Circadian Rhythms in Neurospora crassa and Other Filamentous Fungi

Abstract

Circadian clocks are endogenous cellular timekeepers that control a wide variety of daily physiological and molecular rhythms in most eukaryotic and some prokaryotic organisms (31, 116). These rhythmic events allow organisms to best adapt to the natural environment on earth (88). All circadian rhythms share three basic properties. First, circadian rhythms persist under constant conditions with a period length of about 24 h. Second, they can be entrained (reset) by environmental cues, with light and temperature being the most important signals. Third, the rhythms are temperature compensated, meaning that the period length of the rhythm is stable over a wide range of physiological temperatures. The field of chronobiology has recently experienced unpre- cedented progress in identifying core molecular components involved in the generation of circadian rhythms and is begin- ning to identify components of the output pathways from these core oscillators in several model organisms, including the fila- mentous fungus Neurospora crassa (31, 32, 116). Investigations of the Neurospora circadian clock system have elucidated many of the basic mechanisms that underlie circadian rhythms, in- cluding negative feedback, and light and temperature entrain- ment common to all eukaryotic clocks (32, 49, 72). Neurospora continues to be a premier experimental organism for studying circadian rhythms because of its relative simplicity and because it displays an easily assayed circadian rhythm in asexual spore development (conidiation) that has proven extremely useful for measuring the effects of mutations on clock function. The well-described Neurospora frq/wc-based circadian oscillator (FWO) exhibits remarkable conservation with those of higher eukaryotic organisms, reinforcing its use as a model organism to investigate the workings of the clock. NEUROSPORA frq/wc-BASED CIRCADIAN FEEDBACK LOOPS Similar to the circadian oscillators in Drosophila melano- gaster and mammals, the core circadian oscillator of Neuros- pora consists of an autoregulatory negative feedback loop in which FRQ, FRH (an FRQ-interacting RNA helicase),