Two circadian oscillators in one cell.

A CIRCADIAN clock, which continues to oscillate in constant conditions, is almost ubiquitous in eukaryotes as well as some prokaryotes. This class of biological oscillators drives daily rhythms as diverse as photosynthesis in plants and the sleep-wake cycle in man and enables organisms to anticipate environmental changes or segregate in time-incompatible processes. Circadian oscillators share many properties, suggesting that the clock is a single mechanism, preserved throughout evolution, which is capable of controlling all the different circadian functions. Here we show that two rhythms in a unicellular organism can, under certain experimental conditions, run independently, and thus each rhythm must be controlled by its own distinct oscillator.