Two Sets of Piwi Proteins Are Involved in Distinct sRNA Pathways Leading to Elimination of Germline-Specific DNA

Piwi proteins and piRNAs protect eukaryotic germlines against the spread of transposons. During development in the ciliate Paramecium, two Piwi-dependent sRNA classes are involved in the elimination of transposons and transposon-derived DNA: scan RNAs (scnRNAs), associated with Ptiwi01 and Ptiwi09, and iesRNAs, whose binding partners we now identify as Ptiwi10 and Ptiwi11. scnRNAs derive from the maternal genome and initiate DNA elimination during development, whereas iesRNAs continue DNA targeting until the removal process is complete. Here, we show that scnRNAs and iesRNAs are processed by distinct Dicer-like proteins and bind Piwi proteins in a mutually exclusive manner, suggesting separate biogenesis pathways. We also demonstrate that the PTIWI10 gene is transcribed from the developing nucleus and that its transcription depends on prior DNA excision, suggesting a mechanism of gene expression control triggered by the removal of short DNA segments interrupting the gene.