A Happy 3′ Ending to the piRNA Maturation Story Benjamin Czech, Gregory J. Hannon  Cell  Volume 164, Issue 5, Pages 838-840 (February 2016) DOI: 10.1016/j.cell.2016.02.012 Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 The Biogenesis of piRNA 3′ Ends (Left) Bombyx mori piRNA intermediates are derived from Zuc cleavage or Ago3 slicing and are loaded into Siwi. Next, another cleavage event releases 35- to 40-nt-long Siwi-bound pre-piRNAs. Pre-piRNAs are resected to mature piRNA size by the 3′-5′ exonuclease Trimmer (PNLDC1 in mouse). The trimming reaction also requires Papi (Tdrkh in mouse), which, like Trimmer, is anchored on mitochondria, and is followed by 2-O-methylation to yield mature piRNA-Siwi complexes. A similar processing mechanism is likely acting in piRNA 3′ formation of mammals. (Center) piRNA intermediates in C. elegans are products of uncapping and removal of 2 nt from the 5′ termini of the primary transcripts. pre-piRNAs associated with PRG-1 show 2- to 4-nt-extended 3′ ends that are resected by PARN-1. Alternatively, PARN-1 might trim pre-piRNAs prior to PRG-1 binding. In a final step, HENN-1 methylates the 3′ termini of the mature 21U-RNAs. (Right) The generation of piRNA 3′ ends in Drosophila commences with piRNA intermediates originating from several possible inputs. Following loading into Piwi/Aub, Zuc cleavage produces pre-piRNAs that are very similar in size to mature piRNAs, with little to no extra 3′ nucleotides. Papi and the exonuclease Nibbler shape pre-piRNAs to their mature size; however, the mechanistic details remain to be resolved. Last, methylation by Hen1 completes piRNA 3′ end formation and yields mature PIWI-piRNA complexes. Cell 2016 164, 838-840DOI: (10.1016/j.cell.2016.02.012) Copyright © 2016 Elsevier Inc. Terms and Conditions