MicroRNAs and messenger RNA turnover.

Although initially believed to act exclusively as translational repressors, microRNAs (miRNAs) are now known to target complementary messenger RNA (mRNA) transcripts for either translational repression or cleavage via the RNA-induced silencing complex (RISC) ([1], reviewed in ref. 2). The current model postulates that mature miRNAs are incorporated into the RISC, bind target mRNAs based on complementarity, and guide cleavage of mRNA targets with perfect or nearly perfect complementarity and translational repression of targets with lower complementarity (2). The translational repression mechanism of miRNA-mediated gene regulation, which is common in animals but also exists in plants, is not well understood mechanistically. Conversely, miRNA-directed mRNA cleavage by RISC is common in plants, but also occurs in animals (3). This chapter focuses on the mRNA cleavage by miRNA-programmed RISC, and, specifically, on characterizing the products of such cleavage.