PIWI-interacting RNAs (piRNAs) are a class of small non-coding RNAs essential for fertility. In adult mouse testes, most piRNAs are derived from long single-stranded RNAs lacking annotated open reading frames (ORFs). The mechanisms underlying how piRNA sequences are defined during the cleavages of piRNA precursors remain elusive. Here, we show that 80S ribosomes translate the 5´-proximal short ORFs (uORFs) of piRNA precursors. MOV10L1/Armitage RNA helicase then facilitates the translocation of ribosomes into the uORF-downstream regions (UDRs). The ribosome-bound UDRs are targeted by piRNA processing machinery, with the processed ribosome-protected-regions becoming piRNAs. The dual modes of interaction between ribosomes and piRNA precursors underlies the distinct piRNA biogenesis requirements at uORFs and UDRs. Ribosomes also mediate piRNA processing in roosters and green lizards, implying this mechanism is evolutionarily conserved in amniotes. Our results uncover a function for ribosomes on non-coding regions of RNAs and reveal the mechanisms underlying how piRNAs are defined.

Macromolecular complexes composed of proteins or proteins and nucleic acids rather than individual macromolecules mediate many cellular activities. Maintenance of these activities is essential for cell viability and requires the coordinated production of the individual complex components as well as their faithful incorporation into functional entities. Failure of complex assembly may have fatal consequences and can cause severe diseases. While many macromolecular complexes can form spontaneously in vitro, they often require aid from assembly factors including assembly chaperones in the crowded cellular environment. The assembly of RNA protein complexes implicated in the maturation of pre-mRNAs (termed UsnRNPs) has proven to be a paradigm to understand the action of assembly factors and chaperones. UsnRNPs are assembled by factors united in protein arginine methyltransferase 5 (PRMT5)- and survival motor neuron (SMN)-complexes, which act sequentially in the UsnRNP production line. While the PRMT5-complex pre-arranges specific sets of proteins into stable intermediates, the SMN complex displaces assembly factors from these intermediates and unites them with UsnRNA to form the assembled RNP. Despite advanced mechanistic understanding of UsnRNP assembly, our knowledge of regulatory features of this essential and ubiquitous cellular function remains remarkably incomplete. One may argue that the process operates as a default biosynthesis pathway and does not require sophisticated regulatory cues. Simple theoretical considerations and a number of experimental data, however, indicate that regulation of UsnRNP assembly most likely happens at multiple levels. This review will not only summarize how individual components of this assembly line act mechanistically but also why, how, and when the UsnRNP workflow might be regulated by means of posttranslational modification in response to cellular signaling cues.

Highlights d LARP7 interacts simultaneously with the U6 snRNA and U6specific C/D box snoRNAs d Depletion of LARP7 results in reduced 2 0 -O-methylation of the U6 snRNA d Changes in alternative splicing are observed in the absence of LARP7 d A LARP7 mutation causes splicing alterations in Alazami syndrome patients

Highlights d LARP7 is required for 2 0 -O-methylation of U6 snRNA in mouse male germ cells d LARP7 promotes U6 2 0 -O-methylation by box C/D snoRNP via facilitating U6 loading d LARP7-primed U6 2 0 -O-methylation is critical for the fidelity of pre-mRNA splicing d LARP7-primed U6 2 0 -O-methylation is critical for mouse male germ cell development

Cellular spliceosomal UsnRNP assembly is assisted by the PRMT5 and SMN complexes. Prusty et al. demonstrate that perturbations in the assembly machinery of UsnRNPs trigger complex cellular responses, using ribosomes, exosome-mediated RNA degradation, and autophagy to prevent Sm protein aggregation.