LIGHT‐CONTROLLED ADAPTATION KINETICS IN Phycomyces: EVIDENCE FOR A NOVEL YELLOW‐LIGHT ABSORBING PIGMENT

Abstract— When sporangiophores of the fungus Phycomyces blakesleeanus adapt from high to low fiuence rate, dark adaptation (sensitivity recovery) can be accelerated by dim subliminal light [Galland et al. (1989) Photochem. Photobiol. 49, 485–491]. We measured fiuence rate‐response curves for this acceleration under the following conditions. After sporangiophores were initially adapted symmetrically to a fiuence rate of 1 W m‐² (447 nm), they were exposed to unilateral subliminal light (subthreshold for phototropism) of variable wavelength and fiuence rate, and then to unilateral test light (447 nm) of fiuence rate either 10‐³ or 10‐⁵ W m‐². The duration of the subliminal light was chosen so that phototropism would not occur during this period. Phototropic latencies could be shortened by subliminal light that was less intense than the test light by several orders of magnitude. In experiments with the final unilateral light of fiuence rate 10‐³ W m‐², the 447 nm subliminal light had a threshold (for the acceleration effect) of about 10‐¹¹ W m^‐2. Yellow light of wavelength 575 nm, which itself is extremely ineffective for phototropism was extremely effective in shortening phototropic latencies in response in response to the test light. At 575 nm, the threshold was about 2 × 10‐¹² W m‐². Conversely, near‐UV light of wavelength 347 nm, which is highly effective for phototropism, was relatively ineffective (threshold ‐7 × 10‐⁸ W m‐²) in shortening the phototropic latency. Our results suggest the presence of a novel yellow‐light absorbing pigment in Phycomyces that specifically regulates dark adaptation. The mutant strain C109 (madB101), which has a defective blue‐light photoreceptor system [Galland and Lipson, (1985), Photochem. Photobiol. 41, 331–335] was also defective for the subliminal effect of blue (447 nm) and yellow (575 nm) light. To interpret these and other results, we suggest that the blue‐light photoreceptor system might include a flavoprotein with photochromic properties. The two interconvertible forms of the photochrome could be the oxidized and semiquinone states of the flavin receptor. The semiquinone state accordingly would control adaptation and mediate the novel yellow‐light effect.