The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 209

Figure 6.7 The spliceosomal cycle. The spliceosomal complexes formed during a splic-
ing cycle are shown. The snRNAs present in each splicing complex are indicated. The
exons on the pre-mRNA are shown as two black rectangles, with the intron drawn as
a thin line connecting the two. The position of the branch site adenosine is marked.
Some of the proteins involved in each conformational rearrangement are shown.
independently forming part of the active site, or even direct involvement in
catalysis. 83
As detailed above, the ability of RNAs to form strong, specific interac-
tions by base pairing with another RNA is extensively utilized in the
spliceosome. Base-pairing interactions contribute to substrate recognition
(U1 and U2), positioning of the branch site in a strained catalytic conforma-
tion (U2), regulation of the activity of another snRNA (U4), and juxtapo-
sition of reactive substrates (U2, U5, and U6). Similar RNA-RNA
interactions play identical or closely related roles in group II introns, and
it is conceivable that snRNAs have maintained these roles throughout the
evolution of the spliceosome from group II-like introns.