Circadian Rhythms in Neurospora

Abstract

Circadian rhythms are highly conserved, 24‐h physiological cycles that, through the ideal programming of behaviour, enhance fitness by ensuring many organismal functions are optimally synchronised with the appropriate phase of the day. Circadian rhythms are controlled via an intracellular, highly regulated, transcription‐translation based negative feedback loop, or ‘clock’, and this feedback loop directly or indirectly impacts a large portion of the genome. Over half a century, genetic and molecular analysis of the circadian system of Neurospora crassa has uncovered a number of paradigms now seen to be universal in the operation of circadian rhythms. This article summarises what is currently understood about the mechanisms that underly circadian regulation in Neurospora and highlights many of the contributions Neurospora has made to understanding these same mechanisms in higher eukaryotes.

  • Neurospora crassa is a key model system for eukaryotic clocks.
  • The Neurospora clock comprises a transcription‐translation negative feedback loop.
  • Intrinsic protein disorder is essential for clock function in Neurospora.
  • The Neurospora clock extensively regulates physiological output.
  • Circadian output is regulated at the transcriptional as well as posttranscriptional levels.
  • The Neurospora clock is resistant to environmental effects while remaining sensitive to environmental cues.
  • Light, temperature and metabolic conditions are all environmental cues that can impact the oscillations of the circadian system.

Keywords: biological clock; FREQUENCY; white collar complex; oscillations; clock; circadian output; circadian input

Figure 1. The principle components of the molecular circuit that drive circadian timing in Neurospora crassa. The Neurospora TTFL is initiated in the subjective late night/early morning when the positive‐arm, the WCC, comprising white collar‐1 (WC‐1, blue) and white collar‐2 (WC‐2, yellow), transcriptionally activates the gene frequency (frq). The transcriptional activation of frq leads to the transcription and translation of FRQ protein (purple), and FRQ then dimerises and associates with frequency‐interacting RNA helicase (FRH, orange), forming the FFC. FRQ is then phosphorylated (light blue stars) primarily by casein kinase 1a (CK1a, dark green) as well as other kinases, and enters the nucleus. The FFC then interacts with the WCC, repressing WCC activation, likely via the phosphorylation of the WCC, decreasing transcription at the frq locus. FRQ undergoes further phosphorylation until it can no longer interact with the WCC and it is recognised by FWD‐1 (red), ubiquitinated (yellow lightning) and degraded (purple‐hashed FRQ). Without the interaction of the FFC, the WCC is dephosphorylated by PP2a and other phosphatases (light green) and is able to restart the cycle.
Figure 2. Intrinsic protein disorder in negative‐arm proteins is essential for the proper timing of the circadian clock in Neurospora crassa. When FRQ (purple line), an IDP, is initially translated, it is natively unfolded and therefore available for protein degradation by default by the proteasome. The interaction between FRQ and FRH (orange) leads to the prevention of FRQ degradation. As the day progresses, FRQ is phosphorylated, impacting both the conformation of FRQ and its interacting partners, including the WCC. Further phosphorylation leads to the repression of FFC/WCC interaction and finally to the binding of FWD‐1 (red), triggering the ubiquitination of FRQ, leading to FRQ degradation.
Figure 3. Circadian regulation over physiology (the clocks ‘output’) is driven by both transcriptional and posttranscriptional mechanisms in Neurospora crassa. The regulation of circadian output begins in the nucleus when the WCC transcriptionally activates numerous gene promoters in addition to the activation of the frq promoter (Figure ). The genes that are transcriptionally activated by the WCC that do not play a role in the core circadian clock are called clock‐controlled genes (ccgs) and are the drivers of circadian behaviour. As much as 40% of the Neurospora transcriptome is under circadian control. Some of these ccgs are also transcription factors (circadian transcription factors, cTFs, green) that can act on further gene promoters, creating a circadian transcriptional cascade. Circadian transcriptional activation can only explain half of oscillating proteins. Unknown sources of circadian posttranscriptional regulation (black question mark) stimulate the oscillation of the remaining circadian proteins, and the clock in Neurospora controls as much as 30% of the proteome.
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Hurley, Jennifer M(Aug 2020) Circadian Rhythms in Neurospora. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029143]