Molecular basis of human nuclear and mitochondrial tRNA 3' processing

Eukaryotic transfer RNA (tRNA) precursors undergo sequential processing steps to become mature tRNAs. In humans, ELAC2 carries out 3' end processing of both nucleus-encoded (nu-tRNAs) and mitochondria-encoded (mt-tRNAs) tRNAs. ELAC2 is self-sufficient for processing of nu-tRNAs but requires TRMT10C and SDR5C1 to process most mt-tRNAs. Here we show that TRMT10C and SDR5C1 specifically facilitate processing of structurally degenerate mt-tRNAs lacking the canonical elbow. Structures of ELAC2 in complex with TRMT10C, SDR5C1 and two divergent mt-tRNA substrates reveal two distinct mechanisms of pre-tRNA recognition. While canonical nu-tRNAs and mt-tRNAs are recognized by direct ELAC2-RNA interactions, processing of noncanonical mt-tRNAs depends on protein-protein interactions between ELAC2 and TRMT10C. These results provide the molecular basis for tRNA 3' processing in both the nucleus and the mitochondria and explain the organelle-specific requirement for additional factors. Moreover, they suggest that TRMT10C-SDR5C1 evolved as a mitochondrial tRNA maturation platform to compensate for the structural erosion of mt-tRNAs in bilaterian animals.