Figure 4.
Continual expression of even very low levels of FRQ results in severe disruption of the overt rhythm. frq+ Neurospora cultures bearing an integrated copy of qa-2pfrq were grown on race tubes with different concentrations of QA under conditions for assaying rhythmicity. Race tube cultures were germinated in LL then transferred to DD; thereafter, the mycelial growth front was marked every 24 h (7). Discernible conidial bands are marked with single dots, and the effects of induction of FRQ from qa-2pfrq at inducer concentrations equal to or below those used in Figs. 2 and 3 were noted. (A) Effects of QA addition in the frq-inducible qa-2pfrq strain. (B) Effects of QA addition in the control noninducible strain. No rhythm is apparent at 10−6M QA, which induces FRQ to barely physiological levels (Fig. 3), and induction of FRQ to even lower levels disrupts expression of the rhythm and lengthens the period length (see text).