eIF3a cooperates with sequences 5′ of uORF1 to promote resumption of scanning by post-termination ribosomes for reinitiation on GCN4 mRNA

Yeast initiation factor eIF3 (eukaryotic initiation factor 3) has been implicated in multiple steps of translation initiation. Previously, we showed that the N-terminal domain (NTD) of eIF3a interacts with the small ribosomal protein RPS0A located near the mRNA exit channel, where eIF3 is proposed to reside. Here, we demonstrate that a partial deletion of the RPS0A-binding domain of eIF3a impairs translation initiation and reduces binding of eIF3 and associated eIFs to native preinitiation complexes in vivo. Strikingly, it also severely blocks the induction of GCN4 translation that occurs via reinitiation. Detailed examination unveiled a novel reinitiation defect resulting from an inability of 40S ribosomes to resume scanning after terminating at the first upstream ORF (uORF1). Genetic analysis reveals a functional interaction between the eIF3a-NTD and sequences 5′ of uORF1 that is critically required to enhance reinitiation. We further demonstrate that these stimulatory sequences must be positioned precisely relative to the uORF1 stop codon and that reinitiation efficiency after uORF1 declines with its increasing length. Together, our results suggest that eIF3 is retained on ribosomes throughout uORF1 translation and, upon termination, interacts with its 5′ enhancer at the mRNA exit channel to stabilize mRNA association with post-termination 40S subunits and enable resumption of scanning for reinitiation downstream.