Reactivity-dependent profiling of RNA 5-methylcytidine dioxygenases

Epitranscriptomic RNA modifications can regulate fundamental biological processes, but we lack approaches to map modification sites and probe writer enzymes. Here we present a chemoproteomic strategy to characterize RNA 5-methylcytidine (m5C) dioxygenase enzymes in their native context based upon metabolic labeling and activity-based crosslinking with 5-ethynylcytidine (5-EC), RNA-protein enrichment, and quantitative proteomics. We profile m5C dioxygenases in human cells including ALKBH1 and TET2 and use quantitative nucleoside LC-MS to show that ALKBH1 is the major hm5C and f5C-forming enzyme in RNA, including upon polyadenylated RNA. Further, we map ALKBH1 modification sites transcriptome-wide using 5-EC-based iCLIP analysis to show that ALKBH1 oxidizes m5C in a variety of tRNA anticodon stem loops (ASL), as well as on mRNA and lncRNA, and analyze its substrate specificity using in vitro enzymatic assays. Finally, we apply pyridine borane-mediated sequencing to identify f5C sites in tRNA ASLs. Our work provides powerful chemical approaches for studying RNA m5C dioxygenases and mapping oxidative m5C modifications and reveals the existence of novel epitranscriptomic pathways for regulating RNA function.