Quelling targets the rDNA locus and functions in rDNA copy number control

Here, we detected by northern blotting endogenous short interfering RNA (siRNAs) from the repetitive ribosomal DNA locus (rDNA) that are loaded onto the argonaute protein QDE-2. Moreover, we found a bidirectional transcription that can generate double strand RNA (dsRNA) molecules. Interestingly, quelling mutants have a reduced rDNA gene copy number.Our finding could suggest a new biological function for RNA silencing in the maintenance of the integrity and stability of the Neurospora rDNA locus.RNA silencing phenomena, such as RNA interference, post-transcriptional gene silencing (PTGS) and quelling, are related mechanisms that have been shown to occur in a number of eukaryotic organisms including fungi, plants and animals. The common intermediate in all silencing phenomena is a dsRNA molecule that is processed by the RNAseIII enzyme Dicer into siRNAs of 21–25 nucleotides in length [1]. These siRNAs are subsequently used as guides by the RNA Induced Silencing Complex (RISC) which contains effector proteins belonging to the Argonaute family that are able to cleave in a sequence specific manner transcripts with sequence complementary to siRNAs [2]. The basic features of the mechanism are very conserved in a wide range of eukaryotic species, and it has been suggested that its ancestral function is to limit the expansion of repetitive selfish elements like transposons and viruses [3]. A large body of evidence supports the role of RNA silencing in genome defence. In Caenorhabditis elegans and Chlamydomonas, several components of the RNAi machinery have been found to be necessary in transposon control pathways [4,5]. In plants, the silencing of RNA viruses depends on the RNAi machinery and the silencing of transposons through DNA methylation, mediated by the Argonaute proteins and siRNAs [6-9]. Argonaute's role in transposon silencing is also conserved in flies and vertebrates [10-13].Further to its conserved role in genome defence system in both animals and plants, RNA s