The Goodwin model: Simulating the effect of light pulses on the circadian sporulation rhythm of Neurospora Crassa

The Goodwin oscillator is a minimal model that describes the oscillatory ne gative feedback regulation of a translated protein which inhibits its own t ranscription. Now, over 30 years later this scheme provides a basic descrip tion of the central components in the circadian oscillators of Neurospora, Drosophila, and mammals. We showed previously that Neurospora's resetting b ehavior by pulses of temperature, cycloheximide or heat shock can be simula ted by this model, in which degradation processes play an important role fo r determining the clock's period and its temperature-compensation. Another important environmental factor for the synchronization is light. In this wo rk, we show that on the basis of a light-induced transcription of the frequ ency (frq) gene phase response curves of light pulses as well as the influe nce of the light pulse length on phase shifts can be described by the Goodw in oscillator. A relaxation variant of the model predicts that directly aft er a light pulse inhibition in Jig-transcription occurs, even when the inhi biting factor Z (FRQ) has not reached inhibitory concentrations. This has s o far not been experimentally investigated for frq transcription, but it co mplies with a current model of light-induced transcription of other genes b y a phosphorylated white-collar complex. During long light pulses, the rela xational model predicts that the sporulation rhythm is arrested in a steady state of high frq-mRNA levels. However, experimental results indicate the possibility of oscillations around this steady state and more in favor of t he results by the original Goodwin model. In order to explain the resetting behavior by two light pulses, a biphasic first-order kinetics recovery per iod of the blue light receptor or of the light signal transduction pathway has to be assumed. (C) 2001 Academic Press.