Figure 4.

Schematic representations of trans-splicing and cis-splicing in C. elegans. (A) Spliced leader trans-splicing. A 2,2,7-trimethylguanosine (TMG)-capped 22-nt spliced leader (SL) sequence derived from an SL snRNA in SL snRNP replaces a 7-methylguanosine (m⁷G)-capped outron in a pre-mRNA. A y-shaped outron is excised. (B) Cis-splicing. A lariat-shaped intron is excised and the upstream and downstream exons are ligated. The 5′-splice site (5′SS), 3′-splice site (3′SS) and branch point (BP) are recognized by U1 snRNP (U1), U2 auxiliary factor (U2AF) and U2 snRNP (U2), respectively. Boxes represent exons and solid lines indicate introns and outrons. Dashed lines connect exons that are ligated in the splicing reactions. Cap structures and branch points are indicated with green and black circles, respectively. Almost invariable nucleotide sequences of the splice sites are indicated.