Translation of CircRNAs

TLDR

Circular RNAs (circRNAs) are abundant and evolutionarily conserved RNAs of largely unknown function. Here, we show that a subset of circRNAs is translated in vivo. By ribosome footprinting of fly heads and in vivo/in vitro translation assays, we demonstrate that circRNAs associate with translating ribosomes and that their UTRs enable cap‑independent translation. We found that many ribo‑circRNAs use the host mRNA start codon, are bound by membrane‑associated ribosomes, possess conserved stop codons, that a muscleblind‑derived circRNA encodes a detectable protein, and that starvation and FOXO likely regulate translation of a circMbl isoform, collectively providing strong evidence for circRNA translation and an unexplored layer of gene activity.

Abstract

Circular RNAs (circRNAs) are abundant and evolutionarily conserved RNAs of largely unknown function. Here, we show that a subset of circRNAs is translated in vivo. By performing ribosome footprinting from fly heads, we demonstrate that a group of circRNAs is associated with translating ribosomes. Many of these ribo-circRNAs use the start codon of the hosting mRNA, are bound by membrane-associated ribosomes, and have evolutionarily conserved termination codons. In addition, we found that a circRNA generated from the muscleblind locus encodes a protein, which we detected in fly head extracts by mass spectrometry. Next, by performing in vivo and in vitro translation assays, we show that UTRs of ribo-circRNAs (cUTRs) allow cap-independent translation. Moreover, we found that starvation and FOXO likely regulate the translation of a circMbl isoform. Altogether, our study provides strong evidence for translation of circRNAs, revealing the existence of an unexplored layer of gene activity.