Group I and group II introns are common in lower eukaryotes, such as fungi and yeast, although the lat-
ter is abundant in plants as well. Remarkably, group I and group II introns have also been found in bacteria,
and constitute the only type of intron found so far among prokaryotic organisms. Group I introns (together
with RNase P) were the very first autocatalytic RNA molecules to be discovered.^37 During experiments on
the splicing of a ribosomal gene in the protozoan Tetrahymena thermophila, Thomas R. Cech and col-
leagues found that the rRNA gene repeatedly spliced in control reactions to which enzymatic extracts had
not been added. In fact, the only cofactors required for group I intron splicing were found to be the
nucleotide guanosine and Mg^2 ions. Subsequent mechanistic study has revealed that group I introns fold
into an elaborate three-dimensional structure that positions both splice sites and binds a guanosine mol-
ecule for use as a nucleophile during the first step of splicing (Figure 7.20).^25 As in spliceosomal process-
ing, group I intron splicing is the result of two sequential SN 2 trans-esterifications that ultimately release
intron and ligate the exons (Figures 7.17 and 7.18). Both the folding of the molecule and subsequent cata-
lysis require Mg^2 ions, which has been shown to play an important and general role in the tertiary fold-
ingof RNA molecules.
Group II introns are highly abundant in the organellar genes of plants, fungi, and yeast (Figure 7.21).
They have been subjects of particular interest because their mechanism of splicing is so closely related to
that of the spliceosome. Like the latter, group II introns utilize a 2-hydroxyl group of a bulged adenosine
RNA Structure and Function 267
Figure 7.20 The structure of a group I intron. (a) The secondary structure and (b) a crystal structure of the tertiary
structure for the Azoarchus group I intron. Corresponding colours indicate specific domains of the
intron. In this structure, the intron and both exons are intact, revealing all active-site components and
their relative locations
(Reprinted from Ref. 25. © (2004), with permission from Macmillan Publications Ltd)