Biology Reference
In-Depth Information
early days of evolution. Nevertheless, with the description of
miRNAs this has shown to be vastly untrue. The once so-called
genetic wasteland has shown to partly code for the very important
primary-miRNA (pri-miRNA) transcript which is the basis for
miRNAs. The (partial) highly conserved genetic information of
the pri-miRNA transcripts is encoded on both exons and introns of
coding and noncoding genes [
2
]. The intronic and exonic pri-
miRNAs have been reported to be 80-87 and 13-20 % [
3
].
Although most miRNA sequences are encoded on introns of
the host genome, some miRNAs are also encoded on exons as they
show to have an overlap with transcription units [
3
]. The several
kilobases long pri-miRNA transcript is a product of RNA poly-
merase II and III, whereas most of the pri-miRNAs are transcribed
by RNA polymerase II inside the nucleus [
4
]. The pri-miRNA
transcript is several kilobases long, poladenylated at its 3
′
end,
capped with a 7-methylguanosine cap at its 5
end, hallmarks of
polymerase II transcription, and contains a characteristic stem-
loop structure [
5
,
6
].
Depending on the genomic loci, miRNAs can be categorized
into three types: intragenic (intra-miR), intergenic (inter-miR), and
polycistronic (poly-miR). The evolutionarily conserved (“old”)
intragenic miRNAs are commonly transcribed in conjunction with
their host genes, whereas it has recently been proposed that non-
conserved (“young”) intragenic miRNAs, which dominate in human
genome, are unlikely to be co-expressed with their host genes [
7
,
8
].
The exact mechanism of how non-conserved intragenic miRNAs are
regulated is not completely understood [
8
]. Inter-miR are believed
to be expressed independently by unidentifi ed promoters and are
located in noncoding regions [
9
]. Poly-miR are encoded as a cluster
in the genome and are transcribed in a single primary transcript with
multiple hairpins giving rise to different miRNAs [
10
]. After the pri-
miRNA transcript has been intranuclear generated it is cleaved into
several precursor-miRNAs (pre-miRNAs) by the 500-650 kDa large
microprocessor complex. The core of this microprocessor complex
consists of the intranuclear RNase III enzyme Drosha (RNASEN)
and the double-stranded RNA-binding domain (dsRBD) protein
DiGeorge syndrome critical region gene 8 (DGCR8, Pasha =
Pa
rtner
of Dro
sha
in
Drosophila melanogaster
and
Caenorhabditis elegans
),
which are together properly cleaving synthetic miRNA-substrates in
vitro [
11
,
12
]. In vivo however, additional accessory factors of the
microprocessor complex are required for processing a subset of
pri-miRNAs. DEAD-box RNA helicases p68 (DDX5) and p72
(DDX17) for example have been shown to be among those acces-
sory factors [
13
-
15
]. DDX5 facilitates SMAD (
s
mall and
m
others
a
gainst
d
ecapentaplegic homolog) protein-mediated positive reg-
ulation of Drosha [
16
]. SMAD proteins are signal transducers of
the transforming growth factor beta (TGFbeta) signaling pathway
and can control Drosha-mediated miRNA maturation [
17
,
18
].
′
