eIF1 and eIF5 dynamically control translation start site fidelity.

Human translation initiation requires accurate recognition of translation start sites. While AUG codons are canonical start sites, non-AUG codons are also used, typically with lower efficiency. The initiator tRNA and initiation factors eIF1 and eIF5 control recognition. How they distinguish different start sites yet allow flexible recognition remains unclear. Here we used real-time single-molecule assays and an in vitro reconstituted human system to reveal how eIF1 and eIF5 direct start site selection. eIF1 binds initiation complexes in two modes: stable binding during scanning, followed by transient, concentration-dependent rebinding after start site recognition. Termination of eIF1 rebinding requires transient and concentration-dependent binding by eIF5, which allows the formation of translation competent ribosomes. Non-AUG start sites differentially stabilize eIF1 and destabilize eIF5 binding, blocking initiation at multiple points. We confirmed these opposing effects in human cells. Collectively, our findings uncover that eIF1 and eIF5 directly compete to bind initiation complexes and illuminate how their dynamic interplay tunes the fidelity of start site recognition, which has broad connections to health and disease.