Abstract
MicroRNAs (miRNAs) repress target genes through a poorly defined antisense mechanism. Cell-free and cell-based assays have supported the idea that miRNAs repress their target mRNAs by blocking initiation of translation, whereas studies in animal models argued against this possibility. We examined endogenous targets of the let-7 miRNA, an important regulator of stem cell fates. We report that let-7 represses translation initiation in Caenorhabditis elegans, demonstrating this mode of action for the first time in an organism. Unexpectedly, although the lin-4 miRNA was previously reported to repress its targets at a step downstream of translation initiation, we also observe repression of translation initiation for this miRNA. This repressive mechanism, which frequently but not always coincides with transcript degradation, requires the GW182 proteins AIN-1 and AIN-2, and acts on several mRNAs targeted by different miRNAs. Our analysis of an expanded set of endogenous miRNA targets therefore indicates widespread repression of translation initiation under physiological conditions and establishes C. elegans as a genetic system for dissection of the underlying mechanisms.