A ribosomal kinetic checkpoint governs selective mRNA recruitment

Translation initiation begins with recruitment of mRNA to the ribosome, yet how mRNA engagement is converted into productive initiation remains unclear. Using real-time fluorescence assays with purified components, we show that mRNA recruitment proceeds through a branched kinetic pathway on the 40S subunit. Following rapid sampling, mRNAs partition into either a productive accommodated state or an arrested state that stabilizes ribosome binding before accommodation. mRNA structure, eIF3, and eIF3j bias recruitment toward arrest, whereas eIF4F promotes accommodation in an ATP-dependent manner coupled to displacement of eIF3j from the mRNA entry channel. Unstructured mRNAs accommodate independently of eIF4E, whereas structured mRNAs require an upstream eIF4E-dependent step, enabling selective recruitment under limiting eIF4E. Arrested complexes can convert directly into the accommodated state without dissociation, revealing a reversible standby intermediate poised for activation. Together, our findings establish mRNA accommodation as a ribosome-intrinsic checkpoint governing initiation and provide a framework for selective translation.