Co-dependent assembly of TREX-piRNA precursor complexes and piRNA cluster heterochromatin. Drosophila melanogaster

The PIWI-interacting RNAs (piRNAs) direct germline transposon silencing. Drosophila piRNA precursors are derived from heterochromatic clusters marked by the HP1 homolog Rhino, which suppresses cluster transcript splicing and promotes piRNA biogenesis. We show that Rhino promotes co-dependent binding of the THO complex and UAP56 to unspliced cluster transcripts. THO complex and UAP56 are essential RNA splicing and export factors that interact within the transcription and export (TREX) complex. However, mutations that destabilize TREX complex are viable and support gene expression, but are sterile, disrupt transposon silencing, and block piRNA production. Rhino binds to H3K9me3, a broadly distributed heterochromatin histone modification, and mutations that destabilize TREX complex and disrupt piRNA production also trigger ectopic Rhino binding to heterochromatic and euchromatic H3K9me3 sites outside piRNA clusters. Rhino thus promotes assembly of pre-piRNP with TREX complex, and these complexes restrict Rhino to H3K9me3 marks on the cluster heterochromatin.