MicroRNAs act sequentially and asymmetrically to control chemosensory laterality in the nematode

Animal microRNAs ( miRNAs) are gene regulatory factors that prevent the expression of specific messenger RNA targets by binding to their 3' untranslated region(1-3). The Caenorhabditis elegans lsy-6 miRNA ( for lateral symmetry defective) is required for the left/right asymmetric expression of guanyl cyclase (gcy) genes in two chemosensory neurons termed ASE left (ASEL) and ASE right (ASER)(4,5). The asymmetric expression of these putative chemoreceptors in turn correlates with the functional lateralization of the ASE neurons(6). Here we find that a mutation in the die-1 zinc-finger transcription factor disrupts both the chemosensory laterality and left/right asymmetric expression of chemoreceptor genes in the ASE neurons. die-1 controls chemosensory laterality by activating the expression of lsy-6 specifically in ASEL, but not in ASER, where die-1 expression is downregulated through two sites in its 30 untranslated region. These two sites are complementary to mir-273, a previously uncharacterized miRNA, whose expression is strongly biased towards ASER. Forced bilateral expression of mir-273 in ASEL and ASER causes a loss of asymmetric die-1 expression and ASE laterality. Thus, an inverse distribution of two sequentially acting miRNAs in two bilaterally symmetric neurons controls laterality of the nematode chemosensory system.