vvd, a gene regulating light responses in Neurospora, encodes a novel member of the PAS/LOV protein superfamily. VVD defines a circadian clock-associated autoregulatory feedback loop that influences light resetting, modulates circadian gating of input by connecting output and input, and regulates light adaptation. Rapidly light induced, vvd is an early repressor of light-regulated processes. Further, vvd is clock controlled; the clock gates light induction of vvd and the clock gene frq so identical signals yield greater induction in the morning. Mutation of vvd severely dampens gating, especially of frq, consistent with VVD modulating gating and phasing light-resetting responses. vvd null strains display distinct alterations in the phase-response curve to light. Thus VVD, although not part of the clock, contributes significantly to regulation within the Neurospora circadian system.

The endogenous clock that drives circadian rhythms is thought to communicate temporal information within the cell via cycling downstream transcripts. A transcript encoding a glycine-rich RNA-binding protein, Atgrp7, in Arabidopsis thaliana undergoes circadian oscillations with peak levels in the evening. The AtGRP7 protein also cycles with a time delay so that Atgrp7 transcript levels decline when the AtGRP7 protein accumulates to high levels. After AtGRP7 protein concentration has fallen to trough levels, Atgrp7 transcript starts to reaccumulate. Overexpression of AtGRP7 in transgenic Arabidopsis plants severely depresses cycling of the endogenous Atgrp7 transcript. These data establish both transcript and protein as components of a negative feedback circuit capable of generating a stable oscillation. AtGRP7 overexpression also depresses the oscillation of the circadianregulated transcript encoding the related RNA-binding protein AtGRP8 but does not affect the oscillation of transcripts such as cab or catalase mRNAs. We propose that the AtGRP7 autoregulatory loop represents a ''slave'' oscillator in Arabidopsis that receives temporal information from a central ''master'' oscillator, conserves the rhythmicity by negative feedback, and transduces it to the output pathway by regulating a subset of clock-controlled transcripts.

The frequency (frq) gene, the central component of the frq-based circadian negative feedback loop, regulates various aspects of the circadian clock in Neurospora. However, the biochemical function of its protein products, FRQ, is poorly understood. In this study, we demonstrated that the most conserved region of FRQ forms a coiled-coil domain. FRQ interacts with itself in vivo, and the deletion of the coiled-coil region results in loss of the interaction. Point mutations, which are designed to disrupt the coiled-coil structure, weaken or completely abolish the FRQ self-association and lead to the arrhythmicity of the overt rhythm. Mutations of the FRQ coiled-coil that inhibit self-association also prevent its interaction with two other key components of the Neurospora circadian clock, namely WC-1 and WC-2, the two PAS domain-containing transcription factors. Taken together, these data strongly suggest that the formation of the FRQ±FRQ and FRQ±WC complexes is essential for the function of the Neurospora clock.

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A light-entrainable circadian clock controls development and physiology in Neurospora crassa. Existing simple models for resetting based on light pulses (so-called nonparametric entrainment) predict that constant light should quickly send the clock to an arrhythmic state; however, such a clock would be of little use to an organism in changing photoperiods in the wild, and we confirm that true, albeit dampened, rhythmicity can be observed in extended light. This rhythmicity requires the PAS/LOV protein VIVID (VVD) that acts, in the light, to facilitate expression of an oscillator that is related to, but distinguishable from, the classic FREQUENCY/WHITE-COLLAR complex (FRQ/WCC)-based oscillator that runs in darkness. VVD prevents light resetting of the clock at dawn but, by influencing frq RNA turnover, promotes resetting at dusk, thereby allowing the clock to run through the dawn transition and take its phase cues from dusk. Consistent with this, loss of VVD yields a clock whose performance follows the simple predictions of earlier models, and overexpression of VVD restores rhythmicity in the light and sensitivity of phase to the duration of the photoperiod.[Keywords: Circadian clock; vivid; white-collar; frequency; limit cycle; Neurospora] Supplemental material is available at http://www.genesedev.org.