Saadat Mohsenzadeh

Circadian rhythms and protein turnover: The effect of temperature on the period lengths of clock mutants simulated by the Goodwin oscillator

Recent reports that the circadian clock in both Drosophila and Neurospora consists of a negative feedback loop between clock gene (per, frq) activity and amount of clock protein make the Goodwin oscillator a timely model for circadian rhythms. This model is characterized by a negative feedback loop in the clock gene expression with synthesis/degradation reactions associated with each of the intermediates in the loop. The model predicts that predominantly degradation processes of clock mRNA or clock protein control the circadian period and temperature compensation. Assuming a turnover homeostasis of the clock protein, the model explains temperature effects on period length in per and frq mutants. Recent results obtained in Drosophila [1] and Neurospora [2] show that the circadian clock consists basically of a negative feedback loop. This makes the Goodwin model [3, 4] (Fig. 1) and related oscillators [5] useful models for kinetic studies of circadian rhythms. The Goodwin oscillator shows many properties also observed in circadian clocks, such as temperature compensation [6, 7], phase response curves for temperature steps and pulses, and entrainment by tern-

Effects of light on protein secretion in Neurospora crassa.

The relative concentrations of secreted proteins in liquid cultures of Neurospora crassa differ in constant darkness compared to constant light (2500lx). Light reduces the concentrations of some polypeptides markedly and increases the concentrations of protein species of 67, 40, 18 and 13 kDa. The “blind” wc-2 mutant of Neurospora does not show light dependent differences in amounts of secreted proteins. One of the light-sensitive extracellular proteins is shown to be a protease of 17,5 kDa.