Analyses of piRNA-mediated transcriptional transposon silencing in Drosophila: nuclear run-on assay on ovaries.

In the Drosophila germline, retrotransposons are silenced by the PIWI-interacting RNA (piRNA) pathway. piRNA pathway mutations lead to overexpression and mobilization of retrotransposons in the germline. In different organisms, small RNAs were shown to be implicated in the posttranscriptional degradation of mRNA and/or transcriptional repression of the homologous locus. In Drosophila, the mechanism of piRNA-mediated silencing is still far from being understood. Transcriptional silencing implies a piRNA-mediated formation of repressive chromatin which diminishes the transcriptional capacity of the target locus. Nuclear Run-On (NRO) assay allows a direct estimation of the density of transcribing polymerases at specific genomic regions. Here we describe the NRO protocol on Drosophila ovarian tissues which can be useful for investigation of the transcriptional silencing in the female germline.