Terminal modification, sequence, length, and PIWI-protein identity determine piRNA stability

In animals, PIWI-interacting RNAs (piRNAs) silence transposons, fight viral infections, and regulate gene expression. piRNA biogenesis concludes with 3′ terminal trimming and 2′-O-methylation. Both trimming and methylation influence piRNA stability. Our biochemical data show that multiple mechanisms destabilize unmethylated mouse piRNAs, depending on whether the piRNA 5′ or 3′ sequence is complementary to a trigger RNA. Unlike target-directed degradation of microRNAs, complementarity-dependent destabilization of piRNAs in mice and flies is blocked by 3′ terminal 2′-O-methylation and does not require base pairing to both the piRNA seed and the 3′ sequence. In flies, 2′-O-methylation also protects small interfering RNAs (siRNAs) from complementarity-dependent destruction. By contrast, pre-piRNA trimming protects mouse piRNAs from a degradation pathway unaffected by trigger complementarity. In testis lysate and in vivo, internal or 3′ terminal uridine- or guanine-rich tracts accelerate pre-piRNA decay. Loss of both trimming and 2′-O-methylation causes the mouse piRNA pathway to collapse, demonstrating that these modifications collaborate to stabilize piRNAs.