Reactive oxygen species can modulate circadian phase and period in Neurospora crassa.

Reactive oxygen species (ROS) may serve as signals coupling metabolism to other cell functions. In addition to being by-products of normal metabolism, they are generated at elevated levels under environmental stress situations. We analyzed how reactive oxygen species affect the circadian clock in the model organism Neurospora crassa. In light/dark cycles, an increase in the levels of reactive oxygen species advanced the phase of both the conidiation rhythm and the expression of the clock gene frequency. Our results indicate a dominant role of the superoxide anion in the control of the phase. Elevation of superoxide production resulted in the activation of protein phosphatase 2A, a regulator of the positive element of the circadian clock. Our data indicate that even under nonstress conditions, reactive oxygen species affect circadian timekeeping. Reduction of their basal levels results in a delay of the phase in light/dark cycles and a longer period under constant conditions. We show that under entrained conditions the phase depends on the temperature and reactive oxygen species contribute to this effect. Our results suggest that the superoxide anion is an important factor controlling the circadian oscillator and is able to reset the clock most probably by activating protein phosphatase 2A, thereby modulating the activity of the White Collar complex.