Nematode spliced leaders--ubiquity, evolution and utility.

Most nematode messenger RNAs (mRNAs) have at their 5' end a common 22 nucleotide leader sequence, the trans-spliced leader or SL1. The presence of this leader on some but not all mRNAs raises several questions: What is the role of the spliced leader in mRNA maturation, stability and translation? Why do some genes have a spliced leader and others not? What is the evolutionary origin of this trans-splicing mechanism? Recently, additional trans-spliced leaders (SL2, 3, 4, 5) have been described. What role do these variants play in nematode gene expression? While definitive answers to these questions remain elusive, it is clear that the spliced leader will significantly facilitate the cloning and sequence analysis of most nematode mRNAs.