**8. Cellular responses to photooxidative stress are the candidate evolutionary origin of circadian clocks**

Cellular reactive oxygen species (ROS) were originally thought to solely act as toxic metabolites because they react with components of DNA, proteins, and lipids and exert oxidative stress. However, ROS are also ideally suited as signaling molecules because they are small and can easily diffuse to short distances within a cell [60]. In addition, mechanisms for ROS production and the rapid removal (such as via catalase) are present in almost all cell types [61]. Much evidence has accumulated indicating significant roles of ROS in circadian clock controls that have resulted in the functional coupling of the circadian clock and DDR. For example, in *Drosophila*, a genome-wide screen identified several redox molecules as essential for the light entrainment of the circadian clock [62]. Similarly, a study in mammals showed that changes in reduced NADPH and NADH levels altered the affinity of the NPAS2:BMAL1 complex for its target DNA *in vitro* [63]. Thus, redox state may be an important determinant of circadian oscillations in mammalian cells. Nuclear factor erythroid-derived 2-like 2 (NRF2) is one of the components involved in the major cellular antioxidant defense pathways [64]. It induces a transcriptional program that maintains cellular redox balance and protects cells from oxidative insults. Importantly, it has been reported in mouse that cellular clock generates circadian rhythm in NRF2 level, which is essential in regulating the rhythmic expression of antioxidant genes involved in glutathione redox homeostasis in the lung [65].

In zebrafish, the transcriptional induction of *zCry1a* and *zPer2* genes has been proposed to be required for the light entrainment of cellular clocks [45, 66, 67]. The lightdependent transcription of *zCry1a* and *zPer2* is controlled through the production and removal of cellular ROS [66, 68]. The light-induced ROS stimulate the intracellular ERK signaling pathway and transduce photic signals to the transactivation of *zCry1a* and *zPer2* (**Figure 2**). Importantly, light increases the intracellular catalase activity by increasing the expression *of catalase*, an event that occurs after the maximum expression of the *zCry1a* and *zPer2* genes has been reached. This increased catalase activity diminishes the light-induced cellular ROS levels, resulting in decreased expression levels of *zCry1a* and *zPer2* genes. These findings provide evidence that ROS induced by light are the second messenger coupling photoreception to the entrainment of the circadian clock in zebrafish and further indicate that cellular responses to photooxidative stress would be the evolutionary origin of circadian clocks.
