RiboTRIBE: Monitoring Translation with ADAR-meditated RNA Editing

RNA translation is tightly regulated to ensure proper protein expression in cells and tissues. Translation is often assayed with biochemical assays such as ribosome profiling and TRAP, which are effective in many contexts. These assays are however not ideal with limiting amounts of biological material when it can be difficult or even impossible to make an extract with sufficient signal or sufficient signal:noise. Because of our interest in translational regulation within the few Drosophila adult circadian neurons, we fused the ADAR catalytic domain (ADARcd) to several small subunit ribosomal proteins and assayed mRNA editing in Drosophila S2 cells. The strategy is named RiboTRIBE and is analogous to a recently published APOBEC-based method. The list of RiboTRIBE-edited transcripts overlaps well with ribosome profiling targets, especially with more highly ranked targets. There is also an enriched number of editing sites in ribosome-associated mRNA comparing to total mRNA, indicating that editing occurs preferentially on polyribosome-associated transcripts. The use of cycloheximide to freeze translating ribosomes causes a substantial increase in the number of RiboTRIBE targets, which is decreased by pretreating cells with the chain terminating drug puromycin. NOTE: Additional experiments performed after first submitting this manuscript to BioRxiv estimate that less than 5% of Rps28b-ADAR is ribosome-associated. This is because the vast majority of the fusion protein sediments at the top of a polyribosome gradient. We therefore suggest that most editing reported in the manuscript is not catalyzed by ribosome-associated ADAR (10/2/2021).