TRANSSPLICING 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 (appr oximately 100 nt) SL RNA via trans-splicing. Parallel in vitro and in vivo experiments have begun to clarify both the mechanism and biologic al role of trans-splicing. In vitro analysis (in cell free extracts) h as shown that trans-splicing is remarkably similar to the snRNP mediat ed removal of intervening sequences from pre-mRNAs (cis-splicing). Add itionally, this analysis has suggested a mechanism that may explain ho w the two substrates of trans-splicing (the SL RNA and pre-mRNA) effic iently associate with one another in the absence of sequence complemen tarity. In vivo experiments suggest that a major biological function o f trans-splicing in nematodes may be to process polycistronic transcri ption units. Results obtained from the study of both parasitic and fre e-living species are discussed, and trans-splicing in nematodes is com pared and contrasted to the analogous process in trypanosomatid protoz oans.