PIWI-interacting RNAs (piRNAs) silence retrotransposons in Drosophila germ lines by associating with the PIWI proteins Argonaute 3 (AGO3), Aubergine (Aub) and Piwi1,2. piRNAs in Drosophila are produced from intergenic repetitive genes and piRNA clusters by two systems: the primary processing pathway and the amplification loop1,2,3,4,5,6,7. The amplification loop occurs in a Dicer-independent, PIWI-Slicer-dependent manner3,4,8. However, primary piRNA processing remains elusive. Here we analysed piRNA processing in a Drosophila ovarian somatic cell line where Piwi, but not Aub or AGO3, is expressed; thus, only the primary piRNAs exist. In addition to flamenco, a Piwi-specific piRNA cluster3, traffic jam (tj)9, a large Maf gene, was determined as a new piRNA cluster. piRNAs arising from tj correspond to the untranslated regions of tj messenger RNA and are sense-oriented. piRNA loading on to Piwi may occur in the cytoplasm. zucchini10, a gene encoding a putative cytoplasmic nuclease, is required for tj-derived piRNA production. In tj and piwi mutant ovaries, somatic cells fail to intermingle with germ cells and Fasciclin III is overexpressed. Loss of tj abolishes Piwi expression in gonadal somatic cells. Thus, in gonadal somatic cells, tj gives rise simultaneously to two different molecules: the TJ protein, which activates Piwi expression, and piRNAs, which define the Piwi targets for silencing.
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We thank Y. Niki, D. Godt, H. Lin, E. Matunis, S. Kobayashi, Y. Kitadate, Y. Kageyama and H. Sano for providing reagents. We also thank Bloomington and Kyoto Drosophila Stock Center for the supply of Drosophila strains. We thank K. Yamada, E. Hattori, K. M. Nishida and T. N. Okada for technical assistance; S. Takahashi and K. Kataoka for discussions and suggestions; and other members of the Siomi laboratory for discussions and comments on the manuscript. We also thank K. Greer and D. McGowan for encouragement. This work was supported by MEXT grants to H.S. and NEDO (New Energy and Industrial Technology Development Organization) grants to M.C.S., T.M. and K.A. M.C.S. is supported by CREST from JST. M.C.S. is Associate Professor of Global COE for Human Metabolomics Systems Biology by MEXT.
Author Contributions K.S., S.I., Y.K. and H.K. conducted biochemical experiments. T.M., Y.O., E.S. and K.A. performed bioinformatics. K.S., T.M., H.S. and M.C.S designed experiments, interpreted data and prepared the manuscript.
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Saito, K., Inagaki, S., Mituyama, T. et al. A regulatory circuit for piwi by the large Maf gene traffic jam in Drosophila. Nature 461, 1296–1299 (2009). https://doi.org/10.1038/nature08501
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