Silencio/CG9754 connects the Piwi-piRNA complex to the cellular heterochromatin machinery

The repression of transposable elements in eukaryotes often involves their transcriptional silencing via targeted chromatin modifications. In animal gonads, nuclear Argonaute proteins of the PIWI-clade complexed with small guide RNAs (piRNAs) serve as sequence specificity determinants in this process. How binding of nuclear PIWI-piRNA complexes to nascent transcripts orchestrates heterochromatin formation and transcriptional silencing is unknown. Here, we characterize CG9754/Silencio as an essential piRNA pathway factor that is required for Piwi-mediated transcriptional silencing in Drosophila. Ectopic targeting of Silencio to RNA or DNA is sufficient to elicit silencing independent of Piwi and known piRNA pathway factors. Instead, Silencio requires the H3K9 methyl-transferase Eggless/SetDB1 for its silencing ability. In agreement with this, SetDB1 but not Su(var)3-9 is required for Piwi-mediated transcriptional silencing genome-wide. Due to its interaction with the target-engaged Piwi-piRNA complex we suggest that Silencio acts as linker between the sequence specificity factor Piwi and the cellular heterochromatin machinery. Overall design: Impact of depletion of CG9754/Silencio, HP1a, SetDB1, Su(var)3-9, G9a on transcription and the H3K9m3 patterns in ovarian somatic cells (OSC)