Biology Reference
In-Depth Information
likely via hydrolytic cleavage reactions on fully spliced mRNAs in a spliced
exon reopening (SER) reaction. 24,50 While many aspects of this reaction
including the RNA requirements for its catalysis and its relationship, if
any, to the SER reactions observed in self-splicing ribozymes remain to
be elucidated, the existing data draw strong parallels between the
spliceosomal active site and that of group II self-splicing introns. 12-14,24 Col-
lectively, the evidence further strengthens the likelihood that the U6
sequence elements equivalent to those forming the active site of group II
introns may be involved in forming the spliceosomal active site.
Another interesting conclusion that can be drawn from the aberrant reac-
tions described above, in which U2 is taken as the 5 0 splice site, is the flex-
ibility of the spliceosomal active site with respect to the choice of splice sites.
This is consistent with previously observed results, in which mutations in U6
sequences upstream of the invariant ACAGAGA box in nematodes resulted
in the aberrant use of these mutant sequences as the 5 0 splice site. 51 More
recently, Smith and colleagues 52 systematically probed the positioning of
the branch site adenosine within the active site. The authors developed a
model in yeast by introducing an additional, mutant copy of U2 snRNA that
could only function in the splicing of a pre-mRNA bearing complementary
mutations in its branch site. They also showed that while there is consider-
able flexibility in the sequence of the branch site-U2 base-pairing interac-
tions that flank the branch site adenosine, the location of the bulged
adenosine within this duplex could be moved only one nucleotide upstream
or downstream of its original position. Additional analyses indicated that this
narrow window is at least partially defined by the distance between the
branch site adenosine and the base-paired U2/U6 helix I ( Fig. 6.3 ), which
likely reflects its positioning in relationship to the rest of the catalytically crit-
ical elements in the active site such as the ACAGAGA box. Thus, although
there is a significant degree of sequence flexibility in the choice of splice sites,
maintaining the spatial positioning of the RNA elements of the active site in
relationship to the reacting groups seems to be crucial for splicing.
4. THE THREE-DIMENSIONAL POSITIONING OF snRNAs
IN THE CATALYTIC CORE
Although under certain conditions U2 snRNAmay be dispensable for
splicing as discussed above, the very low efficiency of these aberrant reac-
tions suggests that U2 is essential for physiological splicing. In addition to
forming base-pairing interactions with the branch site of introns within
Search WWH ::




Custom Search