BTBD18 licenses piRNA-generating loci to support mouse germ cell development

PIWI-interacting RNAs (piRNAs) are small non-coding RNAs essential for animal germ cell development. Despite intense investigation of post-transcriptional processing, piRNA biogenesis at the chromatin level remains enigmatic. Here we document that BTBD18 is a pachytene nuclear protein in mouse testes that occupies intergenic pachytene piRNA-producing loci with remarkable genome-wide specificity. Deficiency of Btbd18 in mice results in disrupted piRNA biogenesis and male sterility. Transcriptome profiling, chromatin accessibility and RNA polymerase II occupancy demonstrate that BTBD18 facilitates expression of pachytene piRNA precursors by promoting transcription elongation. Therefore, we conclude that BTBD18 functionally licenses genomic sources of piRNAs for transcription activation in mice, and propose that piRNA precursor biogenesis is regulated by transcriptional elongation. Overall design: RNA-seq, small RNA-seq, ChIP-seq, and ATAC-seq were performed using mouse testes or purified testicular cells with indicated genotypes.