Protein-DNA contacts and conformational changes in the Tn10 transpososome during assembly and activation for cleavage.

IHF or supercoiling is required early in Tn10 transposition, but at later stages they inhibit the reaction in a classic homeostatic loop. We investigated the mechanism of transpososome assembly and regulation using hydroxyl radical DNA protection and interference. We present a three-dimensional molecular model for the IHF-bent end of Tn10 wrapped around a transposase core. Contacts span some 80 bp at the transposon end, but after assembly of an active complex containing metal ion, most contacts become dispensable. These include transposase contacts beyond the IHF site that chaperone assembly of the complex and are needed for efficient cleavage. Single and double-end breaks do not affect the complex but divalent metal ions promote large conformational changes at bp +1 and the flanking DNA.