Panoramix enforces piRNA-dependent cotranscriptional silencing

Abstract

The Piwi-interacting RNA (piRNA) pathway is a small RNA–based innate immune system that defends germ cell genomes against transposons. In Drosophila ovaries, the nuclear Piwi protein is required for transcriptional silencing of transposons, though the precise mechanisms by which this occurs are unknown. Here we show that the CG9754 protein is a component of Piwi complexes that functions downstream of Piwi and its binding partner, Asterix, in transcriptional silencing. Enforced tethering of CG9754 to nascent messenger RNA transcripts causes cotranscriptional silencing of the source locus and the deposition of repressive chromatin marks. We have named CG9754 “Panoramix,” and we propose that this protein could act as an adaptor, scaffolding interactions between the piRNA pathway and the general silencing machinery that it recruits to enforce transcriptional repression.

Multiple LINEs of retrotransposon silencing mechanisms in the mammalian germline

“…Specifically, the nuclear Piwi-piRNA complex recognizes and binds to nascent transcripts of transposable elements, thereby marking their genomic sites. Silencio, an adaptor protein, interacts with target-engaged Piwi-piRNA complexes and directly or indirectly recruits SetDB1, an H3K9 tri-methyltransferase, leading to the formation of heterochromatin at transposon-rich genomic regions [64, 65]. Drosophila lacks DNA methylation, and defects in the fly piRNA pathway cause loss of repressive H3K9me3 at transposable elements resulting in their activation [63, 66].…”

piRNA-mediated regulation of transposon alternative splicing in the soma and germ line

“…Unexpectedly, we show that the piRNA pathway components do not act to reduce P -element transposon transcript levels during P-M hybrid dysgenesis, a syndrome that affects germline development in Drosophila 3,4 . Instead, splicing regulation is mechanistically achieved in concert with piRNA-mediated changes to repressive chromatin states, and relies on the function of the Piwi-piRNA complex proteins Asterix/Gtsf1 5–7 and Panoramix/Silencio 8,9 , as well as Heterochromatin Protein 1a (Su(var)205/HP1a). Furthermore, we show that this machinery, together with the piRNA Flamenco cluster 10 , not only controls the accumulation of Gypsy retrotransposon transcripts 11 but also regulates splicing of Gypsy mRNAs in cultured ovarian somatic cells, a process required for the production of infectious particles that can lead to heritable transposition events 12,13 .…”

“…Piwi-interacting proteins such as Asterix/Gtsf1 (Arx) and Panoramix/Silencio (Panx) are dispensable for piRNA biogenesis but are essential for establishing piwi-mediated chromatin changes, possibly by acting as a scaffold to recruit histone modifying enzymes and chromatin-binding proteins to target loci 8,9 . To test the role of these chromatin regulators on P -element splicing, we performed germline-specific RNAi knockdown experiments in the Harwich background.…”

Oncogenic transformation of Drosophila somatic cells induces a functional piRNA pathway

“…Differentially expressed genes were considered significant when varying at least twofold with a padj-value of <0.05. Transposon reads were mapped to UCSC consensus transposon sequences with Bowtie2 (version 2.2.3) (options: -fr -k 10 -N 1 -X 1000) and normalized to genome mappers as described by Yu et al (2015).…”

“…Nuclear PIWI proteins, along with cofactors (Asterix and Panoramix in flies), recruit histone-modifying enzymes, including deacetylases and methyltransferases, to establish a chromatin signature correlated with transcriptional repression (Dönertas et al 2013;Muerdter et al 2013;Ohtani et al 2013;Rozhkov et al 2013;Sienski et al 2015;Yu et al 2015).…”