Spliced leader RNA of trypanosomes: in vivo mutational analysis reveals extensive and distinct requirements for trans splicing and cap4 formation.

In trypanosomes mRNAs are generated through trans splicing. The spliced leader (SL) RNA, which donates the 5′‐terminal mini‐exon to each of the protein coding exons, plays a central role in the trans splicing process. We have established in vivo assays to study in detail trans splicing, cap4 modification, and RNP assembly of the SL RNA in the trypanosomatid species Leptomonas seymouri. First, we found that extensive sequences within the mini‐exon are required for SL RNA function in vivo, although a conserved length of 39 nt is not essential. In contrast, the intron sequence appears to be surprisingly tolerant to mutation; only the stem‐loop II structure is indispensable. The asymmetry of the sequence requirements in the stem I region suggests that this domain may exist in different functional conformations. Second, distinct mini‐exon sequences outside the modification site are important for efficient cap4 formation. Third, all SL RNA mutations tested allowed core RNP assembly, suggesting flexible requirements for core protein binding. In sum, the results of our mutational analysis provide evidence for a discrete domain structure of the SL RNA and help to explain the strong phylogenetic conservation of the mini‐exon sequence and of the overall SL RNA secondary structure; they also suggest that there may be certain differences between trans splicing in nematodes and trypanosomes. This approach provides a basis for studying RNA‐RNA interactions in the trans spliceosome.