ADMA-histones define genomic loading of Rhino at the initial step of piRNA cluster formation

In Drosophila germ cells, piRNAs arise from dual-strand piRNA clusters. These clusters are occupied by H3K9me3, but are transcribed from internal sites in a manner dependent on the binding of HP1 homolog Rhino to H3K9me3 on the clusters. However, how initial loading of Rhino onto the clusters occurs remains unknown. Here, we used cultured ovarian somatic cells (OSCs), which lack endogenous Rhino and Rhino stabilizer Kipferl, the absence of which renders the dual-strand clusters inert, and found that exogenous Rhino tends to bind to the ends of dual-strand clusters with asymmetric dimethylarginine histones (ADMA-histones). Depletion of the arginine methyltransferases responsible for ADMA modification affected the genomic localization of Rhino in OSCs and in the ovary. We also identified genomic regions, termed INSECTs, where ADMA-dependent Rhino propagation begins. We propose that ADMA-histones define the initial genomic loading of Rhino and stabilize Rhino−genome association during cluster formation.