The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 207

Figure 6.6 Protein-free catalytic activity of human snRNAs. The snRNA sequences used
in the catalytic reactions are shown and correspond to the central domain of U6 and the
5 0 domain of U2 snRNAs. The AGC and ACAGAGA sequences are highlighted in gray. The
positioning of the two substrate oligonucleotides, exon1 and exon2, relative to U6 are
shown. Thunderbolts mark the locations of crosslinked interactions. Exon2 binds the 5 0
end of U6 through an engineered base-paired interaction. The reaction pathway is
shown at the bottom. 79 The “exonic” sequences in the substrates are shown in gray
boxes. Depicted are the sequences of both the bipartite substrates (exon1 and exon2,
left) and a

-like substrate (right) that undergo a similar splicing reaction
when incubated with the U6 and U2 snRNAs. 80

“ mini pre-mRNA ”

the one formed when the bipartite substrate was used ( Fig. 6.6 ). 80 Mutations
in the invariant ACAGAGA and AGC sequences and the U6 ISL either
completely blocked the formation of product or resulted in significantly
reduced efficiency, suggesting that the sequences play a role in catalysis of
this reaction similar to authentic spliceosomal catalysis. 79 In addition,
crosslinking and mutational complementation analyses and binding assays
proved that the reaction occurred in the vicinity of the ACAGAGA
sequence and provided a further parallel with spliceosomal catalysis. 79,81
Finally, similar to both spliceosomal and group II intron catalysis, the