Protection of Germline Gene Expression by the C. elegans Argonaute CSR-1

• An in vivo RNA tethering assay enables the assessment of AGO CSR-1 functions
• Tethering CSR-1 to a transcript protects from silencing by the piRNA pathway
• Tethering CSR-1 to a silent transcript reactivates its expression
• Remarkably, CSR-1 can activate a silent gene in trans over multiple generations

SummaryIn Caenorhabditis elegans, the Piwi-interacting small RNA (piRNA)-mediated germline surveillance system encodes more than 30,000 unique 21-nucleotide piRNAs, which silence a variety of foreign nucleic acids. What mechanisms allow endogenous germline-expressed transcripts to evade silencing by the piRNA pathway? One likely candidate in a protective mechanism is the Argonaute CSR-1, which interacts with 22G-small RNAs that are antisense to nearly all germline-expressed genes. Here, we use an in vivo RNA tethering assay to demonstrate that the recruitment of CSR-1 to a transcript licenses expression of the transcript, protecting it from piRNA-mediated silencing. Licensing occurs mainly at the level of transcription, as we observe changes in pre-mRNA levels consistent with transcriptional activation when CSR-1 is tethered. Furthermore, the recruitment of CSR-1 to a previously silenced locus transcriptionally activates its expression. Together, these results demonstrate a rare positive role for an endogenous Argonaute pathway in heritably licensing and protecting germline transcripts.

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