Biology Reference
In-Depth Information
presence of a miRNA in the BK virus,
49
and a study of the Simian virus
40 (SV40)
53
identified one pre-miRNA expressed from the opposite
strand of the T antigen. RNA viruses like yellow fever virus, human
immunodeficiency virus (HIV), and hepatitis C virus (HCV) appear to
lack miRNAs.
49
As is the case with most miRNAs, the functions of viral
miRNAs are only beginning to be characterized. The available data point
to the role of viral miRNA in regulating viral gene expression. For
instance, the EBV miRNA miR-BART2 is expressed from the comple-
mentary strand of the BALF5 transcript, which encodes EBV DNA poly-
merase, suggesting that the miRNA is involved in the cleavage and
downregulation of the viral polymerase.
49
This function of miR-BART2
has been very recently demonstrated.
54
Similarly, the SV40 miRNA was
shown to downregulate early viral mRNAs by cleavage, leading to
reduced T antigen expression and lower interferon-
, and allowing the
virus to evade the immune response.
53
Finally, the KSHV-encoded
miR-K12-10 is located in the open reading frame (ORF) of the kaposin
gene, which triggers cell transformation, so processing of the kaposin
ORF would downregulatate the kaposin expression.
49,51
γ
3.2. Piwi-Interacting RNAs
The novel class of Piwi-interacting associated small RNAs (piRNAs) has
only recently been discovered. They are generated by a Dicer-
independent mechanism, are longer than the miRNAs (24-30 nt); and
associate with members of the Piwi subfamily of Argonaute pro-
teins.
13-15
Piwi proteins are required for male and female fertility,
55
and
are involved in the silencing of the transposons and repeat elements in
Drosophila
.
56
In fact, a cloning study initially identified small RNAs
derived from repeat elements in
Drosophila
and called them repeat-
associated small interfering RNAs (rasiRNAs)
57
; these were later found
to associate with Piwi proteins
16-18
and, for this reason, were renamed
piRNAs. In mammals, piRNAs are needed for spermatogenesis and
appear responsible for the stability of a subset of mRNAs.
58,59
Various
subsets appear to exist, associating with different Piwi proteins in a
developmental stage-specific manner.
13,60
The presence of Piwi protein
in zebrafish has also been demonstrated; Ziwi, the Piwi homolog in
