The effects of light on a circadian rhythm of conidiation in neurospora

The expression of a circadian rhythm of conidiation by timex, a strain of Neurospora crassa, is inhibited by growth in continuous white light. The action spectrum for this effect has a strong peak (with minor subpeaks) in the blue region of the visible spectrum, and a broad shoulder in the near ultraviolet. This action spectrum suggests that a carotenoid or flavin compound may be the photoreceptor, but does not allow one to determine conclusively whether the receptor is indeed a carotenoid, flavin, or some other unrelated pigment. Two lines of evidence suggest that a carotenoid is not the photoreceptor. First, the in vivo absorption spectrum of timex (representing the sum of the spectra of the individual pigments present, predominantly carotenoids) has peaks at wavelengths 10 to 20 mmu longer than those of the action spectrum peaks. Second, an albino-timex has normal photosensitivity, a situation requiring that the photoreceptor, if carotenoid, be a quantitatively minor constituent of the total carotenoid complement.The magnitude and direction of phase-shift resulting from a standard dose of white light given at different times in the daily cycle of timex varies in the manner reported for other organisms. Additional phase-shift experiments have shown that there are no major transients in the attainment of a new equilibrium after a phase-shifting perturbation, and that 2 light reactions (rapidly and slowly saturating) may be involved in the phase-shift response.