Trans-splicing of nematode premessenger RNA.

In nematodes, many mRNAs contain a common 5' terminal 22-nt sequence. This sequence, the spliced leader (SL), is acquired from a small (approximately 100 nt) SL RNA via trans-splicing. Parallel in vitro and in vivo experiments have begun to clarify both the mechanism and biological role of trans-splicing. In vitro analysis (in cell free extracts) has shown that trans-splicing is remarkably similar to the snRNP mediated removal of intervening sequences from pre-mRNAs (cis-splicing). Additionally, this analysis has suggested a mechanism that may explain how the two substrates of trans-splicing (the SL RNA and pre-mRNA) efficiently associate with one another in the absence of sequence complementarity. In vivo experiments suggest that a major biological function of trans-splicing in nematodes may be to process polycistronic transcription units. Results obtained from the study of both parasitic and free-living species are discussed, and trans-splicing in nematodes is compared and contrasted to the analogous process in trypanosomatid protozoans.