266 Chapter 7
Figure 7.19 The four classes of RNA splicing
either be stitched together sequentially, or certain exons can be skipped and left out of the mature message
(see Figure 6.4). As a result of this process, called alternative splicing, a single pre-mRNA gene can gen-
erate many different types of proteins, thereby providing a form of combinatorial diversity that is not
genetically encoded.^34
7.2.2.2 Self-Splicing and Other Splicing Pathways. While most eukaryotic splicing is carried out
by the spliceosome, there are specialized genes and introns that are spliced through different mechanisms
(Figure 7.19). For example, certain tRNA genes contain introns in the anticodon, and these are removed
by the sequential action of protein endonucleases and ligases. But perhaps the most remarkable pathways
for splicing involve introns that are inherently reactive, and which can splice themselves out of flanking
exons without the aid of spliceosomal machinery. These self-splicing intronsfall into two categories, the
group I and group II introns.35,36The discovery of these autocatalytic RNA molecules, or ribozymes,
along with other families of catalytic RNA molecules, was one of the most exciting developments in 20th
century biochemistry (Section 7.6.2).
Figure 7.18 The two steps of RNA splicing catalyzed by the spliceosome and group II introns. The 5-exon is shown
in red, the 3-exon is shown in grey, and the nucleophilic adenosine is indicated. The intron is shown as
a black line. Note that the lariat structure is connected by 2–3–5linkages to the adenosine