Biology Reference
In-Depth Information
This cleavage site is a highly preferred editing site and was shown to be generated
in dsRNA
in vitro
by editing with ADAR1, ADAR2, or
Drosophila
ADAR
(Scadden and O'Connell, 2005).
X. INOSINE-CONTAINING RNA REGULATES GENE EXPRESSION
IN TRANS
An affinity column of immobilized inosine-containing dsRNA was used to purify
proteins that specifically interact with inosine-containing dsRNA(Scadden, 2005).
This led to the isolation of factors which are present in stress granules (SGs),
including poly(A)-binding protein (PABP), and components of the eukaryotic
initiation complex (eIF-4A, 4G, and 4E) (Scadden, 2007). Cytoplasmic SGs are
formed when cells undergo environmental stress and proteins required for survival
must be rapidly synthesized, while synthesis of other nonessential proteins is halted
(reviewed in Anderson and Kedersha, 2009). Therefore, SGs are comprised of a
host of RNA processing proteins and cellular mRNAs that are maintained in a
translationally silent state until the stress is relieved. The observation that SG-
associated proteins also interact with inosine-containing dsRNA led to the hypoth-
esis that inosine-containing dsRNA could affect gene mRNA expression levels
through the induction of SGs. This hypothesis proved to be true for several different
dsRNAs containing I-U base pairs and to a lesser extent dsRNAs with G-U base
pairs, including an endogenously edited miRNA-142.
Inosine-containing duplexes transfected into cells resulted in a general
decrease in mRNA levels of both endogenous mRNAs and reporter mRNAs. This
was accompanied by a decrease in translation which was due to inhibition of
translation initiation. A model was proposed whereby editing of dsRNAs by
ADAR produces inosine-containing dsRNAs which are sequestered into SGs with
cellular mRNAs and translation initiation factors leading to a decrease in transla-
tion within the cell (Scadden, 2007). The decrease in translation factors results in
increased traffic of cellular mRNAs to SGs that are subsequently either degraded in
processing (P) bodies or held in a translationally silent state. The translation block is
relieved when inosine-containing dsRNAs are degraded by Tudor-SN (Scadden,
2005), a component of the RISC complex. However, further work is required to
identify the endogenous inosine RNAs that are responsible for this.
XI. EDITING AND RNA INTERFERENCE
RNA interference is the process whereby double-stranded short interfering RNA
(siRNA) or microRNA (miRNA) of approximately 21-23 nucleotides (nt) can
target the degradation or prevent the translation of an mRNA which contains
