Comprehensive modelling of the Neurospora circadian clock and its temperature compensation.

Circadian clocks provide an internal measure of external time allowing organisms to anticipate and exploit predictable daily changes in the environment. Rhythms driven by circadian clocks have a temperature compensated periodicity of approximately 24 hours that persists in constant conditions and can be reset by environmental time cues. Computational modelling has aided our understanding of the molecular mechanisms of circadian clocks, nevertheless it remains a major challenge to integrate the large number of clock components and their interactions into a single, comprehensive model that is able to account for the full breadth of clock phenotypes. Here we present a comprehensive dynamic model of the Neurospora crassa circadian clock that incorporates its key components and their transcriptional and post-transcriptional regulation. The model accounts for a wide range of clock characteristics including: a periodicity of 21.6 hours, persistent oscillation in constant conditions, arrhythmicity in constant light, resetting by brief light pulses, and entrainment to full photoperiods. Crucial components influencing the period and amplitude of oscillations were identified by control analysis. Furthermore, simulations enabled us to propose a mechanism for temperature compensation, which is achieved by simultaneously increasing the translation of frq RNA and decreasing the nuclear import of FRQ protein.

VIVID interacts with the WHITE COLLAR complex and FREQUENCY-interacting RNA helicase to alter light and clock responses in Neurospora.

The photoreceptor and PAS/LOV protein VIVID (VVD) modulates blue-light signaling and influences light and temperature responses of the circadian clock in Neurospora crassa. One of the main actions of VVD on the circadian clock is to influence circadian clock phase by regulating levels of the transcripts encoded by the central clock gene frequency (frq). How this regulation is achieved is unknown. Here we show that VVD interacts with complexes central for circadian clock and blue-light signaling, namely the WHITE-COLLAR complex (WCC) and FREQUENCY-interacting RNA helicase (FRH), a component that complexes with FRQ to mediate negative feedback control in Neurospora. VVD interacts with FRH in the absence of WCC and FRQ but does not seem to control the exosome-mediated negative feedback loop. Instead, VVD acts to modulate the transcriptional activity of the WCC.

The PAS/LOV protein VIVID controls temperature compensation of circadian clock phase and development in Neurospora crassa.

Circadian clocks are cellular timekeepers that regulate aspects of temporal organization on daily and seasonal time scales. To allow accurate time measurement, the period lengths of clocks are conserved in a range of temperatures--a phenomenon known as temperature compensation. Temperature compensation of circadian clock period aids in maintaining a stable "target time" or phase of clock-controlled events. Here we show that the Neurospora protein VIVID (VVD) buffers the circadian system against temperature fluctuations. In vvd-null mutants, the circadian period of clock-controlled events such as asexual sporulation (conidiation) is temperature compensated, but the phase of this clock time marker is not. Consistent with delayed conidiation at lower temperatures in vvd(KO) strains, the levels of vvd gene products in the wild type increase with decreasing temperatures. Moreover, vvd(C108A) mutants that lack the light function of VVD maintain a dark activity that transiently influences the phase of conidiation, indicating that VVD influences the time of conidiation downstream from the clock. FREQUENCY (FRQ) phosphorylation is altered in a vvd(KO) strain, suggesting a mechanism by which VVD can influence the timing of clock-controlled processes in the dark. Thus, temperature compensation of clock-controlled output is a key factor in maintaining temperature compensation of the entire circadian system.