Photoreception in Neurospora crassa: correlation of reduced light sensitivity with flavin deficiency.

Abstract

The effect of flavin deficiency on blue light responses in Neurospora crassa was studied through the use of two riboflavin mutants (rib-i and rib-2). The photoresponses assayed were the, suppression of circadian conidiation, the phase shifting of the circadian conidiation rhythm, and the induction of carotenoid synthesis. Physiological responses to blue light are known in a large number oforganisms. Examples include phototropism in coleoptiles of monocots (1) and sporangiophores of fungi (2), induction of carotenoid synthesis in fungi (3), and entrainment of circadian pupal emergence in fruit flies (4). The photoreceptor pigments involved remain unidentified but most ofthe available evidence supports the hypothesis that a flavin is acting as the photoreceptor for many of these blue light responses. This evidence (reviewed in refs. 5 and 6) includes (i) the similarity between the absorption spectrum of riboflavin and the action spectra of both the blue light responses and light-induced absorbance changes involving a b-type cytochrome and (ii) the effects on blue light responses of inhibitors such as phenylacetic acid and potassium iodide which interact with flavins.Spectrophotometric studies with whole cells and cell fractions suggest that a primary photoreception event for the fungus Neurospora crassa is a flavin-mediated reduction of a b-type cytochrome (7,8). Experiments supporting cytochrome b involvement have been done with a cytoplasmic mutant, poky, which exhibits a low.cytochrome b content, a decrease in blue light-induced absorbance changes, and a decreased sensitivity to light for the suppression of circadian conidiation (9, 10).One approach to elucidation ofthe nature ofa photoreceptor is to use a system in which the photoreceptor levels can be perturbed. Ifthe photoreceptor is indeed a flavin, then a decrease in photoreceptor levels should occur in the case ofa generalized flavin deficiency and might result in a decrease of light sensitivity. To detect this effect, one needs a system having a blue light response that can be monitored quantitatively and in which a flavin deficiency can be specifically induced. Riboflavin-requiring mutants have been isolated in Neurospora (11,12)

Cryptochrome: The Second Photoactive Pigment in the Eye and Its Role in Circadian Photoreception

“…A Neurospora strain defective in flavin biosynthesis has a severely reduced sensitivity to photoentrainment (89), suggesting that the photoreceptor in this organism may be a flavoprotein, although it is not known whether this organism possesses a cryptochrome. In an interesting experiment, Drosophila raised on an aseptic diet without β-carotene (the precursor of retinal) had essentially no visual responsiveness yet maintained normal circadian photosensitivity (90), leading to the conclusion that the circadian photoreceptor in Drosophila is not opsin-based.…”

Genetic Analysis of Phototropism of Neurospora crassa Perithecial Beaks Using White Collar and Albino Mutants

“…Genetic studies have been carried out with N. crassa (9,17,19,20), Phycomyces blakesleeanus (2, 3, 18, 25), and Trichoderma (11) which are aimed at eventually identifying the blue light photoreceptor(s) (designated 'cryptochrome' [6]). In N. crassa, photoinduced carotenoid biosynthesis was investigated using wc and al mutants (9).…”

“…In the fungus Neurospora crassa, blue light has been shown to regulate a number of important processes including carotenoid biosynthesis (8, 24), photosuppression and phase shifting of the circadian rhythm of conidiation (5), photoinduction of protoperithecia formation (15), and under certain conditions promotion of conidiation (16). In Neurospora sitophila, positive phototropism of perithecial beaks (also referred to as perithecial necks) has also been demonstrated (1), but it was not determined whether this phototropic effect is a blue light response.Genetic studies have been carried out with N. crassa (9,17,19,20), Phycomyces blakesleeanus (2, 3, 18, 25), and Trichoderma (11) which are aimed at eventually identifying the blue light photoreceptor(s) (designated 'cryptochrome' [6]). In N. crassa, photoinduced carotenoid biosynthesis was investigated using wc and al mutants (9).…”

Keeping pace with Neurospora circadian rhythms

“…In Neurospora, a flavin-mediated response involving an as yet unidentified blue-light photoreceptor is observed in all known light-regulated events (Klemm & Ninnemann, 1979 ;Paietta & Sargent, 1981), including light resetting of the circadian clock (Dharmananda, 1980 ;Nakashima, 1982;Fritz et al, 1989Fritz et al, ,1990). While various light-input pathways exist in different organisms, all models for rhythmic entrainment propose that light acts to rapidly alter the activity of a central clock component (Pittendrigh, 1960).…”