Updated publication reference for PubMed record(s): 28346450. To study protein-RNA interactions at single amino acid and single nucleotide resolution we developed a new approach, termed cross-linking of segmentally isotope labeled RNA and tandem mass spectrometry (CLIR-MS/MS). The method was developed using the PTBP1-EMCV IRES complex as a model system and additionally applied to the U1 snRNP complex. Data from both complexes are included in this submission.