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

Abstract

Circadian systems include slave oscillators and central pacemakers, and the cores of eukaryotic circadian clocks described to date are composed of transcription and translation feedback loops (TTFLs). In the model system Neurospora, normal circadian rhythmicity requires a TTFL in which a White Collar complex (WCC) activates expression of the frequency (frq) gene, and the FRQ protein feeds back to attenuate that activation. To further test the centrality of this TTFL to the circadian mechanism in Neurospora, we used low-amplitude temperature cycles to compare WT and frq-null strains under conditions in which a banding rhythm was elicited. WT cultures were entrained to these temperature cycles. Unlike those normal strains, however, frq-null mutants did not truly entrain to the same cycles. Their peaks and troughs always occurred in the cold and warm periods, respectively, strongly suggesting that the rhythm in Neurospora lacking frq function simply is driven by the temperature cycles. Previous reports suggested that a FRQ-less oscillator (FLO) could be entrained to temperature cycles, rather than being driven, and speculated that the FLO was the underlying circadian-rhythm generator. These inferences appear to derive from the use of a phase reference point affected by both the changing waveform and the phase of the oscillation. Examination of several other phase markers as well as results of additional experimental tests indicate that the FLO is, at best, a slave oscillator to the TTFL, which underlies circadian rhythm generation in Neurospora.FRQ-less oscillator ͉ frq ͉ FRQ C ircadian programs in eukaryotes are widely perceived to be the output of multiple oscillatory systems based on cell intrinsic transcription and translation feedback loops (TTFLs) (1-3). In many animals and fungi, heterodimeric PAS domaincontaining transcription factors drive expression of genes encoding proteins that block the activity of their heterodimeric activators; such negative feedback loops generally are believed to make up the cores of these circadian clocks. In addition to these autonomous biological clocks, slave oscillators also exist within the panoply of circadian systems. Early studies on entrainment in Drosophila gave rise to models in which a pacemaker drove a slave oscillator that directly regulated an overt rhythmic event (4), and noncircadian slaves have since been experimentally described (e.g., ref. 5). However, because there are few molecular descriptions of slave oscillators, their existence and properties have so far chiefly been inferred from the behavior of the circadian system when exposed to Zeitgeber period lengths outside its innate frequency.At the core of the TTFL in the circadian model system Neurospora crassa are the products of the frequency ( frq), white collar-1 (wc-1), and wc-2 genes. Similar to animal systems, Neurospora possesses a feedback loop in which a heterodimeric activator, the White Collar complex (WCC) of the PAS proteins WC-1 and WC-2, activates expression of frq and thus FRQ, which in turn ...

Disruption of Cryptochrome partially restores circadian rhythmicity to the arrhythmic period mutant of Drosophila

“…This variation allows us to distinguish whether the locomotor anticipation of lights-off is (i) caused by circadian entrainment via an oscillator, (ii) generated by an ''hourglass'' mechanism where peak activity always occurs after a set number of hours, or (iii) whether locomotor behavior simply responds to light (21,22). In wildtype flies, as T increases from LD 6:6 to LD 12:12 the locomotor activity peak moves progressively later with small interfly variability, particularly at LD 12:12.…”

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Transcriptional and post-transcriptional regulation of the circadian clock of cyanobacteria and Neurospora

“…Yet, partial or residual functions of the frq-based oscillator, in particular temperature-entrainable rhythmicity of conidiation, can also be observed in frq-deficient mutants, suggesting that additional oscillatory systems substitute for frq under specific conditions (Merrow et al 1999;Correa et al 2003;Pregueiro et al 2005;Roenneberg et al 2005). Components of such frq-less oscillators (FLOs) are not yet identified.…”

ELF4Is Required for Oscillatory Properties of the Circadian Clock

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