Biology Reference
In-Depth Information
original
bereft
alleles and recapitulated
bereft
phenotypes in IOB organs.
Further analysis of pupal development revealed that
bereft
mutants specify
the sensory organ precursors for IOBs and also execute this cell lineage
normally. However, the shaft cells of IOB organs succumb to apoptosis,
indicating that miR-263a is antiapoptotic. This interpretation is supported
by the fact that loss of
mir-263a
can be compensated by misexpression of the
antiapoptotic viral protein p35.
The proapoptotic gene
hid
was found to be a key target gene of
miR-263a/b, since heterozygosity for
hid
could substantially rescue
mir-
263
mutants, and miR-263 could suppress luciferase-
hid
3
0
UTR sensors
(
Hilgers
et al
., 2010
). Curiously, miR-263a and miR-263b are rather unusual
family members in that their seed regions have diverged (at position 2).
According to current knowledge, continuous Watson-Crick seed pairing
is the major determinant for animal miRNA target recognition so that seed
changes are expected to redirect targeting capacity (
Bartel, 2009
).
The relevant miR-236a/b target sites in the
hid
3
0
UTR are atypical in
that none of the sites are canonical 2-8 matches. This might be compensated
by the existence of multiple target sites; for example, miR-263a pairs to
4 sites in
hid
exhibiting 3-9 pairing (and 1G:U), 1-7 pairing, or 2-7 pairing
(2 sites).
Two aspects of the miR-263 story are particularly instructive with
respect to general miRNA studies. First, it shows that a miRNA locus
was actually deleted and shown to have a compelling developmental defect
in the “pre-miRNA era” but was not initially deciphered due to lack of
knowledge of the associated small RNA. One may wonder whether other
miRNA loci were studied genetically over the years, but the associated
small RNA was not appreciated. Second, it highlights how genetics led to
elucidation of a miRNA:target relationship that is critical for normal animal
development, but that could not have been found by standard miRNA
target predictions, owing to its noncanonical sites. Thus, one should be
circumspect in utilizing genome-wide target predictions, which comprise
powerful information but may not include all critical targeting relationships.
2.3. Other transposons
Owing to the insertion site preference of
P
elements, the yield of new
genomic insertions has steadily decreased over the years. The use of other
transposons with distinct insertion mechanisms can broaden the distribution
and coverage of these collections. One widely used element is piggyBac,
which exhibits broader insertion range across the genome relative to
P
,
including the capacity to insert within genes and introns (
Fig. 8.1
B;
Bellen
et al
., 2004
). Because of this, piggyBac has broader possibilities to disrupt
gene expression or function. However, a disadvantage of piggyBac is that it
essentially only excises precisely. This disadvantage can be ameliorated by
