Self-splicing. A ribosomal RNA precursor from Tetrahymena, representative of the group I introns, splices itself in the presence of a guanosine cofactor (G, shown in green). A 414-nucleotide intron (red) is released in the first splicing reaction. This intron then splices itself twice again to produce a linear RNA that has lost a total of 19 nucleotides. This L19 RNA is catalytically active.
[Information from T. Cech. RNA as an enzyme. Copyright © 1986 by Scientific American, Inc. All rights reserved.]