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Originally discovered in plants 1,2, the phenomenon of co-suppression by transgenic DNA has since been observed in many organisms from fungi 3 to animals 4-7: introduction of transgenic copies of a gene results in reduced expression of the transgene as well as the endogenous gene. The effect depends on sequence identity between transgene and endogenous gene. Some cases of co-suppression resemble RNA interference (the experimental silencing of genes by the introduction of double-stranded RNA) 8, as RNA seems to be both an important initiator and a target in these processes 9-13. Here we show that co-suppression in Caenorhabditis elegans is also probably mediated by RNA molecules. Both RNA interference 14,15 and co-suppression 16 have been implicated in the silencing of transposons. We now report that mutants of C. elegans that are defective in transposon silencing and RNA interference (mut-2, mut-7, mut-8 and mut-9) are in addition resistant to co-suppression. This indicates that RNA interference and co-suppression in C. elegans may be mediated at least in part by the same molecular machinery, possibly through RNA-guided degradation of messenger RNA molecules.

(C) 2000 Nature Publishing Group