Irna

Posttranscriptional Gene Silencing Is Mediated
by RNA Interference
In higher eukaryotes, including nematodes, fruit flies,
plants, and mammals, a class of small RNAs has been
discovered that mediates the silencing of particular
genes. The RNAs function by interacting with mRNAs,
often in the 3_UTR, resulting in either mRNA degradation
or translation inhibition. In either case, the mRNA,and thus the gene that produces it, is silenced. This form
of gene regulation controls developmental timing in at
least some organisms. It is also used as a mechanism
to protect against invading RNA viruses (particularly
1110 Chapter 28 Regulation of Gene Expression
FIGURE 28–32 Translational regulation of eukaryotic mRNA. One of
the most important mechanisms for translational regulation ineukaryotes
involves the binding of translational repressors (RNA-binding
proteins) to specific sites in the 3_ untranslated region (3_UTR) of the
mRNA. These proteins interact with eukaryotic initiation factors or with
the ribosome (see Fig. 27–22) to prevent or slow translation.
A
AAA(A)n
eIF4G
Translational repressors
eIF4E
AUG
5 eIF3 _ cap
3_ poly(A)
binding
protein
40S Ribosomalsubunit
3_ Untranslated
region (3_UTR)
8885d_c28_1110 2/19/04 7:43 AM Page 1110 mac76 mac76:385_reb:
important in plants, which lack an immune system) and
to control the activity of transposons. In addition, small
RNA molecules may play a critical (but still undefined)
role in the formation of heterochromatin.
The small RNAs are sometimes called micro-RNAs
(miRNAs). Many are present onlytransiently during
development, and these are sometimes referred to as
small temporal RNAs (stRNAs). Hundreds of different
miRNAs have been identified in higher eukaryotes. They
are transcribed as precursor RNAs about 70 nucleotides
long, with internally complementary sequences that
form hairpinlike structures (Fig. 28–33). The precursors
are cleaved by endonucleases to form short duplexesabout 20 to 25 nucleotides long. The best-characterized
nuclease goes by the delightfully suggestive name Dicer;
endonucleases in the Dicer family are widely distributed
in higher eukaryotes. One strand of the processed
miRNA is transferred to the target mRNA (or to a viral
or transposon RNA), leading to inhibition of translation
or degradation of the RNA (Fig. 28–33a).
This generegulation mechanism has an interesting
and very useful practical side. If an investigator introduces
into an organism a duplex RNA molecule corresponding
in sequence to virtually any mRNA, the Dicer
endonuclease cleaves the duplex into short segments,
called small interfering RNAs (siRNAs). These bind to
the mRNA and silence it (Fig. 28–33b). The process is
known as RNA interference (RNAi). Inplants, virtually
any gene can be effectively shut down in this way.
In nematodes, simply introducing the duplex RNA into
the worm’s diet produces very effective suppression of
the target gene. The technique has rapidly become an
important tool in the ongoing efforts to study gene function,
because it can disrupt gene function without creating
a mutant organism. The procedure can be applied
tohumans as well. Laboratory-produced siRNAs have
already been used to block HIV and poliovirus infections
in cultured human cells for a week or so at a time. Although
this work is in its infancy, the rapid progress
makes RNA interference a field to watch for future medical
advances.
La actividad del ARNi endógeno ha sido relacionada con la regulación de la movilidad de los transposones, ladeterminación de perfiles de expresión genética, y el destino celular, a la vez que es un componente crucial de la defensa celular innata contra infecciones virales in vivo. Se han demostrado tres mecanismos de ARNi característicos que controlan la expresión de un gen de interés dado. El ARNi regula la transcripción genética mediante la modificación de la formación de heterocromatina. El ARNi...