Biology Reference
In-Depth Information
a putative helicase domain, a DUF283 domain, a Piwi-Argonaute-
Zwille (PAZ) domain, two tandem RNase III domains, and a
dsRNA-binding domain. PAZ domain interacts with dsRNAs that
present the 2 nt 3
-overhangs and a defi ned minimal stem length
probably necessary for recognizing the end of Drosha cleavage
product and for enabling Dicer cleavage. Processing occurs at
dsRNA ends that associate with the PAZ domain. Then, about two
helical turns along the surface of the protein, the substrate extends
to a single processing centre followed by RNase III domains and
each of the two RNase III domains cleaves one of the two strands.
The reaction leaves new ends with ~2 nt 3
′
′
-overhangs and a
5
-monophosphate on the product ends required during later
stages of silencing [
14
]. The double-stranded products of Dicer
can be incorporated into RNA-induced silencing complex (RISC)
to induce posttranscriptional gene-silencing mechanisms.
Argonaute (Ago) proteins are the central components of all RISCs.
The Ago clade associates with miRNA and siRNA during the RISC
assembly pathway, which involves duplex unwinding and culminat-
ing in the stable association of only one of the two strands with
Ago. The associated guide strand leads in target recognition by
Watson-Crick base pairing, whereas the passenger strand of the
original RNA duplex is discarded.
Functioning as translational repressors, most miRNAs in ani-
mals are only able to bind to the 3
′
-UTR with imperfect comple-
mentarity [
10
]. Despite the mRNA translational repression model,
there sure are miRNAs that bind perfectly complementary to its
corresponding mRNA, which then leads to mRNA cleavage.
Another comparable posttranscriptional gene-silencing mechanism
that is evolutionary conserved and sequence specifi c is called RNA
interference (RNAi) [
15
]. It is derived by processing dsRNA to
small interfering RNAs (siRNAs) initiating mRNA cleavage by
perfect complementary binding.
Concerning molecular characteristics, biogenesis, and effector
functions, miRNAs and siRNAs are similar. Both RNA types share
Dicer as RNase-III processing enzyme and the strongly related
effector complex RISC for posttranscriptional repression. The dif-
ference between miRNA and siRNA is its molecular origin and the
mode of mRNA target recognition. miRNA emerges from specifi c
precursors encoded in the genome into distinct species, whereas
siRNA is sampled almost randomly from longer dsRNA [
10
,
15
].
That dsRNA can be induced exogenously or produced from
endogenously transcribed and annealed RNA strands. By contrast
to the often imperfect binding of miRNA to its target, siRNA
mostly forms perfect duplexes with target mRNA. Consequently,
the distinction between miRNA and siRNA is open for discussion
and may be elucidated by further investigations.
Over the recent years, interest in finding ways to manipu-
late miRNA function increased steadily not only to elucidate
′
