Zeitlupe Senses Blue-Light Fluence To Mediate Circadian
Timing in <i>Arabidopsis
thaliana</i>
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Abstract
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