piRNA 3′ uridylation facilitates the assembly of MIWI/piRNA complex for efficient target regulation in mouse male germ cells

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(MlWI)-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.