Read-through marking' reveals differential nucleotide composition of read-through and truncated cDNAs in iCLIP

We established a modified iCLIP protocol, called 'read-through marking', which facilitates the detection of cDNAs that have not been truncated upon encountering the RNA-peptide complex during reverse transcription (read-through cDNAs). A large proportion of these cDNAs would be undesirable in an iCLIP library, as it could affect the resolution of the method. To this end, we added an oligonucleotide to the 5'-end of RNA fragments-a 5'-marker-to mark the read-through cDNAs. By applying this modified iCLIP protocol to PTBP1 and eIF4A3, we found that the start sites of read-through cDNAs are enriched in adenosines, while the remaining cDNAs have a markedly different sequence content at their starts, preferentially containing thymidines. This finding in turn indicates that most of the reads in our iCLIP libraries are a product of truncation with valuable information regarding the proteins' RNA-binding sites. Thus, cDNA start sites confidently identify a protein's RNA-crosslink sites and we can account for the impact of read-through cDNAs by commonly adding a 5'-marker.