Abstract
PIWI-interacting RNAs (piRNAs) are effectors of transposable element (TE) silencing in the reproductive apparatus. In Drosophila ovarian somatic cells, piRNAs arise from longer single-stranded RNA precursors that are processed in the cytoplasm presumably within the Yb-bodies. piRNA precursors encoded by the flamenco (flam) piRNA cluster accumulate in a single focus away from their sites of transcription. In this study, we identify the exportin complex containing Nxf1 and Nxt1 as required for flam precursor nuclear export. Together with components of the exon junction complex (EJC), it is necessary for the efficient transfer of flam precursors away from their site of transcription. Indeed, depletion of these components greatly affects flam intra-nuclear transit. Moreover, we show that Yb-body assembly is dependent on the nucleo-cytoplasmic export of flam transcripts. These results suggest that somatic piRNA precursors are thus required for the assembly of the cytoplasmic transposon silencing machinery.
Highlights
PIWI-interacting RNAs are effectors of transposable element (TE) silencing in the reproductive apparatus
We first investigated the nucleo-cytoplasmic distribution of flam transcripts in more than 1,700 follicular cells. flam transcripts were revealed by in situ hybridization using the specific flam 508 RNA probe that recognizes a unique sequence of the flam locus[17]
We examined how the PIWI-interacting RNAs (piRNAs) precursors are transferred from their site of transcription to the cytoplasmic structure where they are processed into primary piRNAs
Summary
PIWI-interacting RNAs (piRNAs) are effectors of transposable element (TE) silencing in the reproductive apparatus. Together with components of the exon junction complex (EJC), it is necessary for the efficient transfer of flam precursors away from their site of transcription. RNA precursors transcribed from piRNA clusters are first transferred to cytoplasmic perinuclear granules called Yb-bodies that have been reported to be major sites for piRNA biogenesis[7,8]. Dennis et al.[17] found that flam precursors channel through the nucleoplasm and first accumulate in a structure called Dot COM within the nucleus This nuclear structure faces the cytoplasmic Yb-body and has been proposed to be a nuclear site of accumulation of piRNA precursors coming from various piRNA clusters before their export. The two EJC core factors Mago and Tsunagi/Y14 (Tsu) as well as the EJC accessory proteins RnpS1 and Acinus (Acn) have been implicated in the piRNA pathway biology, their mutations leading to TE derepression[21,23,35,36,37]
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