The Primary Target Of Let‐7 MicroRNA

The discovery that regulatory RNAs control almost every biological pathway has revolutionized our understanding of gene expression. At the forefront, miRNAs have proven to be an essential class of RNA molecules with mis‐regulation of miRNA expression underlying a variety of human diseases, including cancer. Most miRNAs are derived from long primary transcripts that undergo processing by Drosha to produce ~65‐nucleotide precursors that are then cleaved by Dicer, resulting in the mature 22‐nucleotide forms. Serving as guides in Argonaute protein complexes, mature miRNAs use imperfect base pairing to recognize sequences in mRNA transcripts, leading to translational repression and destabilization of the target mRNAs. Because of the imperfect nature of miRNA‐target duplexes, identifying biologically relevant target sites is an outstanding challenge. To address this problem, we have used cross‐linking immunopurification with high‐throughput sequencing (CLIP‐seq) to capture miRNA target sequences in vivo. Focusing on the last larval stage in C. elegans development, we found that more than 3,000 mRNA transcripts have sequences bound by ALG‐1 (Argonaute Like Gene 1). Unexpectedly, the non‐coding let‐7 primary transcripts (pri‐let‐7), which are processed into the mature let‐7 miRNA, were among the RNAs targeted by Argonaute. We found that association of ALG‐1 with pri‐let‐7 promotes processing and this interaction is mediated by mature let‐7 miRNA through a conserved complementary site in its own primary transcript, thus creating a positive‐feedback loop. Argonaute also binds let‐7 primary transcripts in human cells, demonstrating that the miRNA pathway targets non‐coding RNAs in addition to protein‐coding messenger RNAs across species. Our studies reveal a novel role for Argonaute in promoting biogenesis of a targeted transcript, expanding the functions of the miRNA pathway in gene regulation. Support or Funding Information The Pasquinelli lab is supported by NIH grant GM071654