A Nitrate-Induced frq-Less Oscillator in Neurospora crassa

Abstract

When nitrate is the only nitrogen source, Neurospora crassa's nitrate reductase (NR) shows endogenous oscillations in its nitrate reductase activity (NRA) on a circadian time scale. These NRA oscillations can be observed in darkness or continuous light conditions and also in a frq 9 mutant in which no functional FRQ protein is formed. Even in a white-collar-1 knockout mutant, NRA oscillations have been observed, although with a highly reduced amplitude. This indicates that the NRA oscillations are not a simple output rhythm of the whitecollar-driven frq oscillator but may be generated by another oscillator that contains the nit-3 autoregulatory negative feedback loop as a part. In this negative feedback loop, a product in the reaction chain catalyzed by nitrate reductase, probably glutamine, induces repression of the nitrate reductase gene and thus downregulates its own production. This is the first example of an endogenous, nutritionally induced daily rhythm with known molecular components that is observed in the absence of an intact FRQ protein.

Demasking biological oscillators: Properties and principles of entrainment exemplified by the Neurospora circadian clock

“…Conditional arrhythmicity has also been shown for clock mutants in other organisms; e.g., Per2 mutant mice are arrhythmic in constant darkness but not in constant light (22). In addition, circadian oscillations have recently been shown in frq NULL Neurospora strains by following the activity of nitrate reductase (23) and RNA profiles (24) as read-outs. Thus, frq NULL strains possess a residual circadian system that can be entrained given the right zeitgeber stimulus.…”

Rhythmic Conidiation in Constant Light inVividMutants ofNeurospora crassa

“…We have also shown that conidiation rhythms continue in LL in a frq 10 null mutant (Figures 1B and 2B) and that rhythms continue for several days after frq 10 cultures are returned to DD ( Figures 1E and 2E). FRQ-less rhythmicity has been reported many times: rhythms in conidiation (Loros and Feldman 1986;Merrow et al 1999;LakinThomas and Brody 2000;Granshaw et al 2003;LakinThomas 2006a;Lombardi et al 2007) and rhythms at the molecular level (Ramsdale and Lakin-Thomas 2000;Christensen et al 2004;de Paula et al 2006) are found in frq null mutants. The combination of the vvd mutation and constant light is therefore another of many conditions that allow rhythmicity to be assayed in the absence of FRQ function.…”

“…Strains carrying null mutations of frq or wc-1 show oscillator-like behavior when entrained to cycles of temperature pulses (Merrow et al 1999;Lakin-Thomas 2006a). Additionally, rhythms have been found in the frq 10 strain in the levels of nitrate reductase (Christensen et al 2004), the levels of the neutral lipid diacylglycerol (Ramsdale and Lakin-Thomas 2000) and in the mRNA levels of some clock-controlled genes (ccgs) (Correa et al 2003). The existence of a FLO or FLOs is further suggested by the finding that ''clock null'' mutants, such as deletions of frq, wc-1, or wc-2, that may lack conidiation rhythmicity under standard growth conditions will exhibit rhythmicity when grown on media enriched with farnesol or geraniol (Granshaw et al 2003;Lombardi et al 2007) or when combined with certain mutations affecting lipid biosynthesis, such as cel or chol-1 (Lakin-Thomas and Brody 2000).…”

Assignment of an essential role for the Neurospora frequency gene in circadian entrainment to temperature cycles

“…An analogous phenomenon is known in the field of chemical oscillators: When conditions are changed in a mixture capable of robust oscillation, a delayed and sporadic appearance of oscillations often signals the organization of an alternative oscillatory state, one not formed when the regular components are present, which is nucleated after the system has passed an ''induction period'' (26). From this analogy, Christensen et al (27) have suggested that a FLO may not normally exist and, instead, represents an unnatural oscillatory state formed when normal connections cannot be made. Alternatively, perhaps the FLO is always present but not always overtly manifest.…”