Antisense transcription of retro transposable elements in Drosophila: The origin of endogenous small interfering RNA precursors

Unrestricted movement of mobile genetic elements could cause pre-mature lethality in Drosophila melanogaster. Specifically, retro transposons can disrupt genomic integrity through insertions, deletions and chromosomal rearrangements. Therefore, eukaryotes have developed defense mechanisms to silence these elements. In Drosophila, endogenous small interfering (endo-siRNAs) repress retro transposon mobility in somatic cells. The generation of endo-siRNAs requires Dicer-2 processing of double-stranded RNA precursors, yet the origins of this precursor are unknown. Here we show that retro transposons in Dmel-2 cells produce sense and antisense transcripts and identify bonafide transcription start sites for these RNAs. We determine that retro transposon antisense transcripts are less polyadenylated than sense transcripts. RNA-seq and small RNA-seq upon Dicer-2 depletion showed global decrease in endo-siRNAs mapping to retro transposons and increased expression of both S and AS retro transposon transcripts. These data support a model in which double-stranded RNA precursors are derived from convergent transcription and retained in the nucleus. Dicer-2 processes these precursors into endo-siRNAs that silence both sense and antisense retro transposon transcripts. Reduction of sense retro transposon transcripts potentially lowers element specific protein levels required for movement. This mechanism preserves genomic integrity and is especially important for Drosophila fitness because mobile genetic elements are highly active. This submission includes 12 raw data files. Two samples (LacZ control and Dcr-2 RNAi-depleted) are represented by three technical triplicate raw data files. RNA-seq and small RNA-seq data are provided for each sample.