Quantitative profiling of initiating ribosomes in vivo

Cells have evolved exquisite mechanisms to fine-tune the rate of protein synthesis in response to stress. Systemic mapping of start codon positions and precise measurement of the corresponding initiation rate would transform our understanding of translational control. Here we present quantitative translation initiation sequencing (QTI-seq), where the initiating ribosomes can be profiled in real time at single nucleotide resolution. The resultant initiation map not only delineates variations of start codon selection, but also highlights a dynamic range of initiation rates in response to nutrient starvation. The integrated data set provides unique insights into principles of alternative translation and mechanisms controlling different aspects of translation initiation. Using RiboTag mice, QTI-seq permits tissue-specific profiling of initiating ribosomes in vivo. Liver cell-specific ribosome profiling uncovers a robust translational reprogramming of the proteasome system in fasted mice. Our findings illuminate the prevalence and dynamic nature of translational regulation pivotal to physiological adaptation in vivo.