The discovery by Cech and colleagues in 1981 that the intervening sequence of a ribosomal intron of Tetrahymena can catalyze its own excision and the coupling of this reaction with the ligation of exons initiated an intense interest in RNA-catalyzed reactions. This interest was further kindled by the demonstration by Altman and colleagues in 1983 of the first true ribozyme activity, the ability of the RNA component of bacterial RNAse P to act in trans to cleave multiple molecules of precursor tRNA and remain unchanged at the end of the reaction. The aim of this review is to consider a number of RNA-catalyzed reactions and to provide a detailed discussion of the adaptation of these reactions, where demonstrated, to systems showing ribozyme activity in trans. The examples discussed are the RNA component of RNase P, the intervening sequence of Tetrahymena, the hammerhead self-cleavage reaction of one viroid, several virusoids, and the plus RNA of satellite tobacco ringspot virus RNA, and the self-cleavage reaction of the minus RNA of satellite tobacco ringspot virus RNA, of plus and minus RNA of hepatitis delta virus, and of the recently discovered transcript of a plasmid in mitochondria of Neurospora.
