Dissection of a circadian oscillation into discrete domains.

The circadian oscillator in Neurospora is a negative feedback loop involving as principal players the products of the frequency (frq) locus. frq encodes multiple forms of its protein product FRQ, which act to depress the amounts of frq transcript. In this scheme there are two discrete and separable steps to the circadian cycle, negative feedback itself (repression) in which FRQ acts to decrease the levels of its own transcript, and recovery from repression (derepression) in which frq transcript levels return to peak amounts. By introducing an exogenously regulatable frq transgene into a frq loss-of-function strain (frq9), we created an artificial system in which the two separate steps in the circadian cycle can be initiated and followed separately for purposes of observing their kinetics. Under these conditions the frq-FRQ cycle occupies the time scale of a full circadian cycle. During this time, the process of negative feedback of FRQ on frq transcript levels is rapid and efficient; it requires only 3 to 6 h and can be mediated by on the order of 10 molecules of FRQ per nucleus, a level even less than that seen in the normal oscillation. In contrast, recovery from negative feedback requires 14 to 18 h, most of the circadian cycle, during which time de novo FRQ synthesis has stopped, and existing FRQ is progressively posttranslationally modified. Altogether the time required to complete both of these steps is in good agreement with the 22-h observed period length of the normal circadian cycle.