Global profiling of RNA-binding protein target sites by LACE-seq.

RNA-binding proteins (RBPs) have essential functions during germline and early embryo development. However, current methods are unable to identify the in vivo targets of a RBP in these low-abundance cells. Here, by coupling RBP-mediated reverse transcription termination with linear amplification of complementary DNA ends and sequencing, we present the LACE-seq method for identifying RBP-regulated RNA networks at or near the single-oocyte level. We determined the binding sites and regulatory mechanisms for several RBPs, including Argonaute 2 (Ago2), Mili, Ddx4 and Ptbp1, in mature mouse oocytes. Unexpectedly, transcriptomics and proteomics analysis of Ago2 oocytes revealed that Ago2 interacts with endogenous small interfering RNAs (endo-siRNAs) to repress mRNA translation globally. Furthermore, the Ago2 and endo-siRNA complexes fine-tune the transcriptome by slicing long terminal repeat retrotransposon-derived chimeric transcripts. The precise mapping of RBP-binding sites in low-input cells opens the door to studying the roles of RBPs in embryonic development and reproductive diseases.