Is RNA interference a universal phenomenon in eukaryotes?
RNAi was first described in plants and has now been found in a variety of unicellular and multicellular eukaryotes. The mechanism of inhibition entails the cleavage of the dsRNA trigger into smaller dsRNAs of 21–23 base pairs, called small interfering (si)RNAs, which recognize the target mRNA and lead to its destruction. Sensitivity to long endogenous dsRNAs may be maintained by selection against the spread of transposons or viruses or against the spurious expression of other repetitive sequences, all of which are likely to be transcribed in both directions to at least some degree, and thus to produce dsRNA. It is a particularly convenient feature of C. elegans that it is sensitive to external dsRNAs provided very simply either by soaking the worms in a RNA preparation [3] or by feeding them with Escherichia coli bacteria expressing dsRNA from a plasmid [4], as outlined in Figure 1. Sensitivity to external dsRNAs is thought to be required for repression of viruses, although this remain
