pre-piRNA trimming and 2′-O-methylation protect piRNAs from 3′ tailing and degradation in C. elegans

The Piwi-interacting RNA (piRNA) pathway suppresses transposable elements and promotes fertility in diverse organisms. Maturation of piRNAs involves pre-piRNA trimming followed by 2′-O-methylation at their 3′ termini. Here, we report that the 3′ termini of Caenorhabditis elegans piRNAs are subject to nontemplated nucleotide addition, and piRNAs with 3′ addition exhibit extensive base-pairing interaction with their target RNAs. Animals deficient for PARN-1 (pre-piRNA trimmer) and HENN-1 (2′-O-methyltransferase) accumulate piRNAs with 3′ nontemplated nucleotides. In henn-1 mutants, piRNAs are shortened prior to 3′ addition, whereas long isoforms of untrimmed piRNAs are preferentially modified in parn-1 mutant animals. Loss of either PARN-1 or HENN-1 results in modest reduction in steady-state levels of piRNAs. Deletion of both enzymes leads to depletion of piRNAs, desilenced piRNA targets, and impaired fecundity. Together, our findings suggest that pre-piRNA trimming and 2′-O-methylation act collaboratively to protect piRNAs from tailing and degradation.