Figure 1

Secondary structures and processing pathways of nuclear group I intron RNAs. (A) Secondary structure diagrams of the group IC1 intron ribozyme in Tetrahymena (Tth.L1925) and the group IE intron ribozyme in Didymium (Dir.S956-1, GIR2). The paired segments (P1 to P9, P13) are noted. The three core domains and the conserved G-C pair at P7 are highlighted. The 5’ and 3’ exons are shown in blue. (B) The two main processing pathways, self-splicing and full-length intron circularization (FLC). The self-splicing pathway involves two transesterification reactions. The first reaction is initiated by a nucleophilic attack by the hydroxyl group of an exogenous guanosine cofactor (exoG) (1). The second transesterification reaction starts with a nucleophilic attack at the 3’ splice site (SS) (2), resulting in ligated exons (LEs) and linear intron (LI) RNA molecules. The FLC pathway involves hydrolysis and transesterification reactions. A hydrolytic cleavage at the 3’ SS (3) is followed by a nucleophilic attack at the 5’ SS by the terminal guanosine (ωG) (4) resulting in a full-length intron circle and non-ligated exons. The FLC pathway is independent of exoG. eG: exogenous guanosine factor covalently linked at the 5’ end of the free intron RNA; exoG: exogenous guanosine cofactor; FLC: full-length circularization; LE: ligated exon; LI; linear intron; SS: splice site.