The transcription factor Traffic jam orchestrates the somatic piRNA pathway in ovaries.

Unlabelled: Transposable elements (TEs) pose a threat to genome integrity, and the piRNA pathway in animal gonads plays a crucial role in silencing TE activity. While the transcriptional regulation of the piRNA pathway components in germ cells has been documented in mice and flies, the mechanisms orchestrating the transcriptional program of the somatic piRNA pathway in ovaries remains unresolved. Here, we demonstrate that Traffic jam (Tj), an orthologue of a large Maf transcription factor in mammals, is a master regulator of the piRNA pathway in ovarian somatic cells, playing a crucial role in maintaining TE silencing and genomic integrity in somatic tissues. We show that Tj directly binds to the promoters of somatic-enriched piRNA factors such as , , , and , as well as the piRNA cluster, a major locus for TE silencing in somatic cells. Depletion of Tj in somatic follicle cells results in a significant downregulation of these piRNA factors, a complete loss of expression and de-repression of -family TEs, which have gained the ability to activate in ovarian somatic cells allowing them to infect germ cells and be transmitted to future generations. We have identified an enhancer carrying Tj binding motifs located downstream of the promoter that is essential for robust and tissue-specific expression in somatic follicle cells of the adult ovary. This work uncovers a previously unappreciated layer of transcriptional regulation of the piRNA pathway, and we propose that the arms race between the host and TEs has driven the evolution of promoters in piRNA genes and clusters to respond to a unique transcription factor thereby ensuring efficient silencing of -family TEs.

Highlights: Traffic jam (Tj) acts as a master regulator of the somatic piRNA pathway in . Tj directly controls the expression of the piRNA cluster, crucial for transposon silencing. Tj regulates a network of piRNA pathway genes, mirroring the gene-regulatory mechanism of A-MYB in the mouse testis.Cis-regulatory elements with Tj motifs are arranged in a palindromic sequence.