The cellular reactive oxygen species (ROS) mediate stability and amplification of circadian rhythm

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Circadian clocks are composed of a number of feedback loops. The filamentous fungus Neurospora crassa has a major feedback loop that consists of the FREQUENCY (FRQ)/WHITE COLLAR (WC) -1 and -2 oscillator (FWO) and presumptive FRQ-less oscillators (FLOs) such as WC-dependent FLO (WC-FLO). Intracellular ROS are thought to be involved in the circadian clocks. However, the mechanisms underlying this involvement remain unknown. Here, we show that cellular ROS concentrations and the ROS destroying catalases (CATs) activities oscillate in a circadian manner. The amplitudes of ROS increase with each cycle and remain steady thereafter, while those of cellular CAT activity decrease. The cellular ROS are generated by NADPH oxidase (NOX) and oscillate via FLOs using CATs, but their amplitudes are regulated by FWO. cat-1 expression oscillates in the absence of frq but does not in wc-1 or wc-2. Furthermore, genetic and biochemical evidence reveals that a higher ROS concentration promotes the WCC's transcriptional function and enhances conidiation banding. The feedback loop composed of the WCC and CAT-1 via ROS may be a candidate for the WC-FLO. These findings suggest that cellular ROS connect oscillators such as the FWO and FLOs, allowing them to communicate and stabilize the circadian rhythm.