The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 192

sequence, secondary structure, and function. While the possibility of con-
vergent evolution cannot be formally ruled out, the above-mentioned sim-
ilarities suggest that an in-depth knowledge of the function of spliceosomal
snRNAs and their relationship to group II introns are essential for under-
standing the spliceosomal function.
2. STRUCTURAL AND FUNCTIONAL SIMILARITIES
BETWEEN snRNAs AND THE GROUP II SELF-SPLICING
INTRONS
The discovery of group II introns, self-splicing ribozymes that are
found in all three kingdoms of life, coincided with intense experimental
efforts to determine the mechanism of pre-mRNA splicing in eukary-
otes. 9-11 It was shown that both the spliceosome and group II introns per-
form splicing through an identical catalytic strategy involving two
transesterification steps. In both systems, the first step involves nucleophilic
attack on the 5 0 splice site by the 2 0 hydroxyl group of a specific adenosine
residue in the intron called the branch site adenosine ( Fig. 6.1 ). This leads to
a transesterification reaction in which the 2 0 oxygen of the branch site aden-
osine replaces the 3 0 oxygen of the last nucleotide in the upstream exon. The
result of this reaction is the release of the first exon and the formation of an
Figure 6.1 U6 and U2 snRNAs and the mRNA at the time of the first and second steps of
splicing (left and right, respectively). The location of U6, U2, and the U6 intramolecular
stem-loop is shown. The intron is depicted as a light gray line initially connecting the
two exons (dark gray). The position of the 5 0 splice site (5 0 SS), 3 0 splice site (3 0 SS), and
branch site are indicated. Solid arrows point to the site of nucleophilic attack during
each step of splicing. Arrowheads mark the location of hydroxyl radical cleavage sites
generated by a tethered Fe-BABE probe, which is shown as a starburst. 56 Base-pairing
interactions are shown by short black lines. The location of the 2 0 -5 0 linkage formed
after the first step of splicing at the branch site is shown.