Figure 5

Structure diagrams of different stages of the GIR1 lariat capping ribozyme derived from a complex twin-ribozyme group I intron. (A) Twin-ribozyme intron (Dir.S956-1) from the D. iridis Panama 2 isolate. A standard group IE splicing ribozyme (GIR2; Figure 1B) contains an insertion in helix P2, which consists of a homing endonuclease gene (HEG) and the lariat capping group I-like ribozyme (GIR1). (B) The active GIR1 conformation performs a transesterification reaction at the junction between P9 and P10 resulting in a 3-nucleotide 2’,5’ lariat structure at the 5’ end of the HE messenger. (C) The regulatory domain of GIR1 resembles a complex riboswitch, which alternates between a catalytic inactive GIR1 (containing HEG-P1; the off state) and an active GIR1 conformation (containing DP2 and P10; the on state). This rearrangement involves replacement of RNA structures (color coded). BP: branch point; GIR1: group I-like ribozyme; GIR2: group I splicing ribozyme; HE: homing endonuclease; HEG: homing endonuclease gene; IPS: internal processing site.