Target DNA structure plays a critical role in Tn7 transposition

The bacterial transposon Tn7 utilizes four Tn7-encoded proteins, TnsA, TnsB , TnsC and TnsD, to make insertions at a specific site termed attTn7. This target is selected by the binding of TnsD to attTn7 in a sequence-specific manner, followed by the binding of TnsC and activation of the transposase, We show that TnsD binding to attTn7 induces a distortion at the 5' end of t he binding site and TnsC contacts the region of attTn7 distorted by TnsD. P revious work has shown that a target site containing tripler DNA, instead o f TnsD-attTn7, can recruit TnsABC and effect site-specific insertion of Tn7 . We propose that the DNA distortion imposed by TnsD on attTn7, like the al tered DNA structure via tripler formation, serves as a signal to recruit Tn sC, We also show that TnsD primarily contacts the major groove of DNA, wher eas TnsC is a minor groove binding protein. The footprint of the TnsC-TnsD- attTn7 nucleoprotein complex includes and extends beyond the Tn7 insertion site, where TnsC forms a platform to receive and activate the transposase t o carry out recombination.