RESUMEN
Translation initiation is often attributed as the rate-determining step of eukaryotic protein synthesis and key to gene expression control. Despite this centrality, the series of steps involved in this process is poorly understood. Here, we capture the transcriptome-wide occupancy of ribosomes across all stages of translation initiation, enabling us to characterize the transcriptome-wide dynamics of ribosome recruitment to mRNAs, scanning across 5' UTRs and stop codon recognition, in a higher eukaryote. We provide mechanistic evidence for ribosomes attaching to the mRNA by threading the mRNA through the small subunit. Moreover, we identify features that regulate the recruitment and processivity of scanning ribosomes and redefine optimal initiation contexts. Our approach enables deconvoluting translation initiation into separate stages and identifying regulators at each step.