Translation of a small upstream open reading frame functions as a rheostat for the regulation of lin-41 by the Let-7 microRNA in Caenorhabditis elegans

MicroRNAs often exert modest effects on target protein levels yet play essential roles in development. How microRNA function combines with other regulatory mechanisms to control dosage-sensitive targets remains incompletely understood. In Caenorhabditis elegans, the Let-7 microRNA represses translation of its principal developmental timing target, the LIN-41 TRIM-NHL RNA-binding protein, by binding two complementary sites in the lin-41 3'UTR and recruiting the microRNA-induced silencing complex (miRISC) to repress translation. Here we identify an independent layer of translational repression in somatic cells that is mediated by a 5’ regulatory exon containing a seven-amino-acid upstream open reading frame (uORF) and conserved sequence elements. Genome editing indicates that the specific uORF amino acid sequence is not required; instead, our data suggest that uORF translation and 5'UTR structure limit initiation at the downstream lin-41 start codon. Together with Let-7, the 5’ regulatory exon and its embedded uORF provide multitiered control of lin-41 translation, which limits LIN-41 to levels compatible with normal somatic development.