Genome-wide RNA-binding analysis of the trypanosome U1 snRNP proteins U1C and U1-70K reveals cis/trans-spliceosomal network.

Trans-splicing in trypanosomes adds a 39-nucleotide mini-exon from the spliced leader (SL) RNA to the 5' end of each protein-coding sequence. On the other hand, cis-splicing of the few intron-containing genes requires the U1 small nuclear ribonucleoprotein (snRNP) particle. To search for potential new functions of the U1 snRNP in Trypanosoma brucei, we applied genome-wide individual-nucleotide resolution crosslinking-immunoprecipitation (iCLIP), focusing on the U1 snRNP-specific proteins U1C and U1-70K. Surprisingly, U1C and U1-70K interact not only with the U1, but also with U6 and SL RNAs. In addition, mapping of crosslinks to the cis-spliced PAP [poly(A) polymerase] pre-mRNA indicate an active role of these proteins in 5' splice site recognition. In sum, our results demonstrate that the iCLIP approach provides insight into stable and transient RNA-protein contacts within the spliceosomal network. We propose that the U1 snRNP may represent an evolutionary link between the cis- and trans-splicing machineries, playing a dual role in 5' splice site recognition on the trans-spliceosomal SL RNP as well as on pre-mRNA cis-introns.