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Physical identification of branched intron side‐products of splicing in Trypanosoma brucei.

Every mRNA in trypanosomes consists of two exons, a common 5′ capped mini‐exon or spliced leader and a coding‐exon. All evidence suggests that the exons are joined by trans‐splicing of two individual precursor RNAs, the mini‐exon donor RNA or spliced leader precursor RNA (medRNA) and the pre‐mRNA. We studied intermediates of the splicing reaction using denaturing two‐dimensional PAGE and structurally identified a group of small (approximately 180‐300 nt) non‐polyadenylated, Y‐shaped branched RNAs. The branched Y‐shaped RNAs contain the 105 nt medRNA derived intron, joined in a 2′‐5′ phosphodiester bond to small heterogeneously sized RNAs. These non‐polyadenylated branched Y‐shaped RNA molecules are analogous to the lariat shaped introns of higher eukaryotes and presumably represent the released intron‐like by‐products of a trans‐splicing reaction which joins the mini‐exon and the major coding‐exon.