S01-04 piRNA biogenesis pathways in Drosophila germline cells

Gene silencing pathways triggered by small RNAs are generically called RNA silencing. Members of the Argonaute family play important roles in the pathways. In Drosophila, the Argoanute family consists of five distinct members (AGO1, AGO2, AGO3, Piwi, and Aubergine). Piwi, Aubergine, and AGO3 (the PIWI proteins) are specifically expressed in germlines and associated with Piwi-interacting RNAs (piRNAs). piRNAs are 24–30 nt long, only expressed in gonads, and function in genome surveillance through association with the PIWI proteins by silencing mobile repetitive elements. Deep-sequencing analyses of piRNAs in Drosophila gonads suggested that piRNAs are produced by two systems, the primary processing pathway and the amplification loop, from repetitive genes and piRNA clusters in the genome. The amplification loop occurs in a Dicer-independent, PIWI-Slicer-dependent manner. However, the primary processing pathway remains largely conceptual. Recently we show that in ovarian somatic cells, which lack Aub and AGO3 but express Piwi, the primary processing pathway for piRNAs indeed exists. In addition to flamenco, a Piwi-specific piRNA cluster, traffic jam (tj), a large Maf-coding gene, was determined as a new piRNA cluster. piRNAs arising from tj correspond to the untranslated regions of tj mRNA and are all in sense. In tj and piwi mutant larval ovaries, somatic cells fail to intermingle with germ cells. Loss of tj abolishes Piwi expression in gonadal somas. 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.