Multiple small RNA pathways regulate the silencing of repeated and foreign genes in C. elegans

Gene segments from other organisms, such as viruses, are detected as foreign and targeted for silencing by RNAi pathways. A deep-sequencing map of the small RNA response to repeated transgenes introduced to Caenorhabditis elegans revealed that specific segments are targeted by siRNAs. Silencing of the foreign gene segments depends on an antiviral response that involves changes in active and silent chromatin modifications and altered levels of antisense siRNAs. Distinct Argonaute proteins target foreign genes for silencing or protection against silencing. We used a repeated transgene in a genome-wide screen to identify gene disruptions that enhance silencing of foreign genetic elements and identified 69 genes. These genes cluster in four groups based on overlapping sets of coexpressed genes, including a group of germline-expressed genes that are likely coregulated by the E2F transcription factor. Many of the gene inactivations enhance exogenous RNAi. About half of the 69 genes have roles in endogenous RNAi pathways that regulate diverse processes, including silencing of duplicated genes and transposons and chromosome segregation. Of these newly identified genes, several are required for siRNA biogenesis or stability in the oocyte-specific ERGO-1 pathway, including eri-12, encoding an interactor of the RNAi-defective protein RDE-10, and ntl-9/CNOT9, one of several CCR4/NOT complex genes that we identified. The conserved ARF-like small GTPase ARL-8 is required specifically for primary siRNA biogenesis or stability in the sperm-specific ALG-3/4 endogenous RNAi pathway.