Figure 1. The hammerhead (a) and M1 ribozymes (b), and the group I self-splicing intron (c). For the hammerhead N equals A, C, G or U, and H equals A, C or U. Arrows indicate the cleavage sites on the hammerhead and the group I intron. Due to the large size of the M1 ribozyme and the group I intron, only schematic illustrations of their secondary structures are shown. IGS is the internal guide sequence of the group I intron, and thick lines indicate the exons. The invariant G is indicated in (c).

Figure 2. The group II self-splicing intron (a), and the hepatitis (b), hairpin (c) and Neurospora VS ribozymes (d). The circled ‘A’ in the group II intron is the internal adenosine that serves as the initiating nucleophile. Exons are indicated by thick lines. Arrows indicate the locations of cleavage sites. N equals A, C, G or U; P equals A or G; and Y equals C or U.

Figure 3. Therapeutic applications of ribozymes. (a) Cleavage of a complementary messenger RNA (mRNA) by the hammerhead ribozyme. (b) Normal reaction catalysed by ribonuclease P or the M1 ribozyme. (c) An external guide sequence (EGS) can be used to elicit in vivo cleavage of a complementary RNA target by ribonuclease P. (d) The use of the group I ribozyme to trans-splice and repair a defective mRNA.