Widespread Exon Junction Complex Footprints in the RNA Degradome Mark mRNA Degradation before Steady State Translation.

Exon junction complexes (EJCs) are deposited on mRNAs during splicing and displaced by ribosomes during the pioneer round of translation. Nonsense-mediated mRNA decay (NMD) degrades EJC-bound mRNA, but the lack of suitable methodology has prevented the identification of other degradation pathways. Here, we show that the RNA degradomes of Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), worm (Caenorhabditis elegans), and human (Homo sapiens) cells exhibit an enrichment of 5' monophosphate (5'P) ends of degradation intermediates that map to the canonical EJC region. Inhibition of 5' to 3' exoribonuclease activity and overexpression of an EJC disassembly factor in Arabidopsis reduced the accumulation of these 5'P ends, supporting the notion that they are in vivo EJC footprints. Hundreds of Arabidopsis NMD targets possess evident EJC footprints, validating their degradation during the pioneer round of translation. In addition to premature termination codons, plant microRNAs can also direct the degradation of EJC-bound mRNAs. However, the production of EJC footprints from NMD but not microRNA targets requires the NMD factor SUPPRESSOR WITH MORPHOLOGICAL EFFECT ON GENITALIA PROTEIN7. Together, our results demonstrating in vivo EJC footprinting in Arabidopsis unravel the composition of the RNA degradome and provide a new avenue for studying NMD and other mechanisms targeting EJC-bound mRNAs for degradation before steady state translation.