A Genome-wide RNAi screen identifies factors required for distinct stages of C. elegans piRNA biogenesis

In animals, piRNAs, and their associated Piwi proteins, guard germ cell genomes against mobile genetic elements via an RNAi-like mechanism. In C. elegans, 21U-RNAs comprise the piRNA class and these collaborate with 22G RNAs, via unclear mechanisms, to discriminate self from non-self and selectively and heritably silence the latter. Recent work indicates that 21U-RNAs are post-transcriptional processing products of individual transcription units that produce ~26 nucleotide capped precursors. Yet, nothing is known of how the expression of precursors is controlled or of how primary transcripts give rise to mature small RNAs. We conducted a genome-wide RNAi screen to identify components of the 21U biogenesis machinery. Screening by direct, qPCR-based measurements of mature 21U-RNA levels, we identified 22 genes important for 21U-RNA production, termed TOFUs (Twenty-One-u Fouled Up). We also identified 7 genes that normally repress 21U production. By measuring mature 21U-RNA and precursor levels for the 7 strongest hits from the screen, we have assigned factors to discrete stages of 21U-RNA production. Our work has identified factors separately required for the transcription of 21U precursors, and the processing of these precursors into mature 21U-RNAs, thereby providing an essential resource for studying the biogenesis of this important small RNA class. Overall design: Small RNA and capped small RNA sequencing from total RNA of C. elegans subjected to different RNAi and different C. elegans mutants

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Goh WS, Seah JW, Harrison EJ, Chen C, Hammell CM, Hannon GJ