Dear Editor,

RNA tailing, i.e., addition of non-templated nucleotides to the 3’ end of RNAs, represents one of the most frequent RNA modifications that participate in regulation of RNA processing, function, degradation, etc.1 PIWI-interacting RNAs (piRNAs) are a class of animal germline-specific small non-coding RNAs that guide PIWI proteins to suppress transposable elements and regulate protein-coding genes in germ cells, playing indispensable roles in germline development and fertility in animals.2 3′ terminal trimming and subsequent 2′-O-methylation are essential for piRNA processing and maturation, which are catalyzed by 3′-to-5′ exoribonuclease PNLDC1 and HEN methyltransferase 1 (HENMT1) in mice, respectively.2 Most recently, a study reports that loss of 3′ terminal trimming and 2′-O-methylation induces non-templated RNA tailing on piRNA precursors and in turn triggers pre-piRNA decay, thereby severely interfering with piRNA production in worms.3 However, whether and how RNA tailing is required for piRNA functions remains largely unclear. Here, we discovered that mouse PIWIL1 (MIWI)-bound piRNAs undergo non-templated RNA tailing and further showed the functional requirement of 3′ uridylation for the targeting efficacy of MIWI/piRNAs in mouse male germ cells, uncovering an unprecedented role for 3′ tailing in piRNA functions in mouse male germ cells.

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