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Andersen et al. sought to understand how piRNA cluster loci in D. melanogaster are bidirectionally transcribed despite lacking overt promoter DNA sequences and being in heterochromatin that
harbours the classic repressive mark histone H3 lysine trimethylation (H3K9me3). Deleting promoters in DNA flanking the 42AB and 80F piRNA clusters indicated that these piRNA clusters were
not transcribed via readthrough from flanking regions. Instead, cap sequencing (Cap-seq) to pinpoint the 5′ end of transcripts revealed pervasive initiation of transcription from many sites
within the piRNA clusters.
To identify potential molecular mediators of this transcription initiation within piRNA clusters, the authors mined data from a published transposon derepression screen, focusing on hits
that have sequence similarity to known transcription initiation machinery. They identified CG12721 as a potential ovary-specific paralogue of the large subunit of transcription factor IIA
(TFIIA-L). Its role as a driver of piRNA cluster transcription was supported by its enrichment at piRNA clusters, its interactions with other transcription initiation proteins and, because
its knockout led to depleted piRNA expression from the 42AB and 80F piRNA clusters, transposon derepression and fly sterility. Based on this function, the authors named CG12721 'Moonshiner'
in reference to its activity in the face of the transcriptional 'prohibition' of the heterochromatin environment.
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