RNA catalyzed reactions. (1) The two-step self-splicing reaction of group I introns. (2) The two-step self-splicing reaction of group II introns. An internal hydroxyl initiates the attack. (3) Cleavage of the 5′ leader sequence from pre-tRNA by RNase P. (4) Self-cleavage reaction of a number of small pathogenic RNAs and a few other RNAs.