Fig. 1. Schematic illustration of the proposed reactions involved in the trans-splicing of trypanosomatids, trematodes and nematodes. BS, branching site; SL, spliced leader, SS, splice sites.
Trans-splicing is a mechanism of nuclear pre-mRNA maturation that occurs in several branches of eukaryotes, including trypanosomatids, euglenoids, nematodes and flatworms. Trans-splicing appears to reflect an archaic form of RNA processing that is used to donate a leader sequence (spliced leader) from the 5' end of a small mRNA (SL RNA) to pre-mRNAs to form the 5' end of the mature mRNAs (Fig. 1). The fundamental mechanism of trans-splicing parallels that of cis-splicing which is used for intron removal in eukaryotes. One major difference between the two events is that trans-splicing is an intermolecular mechanism that involves the joining of exons from two separately transcribed RNAs, while cis-splicing is an intramolecular mechanism in which intervening sequences in eukaryotic pre-mRNAs are removed. In trypanosomatids, all pre-mRNAs receive the SL, whereas in helminths only a part of the pre-mRNAs is processed by trans-splicing. The functional significance of this unusual mechanism of mRNA 5' end formation is still unclear, but its occurrence in early eukaryotes, such as trypanosomatids, might reflect an ancestral mechanism of gene expression that has been preserved up to the present.