Figure 1. Secondary structures of group I introns. Schematic representations of the secondary structure of group I introns drawn two different ways. Dashed lines refer to exon sequences and the arrows point to the splice sites. The dot in P1 denotes the conserved GU wobble at the 5¢ splice site. The universally conserved GC pair in P7 (major component of the G-binding site) and the conserved motifs (P, Q, R and S) are shown. Note the involvement of exon sequences in both P1 and P10. The internal guide sequences (IGS) are defined as the two portions of intron sequences pairing with the exon sequences.

Figure 2. P4–P6 domain of the Tetrahymena intron. (a) Schematic representation of the Tetrahymena intron as shown by Cate et al. (1997). The adenosine platforms are shown underlined. Non-Watson–Crick base pairs are indicated by dots. One of the two clamp interactions holding the helical stacks in close proximity is emphasized by a double-headed arrow. (b) Schematic representation of the above-mentioned clamp interaction between a GAAA tetraloop (L5b) and its receptor (J6a/6b). The stacked nucleotides of the loop and of the receptor are boxed. Tertiary contacts are shown by dotted lines. Redrawn from Cate et al. (1996) Science 273: 1678–1685.

Figure 3. Group I splicing mechanism. (a) Schematic representation of the splicing reaction of group I intron. Not shown is the need of the intron to fold into an active three-dimensional conformation for the reaction to occur. (b) The chemistry of the first step of the splicing reaction along with proven (Mg) and putative (*) coordination sites of divalent metal ions. The chemistry of the second step is exactly the same except that the nucleophile is the 3¢-OH of the exon 1 (instead of the free guanosine) and the scissile bond is the 3¢ splice site (which means that the 5¢ base will be the terminal G and the 3¢ portion will be the exon 2). Redrawn from Pyle (1993) Science 261: 709–714.