Conformational toggling controls target site choice for the heteromeric transposase element Tn7.

The bacterial transposon Tn7 facilitates horizontal transfer by directing transposition into actively replicating DNA with the element-encoded protein TnsE. Structural analysis of the C-terminal domain of wild-type TnsE identified a novel protein fold including a central V-shaped loop that toggles between two distinct conformations. The structure of a robust TnsE gain-of-activity variant has this loop locked in a single conformation, suggesting that conformational flexibility regulates TnsE activity. Structure-based analysis of a series of TnsE mutants relates transposition activity to DNA binding stability. Wild-type TnsE appears to naturally form an unstable complex with a target DNA, whereas mutant combinations required for large changes in transposition frequency and targeting stabilized this interaction. Collectively, our work unveils a unique structural proofreading mechanism where toggling between two conformations regulates target commitment by limiting the stability of target DNA engagement until an appropriate insertion site is identified.