RNA silencing pathways are now recognized to participate in essential cellular functions ranging from the regulation of mRNA turnover to the suppression of the activity of potentially deleterious transposable elements (TEs). Piwi-interacting RNAs (piRNAs) are germline-specific, small silencing RNAs that suppress TE activity and maintain genome integrity during germline development. In Drosophila ovarian somatic cells, piRNAs are processed from long single-stranded RNAs by a Dicer-independent pathway and are loaded onto Piwi in the cytoplasm. The Piwi-piRNA complexes are then transported into the nucleus to exert TE silencing. This mechanism involves gatekeepers for a functional Piwi-piRNA complex to be imported, which parallels with the Tetrahymena Twi1p-scan RNA pathway used to carry out the programmed DNA elimination.
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