Gene silencing in Neurospora crassa requires a protein homologous to RNA-dependent RNA polymerase

In plants and fungi, the introduction of transgenes can lead to post-transc riptional gene silencing(1,2). This phenomenon, in which expression of the transgene and of endogenous genes containing sequences homologous to the tr ansgene can be blocked, is involved in virus resistance(3-5) and genome mai ntenance(6,7). Transgene-induced gene silencing has been termed quelling in Neurospora crassa and co-suppression in plants. Quelling-defective (qde) m utants of N. crassa, in which transgene-induced gene silencing is impaired, have been isolated(8). Here we report the cloning of qde-1, the first cell ular component of the gene-silencing mechanism to be isolated, which define s a new gene family conserved among different species including plants, ani mals and fungi. The qde-1 gene product is similar to an RNA-dependent RNA p olymerase found in the tomato(9). The identification of qde-1 strongly supp orts models that implicate an RNA-dependent RNA polymerase in the post-tran scriptional gene-silencing mechanism. The presence of qde-1 homologues in a variety of species of plants and fungi indicates that a conserved gene-sil encing mechanism may exist, which could have evolved to preserve genome int egrity and to protect the genome against naturally occurring transposons an d viruses.