Tandem repeat-induced sexual silencing: A Rid-dependent RNAi mechanism for fungal genome defense via translational repression.

Eukaryotes have evolved defense systems to protect their genomes from invasive genetic elements. We report a fungal defense mechanism, tandem repeat-induced sexual silencing (TRISS), active during sexual stages. TRISS is a unique RNA interference (RNAi) pathway distinct from known mechanisms like quelling and meiotic silencing by unpaired DNA (MSUD). Triggered by tandem repeats, it involves RNAi components similar to quelling but uniquely includes the MSUD-specific Argonaute Sms2. TRISS operates independently of recombination. We identified small interfering RNA sassociated with TRISS (trasiRNAs) in tandem repeats. Sms2, guided by trasiRNAs, mediates translational repression of target mRNAs and is crucial for trasiRNA biogenesis. TRISS requires the DNA methyltransferase Rid but not Rid-mediated repeat-induced point mutation (RIP). Both Sms2 and Rid interact with replication protein A (RPA) to recruit the RPA-Qde1/RdRP complex, dependent on the helicase Qde3. This study reveals a potentially conserved strategy linking RIP and RNAi to silence tandem repeats during fungal sexual stages, offering insights into fungal genome defense.