The Goodwin Model: Simulating the Effect of Cycloheximide and Heat Shock on the Sporulation Rhythm ofNeurospora crassa

The Goodwin model is a negative feedback oscillator which describes rather closely the putative molecular mechanism of the circadian clock ofNeurosporaandDrosophila. An essential feature is that one or two clock proteins are synthesized and degraded in a rhythmic fashion. When protein synthesis inN.crassa(wild-typefrq+and long-period mutantfrq7) was inhibited by continuous incubation with increasing concentrations of cycloheximide (CHX) the period of the circadian sporulation rhythmicity is only slightly increased. The explanation of this effect may be seen in the inhibition of protein synthesis and protein degradation. In the model, increasing inhibition of both processes led to very similar results with respect to period length. That protein degradation is, in fact, inhibited by CHX is shown by determining protein degradation inN.crassaby means of pulse chase experiments. Phase response curves (PRCs) of theN.crassasporulation rhythm toward CHX which were reported in the literature and investigated in this paper revealed significant differences betweenfrq+and the long period mutantsfrq7andcsp-1frq7. These PRCs were also convincingly simulated by the model, if a transient inhibition of protein degradation by CHX is assumed as well as a lower constitutive degradation rate of FRQ-protein in thefrq7/csp-1frq7mutants. The lower sensitivities offrq7andcsp-1frq7towards CHX may thus be explained by a lower degradation rate of clock protein FRQ7. The phase shifting by moderate temperature pulses (from 25 to 30°C) can also be simulated by the Goodwin model and shows large phase advances at about CT 16–20 as observed in experiments. In case of higher temperature pulses (from 35 to 42 or 45°C=heat shock) the phase position and form of the PRC changes as protein synthesis is increasingly inhibited. It is known from earlier experiments that heat shock not only inhibits the synthesis of many proteins but also inhibits protein degradation. Taking this into account, the Goodwin model also simulates the PRCs of high temperature (heat shock) pulses.