Biology Reference
In-Depth Information
have characteristics of miRNA precursors: they are
65nt and can fold into
hairpins that contain 5 0 monophosphate and 3 0 hydroxyl residues. Computa-
tional and experimental analyses of short RNA transcripts in flies, nematodes,
and mammals have so far revealed few mirtrons, with 14 found in C. elegans
( Berezikov et al. , 2007; Chung et al. ,2011 ). In vertebrates and flies, pre-
miRNAs are transported from the nucleus to the cytoplasm by a nucleocy-
toplasmic transport factor, Exportin-5, where the RNase III enzyme Dicer
removes the loop region, leaving
21nt double-stranded RNA fragments
with 2nt 3 0 end overhangs ( Kim et al. , 2009b; Krol et al. ,2010 ). Curiously,
C. elegans lacks an obvious homolog of Exportin-5, and the cellular location
of miRNA processing events is yet to be determined in nematodes ( Bohnsack
et al. , 2004; Murphy et al. ,2008 ).
The mature miRNA strand, also known as the guide strand, is then
loaded onto an Argonaute (Ago) protein, a key factor in the RNA-induced
silencing complex (RISC; Fig. 1.1 ; Kim et al. , 2009b; Krol et al. , 2010 ). The
leftover strand, called the star strand, is degraded by an unclear mechanism.
Historically, the more abundant mature miRNA species has been named
the guide and the less frequently detected one the star strand. However, the
abundance of one strand versus the other has been found to reverse under
different conditions and, thus, the term star strand does not always indicate a
nonfunctional processing by-product. In fact, both halves of the Dicer
cleavage product, albeit at unequal levels, are often incorporated into Ago
complexes ( Okamura et al. , 2008; Zisoulis et al. , 2010 ). The bias for loading
one strand versus the other is affected by the thermodynamic stability of the
duplex termini, with the strand containing the weaker paired 5 0 end being
favored, and the identity of the first nucleotide ( Czech and Hannon, 2011 ).
The 27 Ago family proteins in C. elegans exert different roles in small
RNA pathways ( Yigit et al. , 2006 ). The Argonaute-Like-Genes 1 and
2 (ALG-1, ALG-2) are loaded with miRNAs and direct translational silenc-
ing or deadenylation of target mRNAs ( Ding and Groszhans, 2009; Steiner
et al. , 2007; Wu et al. , 2010; Zhang et al. , 2007; Zisoulis et al. , 2010 ). Most
of the other Ago proteins function in the exo- and endo-siRNA pathways,
which involve target mRNA cleavage ( Yigit et al. , 2006 ). The choice of
which Ago is loaded with particular small RNA species is not entirely clear
but seems to depend on the origin and structure of the small RNA duplex
( Jannot et al. , 2008; Steiner et al. , 2007 ).
MiRNAs serve as guides to lead the RISC complex to regulatory targets.
The idea that miRNAs use limited base-pairing complementarity to recog-
nize target sites was originally described for the first discovered miRNA,
lin-4, and its target, lin-14 ( Lee et al. , 1993; Wightman et al. , 1993 ). Several
partially complementary sites in well-conserved regions of the lin-14 3 0 UTR
were predicted to pair with the lin-4 miRNA. The next identified miRNA,
let-7, and 3 0 UTR sequences in its target, lin-41 , also exhibited partial base-
pairing capacity, further implicating the functionality of imperfect duplexes
Search WWH ::




Custom Search