Browsing by keyword "transcriptional silencing"
Now showing items 1-2 of 2
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piRNA-independent transposon silencing by the Drosophila THO complexpiRNAs guide Piwi/Panoramix-dependent H3K9me3 chromatin modification and transposon silencing during Drosophila germline development. The THO RNA export complex is composed of Hpr1, Tho2, and Thoc5-7. Null thoc7 mutations, which displace Thoc5 and Thoc6 from a Tho2-Hpr1 subcomplex, reduce expression of a subset of germline piRNAs and increase transposon expression, suggesting that THO silences transposons by promoting piRNA biogenesis. Here, we show that the thoc7-null mutant combination increases transposon transcription but does not reduce anti-sense piRNAs targeting half of the transcriptionally activated transposon families. These mutations also fail to reduce piRNA-guided H3K9me3 chromatin modification or block Panoramix-dependent silencing of a reporter transgene, and unspliced transposon transcripts co-precipitate with THO through a Piwi- and Panoramix-independent mechanism. Mutations in piwi also dominantly enhance germline defects associated with thoc7-null alleles. THO thus functions in a piRNA-independent transposon-silencing pathway, which acts cooperatively with Piwi to support germline development.
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TREX Function in piRNA Biogenesis and Transposon SilencingThe Piwi interacting RNA pathway (piRNA) transcriptionally and post-transcriptionally silences transposons in the germline to maintain host genome integrity and faithful transmission of the genetic materials. In Drosophilaovaries, maternally loaded piRNAs kick-start piRNA biogenesis and convert precursor transcripts into piRNAs to replenish the piRNA pool during oogenesis. piRNA clusters are the genomic source of piRNA precursors, which are determined by the HP1 homolog Rhino and accessary factors. Rhino specifically binds to piRNA cluster chromatin. I was intrigued by how Rhino localizes to piRNA clusters to specify piRNA precursors. TREX is a conserved mRNA biogenesis complex composed of UAP56 and the THO complex. Identification of UAP56 as a cluster transcript-processing factor established the link between piRNA biogenesis and the general mRNA processing machinery. In my thesis, I investigated the functions of UAP56 and THO in piRNA cluster transcript processing. I characterized an RNP specific to cluster transcripts, defined by binding with both factors, which is distinct from RNP of bulk mRNA transcripts, and found that assembly of these RNPs depends on Rhino. These findings imply that piRNA precursors are specified co-transcriptionally. Additionally, I found that TREX mutants lead to a loss of Rhino binding specificity. I propose that Rhino and TREX co-transcriptionally scan for cluster and transposon sequences to establish loci that produce piRNA precursors. Surprisingly, I also discovered a piRNA-independent function for TREX in transposon silencing. I showed that TREX mutants lead to transcriptionally activation of a number of transposon families without affecting their piRNA biogenesis and piRNA mediated repressive histone modifications. I propose that TREX could mediate a conserved transposon silencing mechanism.
