Animal development requires coordination among cyclic processes, sequential cell fate specifications, and once-a-lifetime morphogenic events, but the underlying timing mechanisms are not well understood. Caenorhabditis elegans undergoes four molts at regular 8 to 10 hour intervals. The pace of the cycle is governed by PERIOD/lin-42 and other as-yet unknown factors. Cessation of the cycle in young adults is controlled by the
let-7 family of microRNAs and downstream transcription factors in the heterochronic pathway. Here, we characterize a negative feedback loop between NHR-23, the worm homolog of mammalian retinoid-related orphan receptors (RORs), and the
let-7 family of microRNAs that regulates both the frequency and finite number of molts. The molting cycle is decelerated in
nhr-23 knockdowns and accelerated in
let-7(-) mutants, but timed similarly in
let-7(-)
nhr-23(-) double mutants and wild-type animals. NHR-23 binds response elements (ROREs) in the
let-7 promoter and activates transcription. In turn,
let-7 dampens
nhr-23 expression across development via a complementary
let-7-binding site (LCS) in the
nhr-23 3' UTR. The molecular interactions between NHR-23 and
let-7 hold true for other
let-7 family microRNAs. Either derepression of
nhr-23 transcripts by LCS deletion or high gene dosage of
nhr-23 leads to protracted behavioral quiescence and extra molts in adults. NHR-23 and
let-7 also coregulate scores of genes required for execution of the molts, including
lin-42. In addition, ROREs and LCSs isolated from mammalian ROR and
let-7 genes function in C. elegans, suggesting conservation of this feedback mechanism. We propose that this feedback loop unites the molting timer and the heterochronic gene regulatory network, possibly by functioning as a cycle counter.