Zeitlupe senses blue-light fluence to mediate circadian timing in Arabidopsis thaliana.

Plants employ a variety of light, oxygen, voltage (LOV) domain photoreceptors to regulate diverse aspects of growth and development. The Zeitlupe (ZTL), Flavin-Kelch-Fbox-1 (FKF1), and LOV-Kelch-Protein-2 (LKP2) proteins dictate measurement of the day length, flowering time, and regulation of the circadian clock by blue-light regulation of protein complex formation. Previous reports indicated that ZTL photochemistry was irreversible, which is inconsistent with its role in marking the day-night transition. A kinetic model of LOV domain function predicts that ZTL has evolved unique photochemical parameters to allow it to function as a sensor of environmental light intensity. Moreover, our model indicates that a photocatalyzed reverse reaction is required for the sensitivity of LOV domains to light fluence. Inclusion of a photocatalyzed rate constant allows the establishment of a photostationary steady state of light-activated proteins, whose relative population is sensitive to daily (circadian) or positional (phototropism) oscillations in light intensity. Photochemical characterization confirms that ZTL undergoes adduct decay on a time scale of hours in contrast to previous reports. The fast photocycle allows detection of the day-night transition facilitating circadian timing. ZTL kinetics reflect an evolutionary adaptation of the ZTL/FKF1/LKP2 family to function in distinct aspects of blue-light signaling.