Abstract
We are using the fungus Neurospora crassa as a model organism to study the circadian system of eukaryotes. Although the FRQ/WCC feedback loop is said to be central to the circadian system in Neurospora, rhythms can still be seen under many conditions in FRQ-less (frq knockout) strains. To try to identify components of the FRQ-less oscillator (FLO), we carried out a mutagenesis screen in a FRQ-less strain and selected colonies with altered conidiation (spore-formation) rhythms. A mutation we named UV90 affects rhythmicity in both FRQ-less and FRQ-sufficient strains. The UV90 mutation affects FRQ-less rhythms in two conditions: the free-running long-period rhythm in choline-depleted chol-1 strains becomes arrhythmic, and the heat-entrained rhythm in the frq10 knockout is severely altered. In a FRQ-sufficient background, the UV90 mutation causes damping of the free-running conidiation rhythm, reduction of the amplitude of the FRQ protein rhythm, and increased phase-resetting responses to both light and heat pulses, consistent with a decreased amplitude of the circadian oscillator. The UV90 mutation also has small but significant effects on the period of the conidiation rhythm and on growth rate. The wild-type UV90 gene product appears to be required for a functional FLO and for sustained, high-amplitude rhythms in FRQ-sufficient conditions. The UV90 gene product may therefore be a good candidate for a component of the FRQ-less oscillator. These results support a model of the Neurospora circadian system in which the FRQ/WCC feedback loop mutually interacts with a single FLO in an integrated circadian system.
Highlights
Circadian rhythms are approximately 24 h cycles of behavior, physiology, etc. that are driven by an endogenous biological clock
The molecular oscillators that drive these circadian rhythms are said to depend on rhythmic expression and feedback regulation of a small set of ‘‘clock genes.’’ there is increasing evidence that there is more to the story than these wellstudied feedback loops
In the fungus Neurospora crassa, rhythms can still be seen in mutants that are missing one of the clock genes, frq
Summary
Circadian rhythms are approximately 24 h cycles of behavior, physiology, etc. that are driven by an endogenous biological clock. It was found that cycling of mammalian clock proteins CLOCK and CRY is not required for rhythmicity in fibroblasts [2]; large reductions in overall transcription rate and levels of clock proteins do not eliminate circadian oscillations in mouse fibroblasts [3]; and rhythmic cAMP signaling is required to sustain rhythmic transcription in mammalian cells [4]. These and other findings are not compatible with the canonical TTFL models
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