Not lost in translation: stepwise regulation of microRNA targets

MicroRNAs (miRNAs) are endogenous, ~22-nucleotide-long, noncoding RNAs that play critical roles in physiology and disease via mechanisms that remain obscure. Although numerous studies implicate miRNAs in repression of translation, more recent reports suggest that the major role of miRNAs is in reduction of target mRNA stability. Because mRNA translation and stability are intimately connected, it has been a challenge to establish whether miRNAs induce translational repression, mRNA decay, or both. If miRNAs reduce both mRNA translation and stability, the timing and contribution of each process to overall repression is unclear. Indeed, it has been debated whether mRNA decay is a cause or consequence of miRNA-mediated translational repression. On the other hand, if these events are mutually exclusive, what determines which mechanism is used? In a recent issue of Science, Bazzini et al (2012) use genome-wide ribosome footprinting and RNA sequencing (RNA-Seq) to demonstrate that in developing zebrafish embryos, miR-430 naturally represses translation initiation of target mRNAs, followed by their deadenylation and decay.