Modification of Blue Light Photoresponses by Riboflavin Analogs in Neurospora crassa

Abstract

The effect of riboflavin analogs on blue Ught responses in a riboflavin mutant of Neurospora crassa was studied. The analogs 1-deazariboflavin and roseoflavin, which have red-shifted absorption, acted as photoreceptors for the photosuppression and phase shifting of circadian conidiation by 540 nm Hght, but were ineffective as photoreceptors for the induction of carotenoid synthesis. These results provide additional evidence impUcating a flavin photoreceptor for at least two blue Ught responses of Newrospora.In a number of organisms, blue light responses appear to be mediated by a flavin or flavoprotein photoreceptor (22,23). For the fungus Neurospora crassa, spectrophotometric studies have implicated a flavin-mediated photoreduction of a b-type Cyt in such responses (3,14). Recent studies with mutant strains of Neurospora have also indicated flavin-Cyt b involvement because a Cyt b (2, 4) or flavin (17, 18) deficiency correlates with a decreased sensitivity to light. The specific flavoprotein(s) involved has not yet been identified, but it does not appear to be nitrate reductase (19), which has been suggested as a potential blue light receptor for Neurospora (12).Another approach that has proven useful in photobiology is the replacement of natural chromophores by analogs. Examples include the use of retinal analogs in the study of Halobacterium membranes (25) and mammalian vision (6). Only a few studies, however, have utilized analogs in the study of blue light responses. Page (16) used L-lyxoflavin, a riboflavin analog, in studies which indicated a flavin photoreceptor for trophocyst formation in Pilobolus. Two recent studies have applied this approach using roseoflavin, a riboflavin analog with red-shifted absorption. In Phycomyces, roseoflavin acted as a photoreceptor for phototropism (15), while in Neurospora it was ineffective as a photoreceptor for carotenogenesis (24).The present work examines the effect of riboflavin analogs on three Neurospora blue light responses: the suppression and phase shifting of circadian conidiation, and the induction of carotenoid synthesis. Two analogs, 1-deazariboflavin and roseoflavin, which have red-shifted absorption (26) as compared to riboflavin (535, 505, and 445 nm absorption maxima, respectively) were chosen for study following preliminary experiments with 17 different analogs. Neurospora will not normally respond to wavelengths above 520 nm for the three photoresponses described above (8,9,21). A response, therefore, of an analog-supplemented culture to longer wavelengths of light (e.g. 540 nm) can be unambiguously interpreted to have been potentiated by the analog.'Present address:

Photoreception in Neurospora : a tale of two White Collar proteins

“…First, the action spectra for several activities, including entrainment of circadian clock and photoinduction of carotenoid biosynthesis, are similar to typical flavin absorption spectra, with peak activity at 465 nm and no response above ~520 nm [48,49,51]. In addition, two Neurospora flavin deficiency mutant strains were found to have reduced photosensitivity for clock resetting and induction of carotenogenesis [52,53]. The fact that wc mutants are the only mutants that have been repeatedly isolated in genetic screens that show loss of almost all light responses [1,2,36] raises the possibility that one or both of the WC proteins are photoreceptors responsible for these light responses.…”

Day/night and circadian rhythm control of con gene expression in Neurospora.

“…However, we have not determined whether light repression of con-6 and con-10 transcription is merely delayed in the bd mutant. Paietta and Sargent (34) isolated mutants in the bd strain that were resistant to light repression of circadian conidiation; they called these mutants lis (light insensitive). These mutations have not been examined for light repression of con-6 and con-10 expression.…”

Neurospora Photoreceptors

“…Consistent with this possibility, f lavin biosynthesis mutants rib-1 and rib-2 showed reduced and f lavin-dependent light responses including induction of carotenogenesis and phase shifting and photosuppression of circadian rhythmicity (Paietta and Sargent, 1981). Also, supplementation of the mutants with 1-deazaribof lavin and roseof lavin yielded light responses having appropriately altered action spectra (Paietta and Sargent, 1983a). A number of f lavoproteins have been suggested as possible blue light photoreceptors including nitrate reductase (Klemm and Ninneman, 1978) and cryptochromes.…”