Eukaryotic circadian systems: cycles in common.

Common regulatory patterns can now be discerned among eukaryotic circadian systems, extending from fungi through to mammals. Complexes of two distinct PAS domain-containing transcription factors play positive roles in clock-associated feedback loops by turning on classic clock proteins such as FRQ, PER and TIM. These in turn appear to act as negative elements, interfering with their own activation and thus giving rise to an oscillatory negative feedback loop. Post-transcriptional control governs the amount and type of FRQ and makes the clock responsive to temperature.