The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 195

Figure 6.3 Known RNA-RNA interactions at the spliceosomal catalytic core at the time
of the first step of splicing. The central domain of human U6 and the 5 0 domain of
human U2 that contain the sequences necessary for splicing in vivo are shown. Numbers
next to the sequences refer to the human numbering system. The base-pairing interac-
tions forming helices I, II, and III are indicated. The exons in pre-mRNA are drawn as rect-
angles, and the intron as a thin solid line. The sequence of the intron branch site is
shown. The conserved stem-loop I of U5 snRNA is shown at the bottom, with the thin
solid and dashed lines denoting noncanonical base pairs with exonic sequences next to
the splice sites. The ACAGAGA and AGC domains are in boldface. The solid curved line
points to two residues that form a functionally essential tertiary interaction. 57 The
dashed curved line points to the binding sites for two functionally required metal ions,
which may be located near each other in activated spliceosomes. 56
3. ROLE OF snRNAs IN THE SPLICEOSOMAL ACTIVE SITE
Analysis of the content of spliceosomes purified at various stages of
assembly and the catalysis cycle indicated that among the five spliceosomal
snRNAs, U1 and U4 leave the spliceosome during the assembly steps.
Therefore, only U2, U6, and U5 snRNAs were present at the time of cat-
alytic activation of the spliceosome ( Fig. 6.3 ). 5,17 Mutagenesis studies dem-
onstrated, at least in vitro , that a conserved loop in U5, although previously
shown to be a functionally important domain of the molecule ( Fig. 6.3 ), was