TRANSSPLICING - AN UPDATE

5'-end maturation of messenger RNAs via acquisition of a trans-spliced leader sequence occurs in several primitive eukaryotes, some of which are parasitic. This type of trans-splicing proceeds though a two-step reaction pathway directly analogous to that of cia-splicing and like cia-splicing it requires multiple U snRNP cofactors. This minireview a ttempts to provide a brief synopsis of our current understanding of th e evolution and biological significance of trans-splicing. Progress in deciphering the mechanism of trans-splicing, particularly as it relat es to current models of cia-splicing, is also discussed.