Conservation of the ancestral function of GTSF1 in transposon silencing in the unicellular eukaryoteParamecium tetraurelia

The PIWI-interacting RNA (piRNA) pathway is crucial for transposon repression and the maintenance of genomic integrity. Gametocyte specific factor 1 (GTSF1), an indispensable auxiliary factor of PIWI, was recently shown to potentiate the catalytic activity of PIWI in many metazoans. Whether the requirement of GTSF1 extends to PIWI proteins beyond metazoans is unknown. In this study, we identified a homolog of GTSF1 in the unicellular eukaryote Paramecium tetraurelia (PtGTSF1) and found that its role as a PIWI-cofactor is conserved. PtGTSF1 interacts with PIWI (Ptiwi09) and Polycomb Repressive Complex 2 (PRC2) and is essential for PIWI-dependent DNA elimination of transposons during sexual development. PtGTSF1 is crucial for the degradation of PIWI-bound small RNAs recognizing the organism’s own genomic sequences. Without PtGTSF1, self-matching small RNAs are not degraded and results in an accumulation of H3K9me3 and H3K27me3, which disturbs transposon recognition and slows down their elimination. Our results demonstrate that the PIWI-GTSF1 interaction also exists in unicellular eukaryotes with the ancestral function of transposon silencing.