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SCIENCE AT THE EDGE SEMINAR QB/GEDD Friday, January 17 at 11:30am Room 1400 Biomedical and Physical Sciences Bldg. Refreshments at 11:15 Amy Pasquinelli Division of Biology UCSD, La Jolla, CA Pinning Down MicroRNA Targets in Vivo The discovery that regulatory RNAs control almost every biological pathway has revolutionized our understanding of gene expression over the past decade. At the forefront, microRNAs (miRNAs) have proven to be an essential class of RNA molecules with mis-regulation of miRNA expression underlying a variety of human diseases. 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, as indicated by the presence of an ALG-1-binding site towards the 3′ end of the transcripts. 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. Moreover, our studies in C. elegans reveal a novel role for Argonaute in promoting biogenesis of a targeted transcript, expanding the functions of the miRNA pathway in gene regulation. Helen Geiger, Administrative Assistant Quantitative Biology Graduate Program and Gene Expression in Development and Disease Biochemistry 603 Wilson Road, Room 212 East Lansing, MI 48824 Email: [log in to unmask]<mailto:[log in to unmask]> Phone: 517-432-9895 QB Website: http://www.qbi.msu.edu/ GEDD Website: http://www.gedd.msu.edu/