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In C. elegans ego-1 mutants ("ego" stands for
"enhancer of glp-1"), RNAi functions normally in somatic cells, but
is defective in germline cells where ego-1 is primarily expressed.
In Arabidopsis SDE-1/SGS-2 mutants ("SGS" stands for
suppressor of gene silencing), siRNAs are produced when dsRNA is
introduced via an endogenously replicating RNA virus, but not when
introduced by a transgene. It has been proposed that perhaps the viral
RdRP is substituting for the Arabidopsis enzyme in these mutants
(1,
3,
17,
18).
Although no homolog of an RdRP has been found in flies or humans, an RdRP
activity has recently been reported in Drosophila embryo lysates
(30).
One model of amplification, termed the "random degradative PCR" model,
suggests that an RdRP uses the guide strand of an siRNA as a primer for
the target mRNA, generating a dsRNA substrate for Dicer and thus more
siRNAs (27,
30).
Evidence supporting this model has been found i
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