Figure 1. Trans- and cis-splicing pathways. Comparable steps in the two reactions are drawn next to each other for direct comparison. The 5¢ exon in the trans-splicing reaction is contained in the SL RNA.

Figure 2. Predicted secondary structures of SL RNAs from different organisms. Each contains a 5¢ terminal stem loop that includes the SL exon and the 5¢ splice site (ss, indicated by an arrow). In the 3¢ intronic region of the molecule, each SL RNA contains a binding site for the common Sm proteins. Secondary structures are not drawn to scale, and subtle features such as bulges are not noted.

Figure 3. Trans-spliceosome assembly. The RNA containing the 3¢ splice site (3¢ acceptor) interacts with U2 snRNP, assisted by interactions with SR proteins bound to splicing enhancers (enh) or by interactions with U1 snRNP bound downstream (exon definition). The SL RNP interacts with the U4/U6·U5 tri-snRNP to form the SL·U4/U6·U5 tetra-snRNP. In a manner dependent on SR proteins, these initial complexes associate functionally, leading to the first catalytic step of the reaction – generation of the free SL exon and the Y-branched intermediate. In the second step of the reaction, the trans-spliced product is formed.