Viruses and RNA interference in mammalian cells

RNA synthesis. It destructs the RNA transcripts (De Carvalho et al., 1992 ; Van Blokland et
al., 1994).
RNAi was first described in 1998 when the researchers found that gene interference
produced by dsRNAs was considerably more effective than were sense or antisense strand
separately. Moreover, the introduction of specific dsRNA sequences in minimal quantities
into the nematode Caenorhabditis elegans could effectively silence the expression of a
target gene in the injected animals, as well as in their progeny (Fire et al., 1998).
General RNAi mechanism
DsRNA (more than 30 bp) is produced from the invading genes, either during viral
replication or by aberrant transcription from promoters located near the transgene insertion
site. Initiative dsRNA can be either exogenic or endogenic. RNAi is carried out in two steps
(Figure 3). Frst, long dsRNA homologous in sequence to the silenced gene (Elbashir et al.,
2001) is converted to 21–23-nucleotide small interfering RNA (siRNA) duplexes by
ribonuclease III (RNase III)-like enzyme Dicer in the cytoplasm (Meister and Tuschl, 2004).
In the second step, the small dsRNA products of the cleavage are assembled into a
multiprotein RNA-induced silencing complex (RISC) with the help of the Dicer in the
cytoplasm. Then the duplexes of siRNA unwound and one single (Nykanen et al., 2001;
Martinez et al., 2002) strand with lowest stability at its 5’-end (Schwarz et al., 2003), guides
RISC to its complementary mRNA. Further, mRNA is degraded by RISC nuclease activity.
As a result, the translation stops (Elbashir et al., 2001). The other anti-guide strand is
degraded during RISC activation (Gregory et al., 2005). Below, more detailed description of
RNAi functional elements is given.
8