Figure 1.

Biochemical reactions naturally catalyzed by RNA. (a) Precursor tRNA hydrolysis by bacterial RNase P yields a phosphate-containing 5' end of the mature tRNA and a 3'-hydroxyl group at the 5' cleavage product. (b-d) Transesterification reactions catalyzed by (b) the small nucleolytic ribozymes, (c) group I introns, and (d) group II introns, in which different chemical groups serve as the attacking nucleophile. In the small nucleolytic ribozymes (b), a defined 2'-hydroxyl attacks the neighboring 3',5'-phosphodiester bond, resulting in a 2',3'-cyclic phosphate and a 5'-hydroxyl in the respective cleavage products. In the first step of group I intron splicing (c), the 3'-hydroxyl of the exogenous guanosine (G) cofactor attacks the 5'-exon-intron junction and sets the 5' exon free, which leads to the covalent attachment of the cofactor to the 5' end of the intron. In a second transesterification reaction, the 5' exon forms a conventional 3',5' bond with the 3' exon, releasing the linear intron with the additional guanosine [1]. In group II introns (d), the conserved branch-point adenosine (A) serves as the nucleophile, leading to the formation of a lariat intron. (e) Peptide-bond formation catalyzed by the ribosome.