GeneCore for preparation of the Methyl-seq libraries. Author contributionsA.Z. contributed to the design, execution and analysis of most experiments. T.A. helped established the IP conditions and performed the mass-spectrometry analysis under the guidance of J.R. and R.C.A. R.B. and Y.K. performed the bioinformatic analysis of the Methyl-seq data or sRNA-seq as well as RNA-seq data, respectively. T.S. prepared the sRNA-seq libraries and together with Y.K. the RNA-seq libraries of P20 testes. M.H and A.C. performed the homology alignment of the SPOC and TFIIS-M domains. L.V. performed the IF staining of HA-MIWI2 and generated the gonocytes microarray dataset. Y.R.P. performed the phylogenetic analysis under guidance of A.S. D.O'C. conceived and supervised this study. D.O'C. and A.Z wrote the final version of the manuscript.

The PIWI protein MIWI2 and its associated PIWI-interacting RNAs (piRNAs) instruct DNA methylation of young active transposable elements (TEs) in the male germline. piRNAs are proposed to recruit MIWI2 to the transcriptionally active TE loci by base pairing to nascent transcripts, however the downstream mechanisms and effector proteins utilized by MIWI2 in directing de novo TE methylation remain incompletely understood. Here, we show that MIWI2 associates with TEX15 in foetal gonocytes. TEX15 is predominantly a nuclear protein that is not required for piRNA biogenesis but is essential for piRNA-directed TE de novo methylation and silencing. In summary, TEX15 is an essential executor of mammalian piRNAdirected DNA methylation.