Biology Reference
In-Depth Information
(
Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001; Lee et al.,
1993; Pasquinelli et al., 2000; Wightman et al., 1993
). Here, we review one of
the first characterized miRNAs, let-7, and describe its role in development and
the intricacies of its biogenesis and function.
1. Introduction
MicroRNAs (MiRNAs) distinguish themselves from other small non-
coding RNAs by several unique features. Since miRNAs and the small
interfering RNAs (siRNAs) that function in RNA interference (RNAi)
associate with common Argonaute proteins and are functionally indistinct
in some species, these RNAs are primarily classified based upon their
biogenesis pathways. MiRNA genes are scattered throughout the genome,
in intra- and intergenic positions, and are transcribed as single-stranded
RNAs capable of forming stem loops that contain mismatches and bulges
(
Fig. 1.1
;
Kim
et al.
, 2009b
). The stem-loop structures are processed from
the nascent transcripts into
65nt hairpin shaped RNAs that undergo final
processing to the
21nt mature miRNAs (
Krol
et al.
, 2010
). In contrast,
siRNAs are usually produced from convergent transcripts forming long
double-stranded RNAs that serve as substrates for RNase processing to
the eventual 20-25nt forms (
Czech and Hannon, 2011
). Perfect double-
stranded RNAs from exogenous and endogenous sources generate exo- and
endo-siRNAs, respectively. In animals, miRNAs and siRNAs are also often
characterized by distinct mechanisms for regulating gene expression. The
miRNAs typically bind to the 3
0
-untranslated regions (3
0
UTRs) of target
messenger RNAs (mRNAs). Binding is usually imperfect, and target
mRNAs are either translationally repressed or deadenylated and degraded
(
Bartel, 2009; Fabian
et al.
, 2010
). This is in contrast to siRNAs, which form
perfectly complementary bonds to any region in target mRNAs and cause
degradation by cleavage at the binding site. This distinction means that one
miRNA can regulate multiple mRNAs with nonidentical target sites, while
siRNAs would be limited to targeting mRNAs with sites of perfect com-
plementarity (
Lim
et al.
, 2005
). However, off target RNAi effects are often
attributable to partial pairing between siRNAs and unintended target sites,
and miRNAs can direct mRNA cleavage when presented with perfectly
complementary target sequences (
Alem
´
n
et al.
, 2007; Doench
et al.
, 2003;
Zeng
et al.
, 2003
).
Typically, miRNA biogenesis begins with transcription by RNA poly-
merase II, either through an independent promoter or as part of a host gene
where the miRNA is embedded within an intron of a protein-coding gene
(
Fig. 1.1
;
Kim
et al.
, 2009b
). Some miRNAs are closely arranged in the
genome and considered a “cluster” when they are synthesized as part of a
