The Circadian Transcription Factor CLOCK Modulates Oxidative Stress Resistance via the ACHL– Relish Axis in Drosophila

Circadian clocks orchestrate temporal regulation of diverse physiological processes, including innate immunity and oxidative stress responses. However, the molecular mechanisms by which core clock components modulate immune tone and redox homeostasis remain elusive. Here, the circadian transcription factor CLOCK (CLK) is identified as a key regulator of oxidative stress resistance in Drosophila melanogaster. Loss of clk significantly enhances survival under oxidative stress, accompanied by constitutive activation of innate immune pathways. Mechanistically, the RNA-binding protein Achilles (ACHL) is identified as a critical downstream effector of CLK. Indeed, CLK drives rhythmic transcription of Achl, and ACHL post-transcriptionally represses the NF-κB homolog Relish by promoting its mRNA degradation, thereby limiting immune overactivation. Disruption of this regulatory cascade, through loss of either clk or Achl, leads to increased Relish abundance, excessive immune gene expression, and enhanced oxidative stress resistance. Genetic suppression of Relish reverses these phenotypes, establishing a functional CLK-ACHL-Relish axis that links circadian output to immune restraint. Importantly, this regulatory mechanism is evolutionarily conserved, as Clock-deficient mammalian cells exhibit increased resistance to oxidative injury. Together, the findings uncover a post-transcriptional immune checkpoint controlled by circadian networks, linking immune quiescence with redox adaptation.