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13 Nucleases That Are RNA

Michael D. Been


A. Discovery of RNA-mediated RNA Cleavage and Ligation: Self-splicing Ribozymes
In some species of Tetrahymena, the gene for the large subunit ribosomal RNA (rRNA) is interrupted by an intron or intervening sequence (IVS) of about 400 nucleotides. The intron sequence is transcribed along with the exon sequences, and it is removed in a posttranscriptional processing event called RNA splicing. As with any precursor RNA that is processed by splicing, the RNA must be cleaved at the 5′ and 3′ ends of the intron (the 5′ and 3′ splice sites), the intron removed, and the exons (generally the structural or coding sequences) ligated to re-form the covalent backbone of the RNA. Cech et al. (1981) found that the Tetrahymena rRNA precursor (pre-rRNA) could be prepared in vitro for use as a substrate in the splicing reaction. Use of this pre-rRNA as a substrate to identify splicing factors led to the discovery of self-splicing RNA: RNA that spliced in vitro in the absence of proteins (Cech et al. 1981; Kruger et al. 1982). Thus, within 1 year of the first “Nuclease” meeting at Cold Spring Harbor Laboratory in 1981, the definition of “enzyme” was under revision. At that meeting, data demonstrating breakage and rejoining of a deproteinized RNA derived from the intron of the Tetrahymena pre-rRNA (Grabowski et al. 1981) were presented by P. Grabowski. Not only did the excised intron RNA catalyze its own breakage and rejoining, but the precursor RNA, from which the intron was excised, appeared to...

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