The ZN domain acts as a dynamic switch coordinating multiple-step aminoacylation in bacterial leucyl-tRNA synthetase.

Aminoacyl-tRNA synthetases (AARSs) safeguard translational fidelity by coordinating amino acid activation and tRNA charging within distinct catalytic and editing domains. In leucyl-tRNA synthetase (LeuRS), the small, centrally located zinc-binding domain (ZN domain) sits at the crossroads of these functional centres, yet its role has remained enigmatic. Here, we present crystal structures of the Escherichia coli LeuRS-tRNALeu complex that capture the ZN domain in a previously unobserved conformation, revealing the pre-activation state. By integrating structural data with kinetic analysis, we propose a model for the aminoacylation reaction in which the ZN domain functions as a dynamic molecular switch that coordinates the different catalytic steps of the reaction cycle. We show that the ZN domain first acts as a wedge to prime the synthetic active site while locking the 3'-end tRNALeu in the editing domain. It subsequently reorients to stabilize the tRNA acceptor stem for aminoacyl transfer, before releasing it for proofreading. These findings resolve the mechanism of long-range domain communication in LeuRS and identify a discrete, druggable intermediate for the design of next-generation antimicrobials.