DNA looping and catalysis; the IHF-folded arm of Tn10 promotes conformational changes and hairpin resolution.

DNA loops and bends are common features of DNA processing machines. The bacterial transposon Tn10 has recruited integration host factor (IHF), a site-specific DNA-bending protein, as an architectural component for assembly of the higher-order nucleoprotein complex within which the transposition reaction takes place. Here, we demonstrate additional roles for the IHF loop during the catalytic steps of the reaction. We show that metal ion-dependent unfolding of the IHF-bent transposon arm is communicated to the catalytic center, inducing a substantial conformational change in the DNA. Partial disruption of the IHF loop shows that this step promotes resolution of the hairpin intermediate on one transposon end and initiation of catalysis at the other. Further evidence suggests that the molecular mechanism responsible may be mechanical stress in the IHF loop, related to a change in the relative position of the transposase contacts that anchor the loop on either side.