The Role of snRNAs in Spliceosomal Catalysis - Progress in Molecular Biology and Translational Science - page 194

Figure 6.2 The structural organization of the group II self-splicing intron aI5g. The loca-
tion of domains I through VI, the two exons, the splice sites, and the branch site (5 0 SS,
3 0 SS, and BS, respectively) are shown. The position of J2/3 and the AGC sequence are
indicated. The metal ion-binding site of domain V is indicated (Mg). Dashed lines con-
nect regions involved in forming the base triples at the catalytic core. Gray circles
denote the functional equivalent of each domain or subdomain in the spliceosome.
e and e 0 sites are involved in an interaction important in the recognition of the 5 0
splice site.
Fig. 6.2 ) is involved in recognition of the 5 0 splice site and thus can be con-
sidered the functional counterpart of the U1 snRNA. 24 Currently, there are
no known functional or structural equivalents for the rest of domain I and for
domains II and III of the group II introns among the spliceosomal snRNAs.
It is likely that these RNA domains are replaced by proteins in the
spliceosome. Similarly, there are no clear equivalents for the U4 snRNA
in group II introns, and the evolutionary origin of this snRNA is completely
unknown. Taken together, the structural and mechanistic similarities
between group II introns and the snRNAs suggest an evolutionary relation-
ship between the two systems and a prominent role for the snRNAs in the
active site of the spliceosome.