IS903, Other outer in mutations did not have this effect. These data are consistent with the terminal bases of the transposon playing multiple a nd distinct roles in transposition.