Circadian rhythms persist without transcription in a eukaryote

Non-transcriptional circadian rhythms Circadian clocks are critical timing regulators of physiology and behaviour that are ubiquitous in eukaryotes. Most mechanistic models of the clock are based on transcription cycles, but evidence for post-translational regulation has recently surfaced in plants and cyanobacteria. Two groups now demonstrate a role for the oxidation of peroxiredoxin proteins in maintaining an entrainable oscillation in human red blood cells and in the unicellular alga Ostreococcus tauri . These data suggest a role for non-transcriptional mechanisms in clock models and open the door to future work exploring the connections between the transcriptional and non-transcriptional circadian machinery. Circadian clocks are critical timing regulators of physiology and behaviour that are ubiquitous in eukaryotes. Most mechanistic models of the this clock are based on transcription cycles, but some evidence for post-translational regulation has recently surfaced in plants and cyanobacteria. This is one of two groups demonstrating a role for the oxidation of peroxiredoxin proteins in maintaining an entrainable oscillation in human red blood cells and a unicellular alga. These data indicate a role for non-transcriptional mechanisms in clock models and open the door to future work exploring the connections between the transcriptional and non transcriptional circadian machinery. Circadian rhythms are ubiquitous in eukaryotes, and coordinate numerous aspects of behaviour, physiology and metabolism, from sleep/wake cycles in mammals to growth and photosynthesis in plants 1 , 2 . This daily timekeeping is thought to be driven by transcriptional–translational feedback loops, whereby rhythmic expression of ‘clock’ gene products regulates the expression of associated genes in approximately 24-hour cycles. The specific transcriptional components differ between phylogenetic kingdoms 3 . The unicellular pico-eukaryotic alga Ostreococcus tauri possesses a naturally minimized clock, which includes many features that are shared with plants, such as a central negative feedback loop that involves the morning-expressed CCA1 and evening-expressed TOC1 genes 4 . Given that recent observations in animals and plants have revealed prominent post-translational contributions to timekeeping 5 , a reappraisal of the transcriptional contribution to oscillator function is overdue. Here we show that non-transcriptional mechanisms are sufficient to sustain circadian timekeeping in