Co-Dependent Assembly of <i>Drosophila</I> piRNA Precursor Complexes and piRNA Cluster Heterochromatin

Abstract

In Drosophila, the piRNAs that direct germline transposon silencing are produced from heterochromatic clusters marked by the HP1 homolog Rhino. We show that Rhino promotes assembly of complexes containing cluster transcripts, UAP56, and the THO complex, and that these complexes are unique to piRNA precursors. UAP56 and THO are ubiquitous RNA processing factors, and null alleles of uap56 and the core THO subunit gene tho2 are lethal. However, the uap56sz15 point mutation, and mutations in the THO subunit genes thoc5 and thoc7, are viable but sterile, and disrupt piRNA biogenesis. We show that the uap56sz15 mutation disrupts UAP56 binding to THO, and thoc5 and thoc7 mutations block THO complex assembly and subunit co-localization with UAP56. Significantly, these mutations also reduce Rhino binding to clusters and trigger Rhino binding to ectopic sites across the genome. Rhino thus promotes assembly of piRNA precursor complexes, and these complexes restrict Rhino to cluster heterochromatin.