Flavins photoreceptors

Another role of bluelight is exhibited by the moth Pectinophora gossypiella. A circadian rhythm of egg hatching can be initiated with a brief light pulse. The action spectrum (similar to Fig. 16 2,27)) again suggests a flavin photoreceptor. 

Fig. 18. Hypothetical orientation of the flavin photoreceptor within the plasma-lemma in cells of Funaria leaves202, based on known angles of flavin transition vectors in the blue105)...

Fig. 22k. The reasonably good fit between the measured points and the triplet-triplet (T i -> Tn) absorption spectrum of flavin (dotted lines, 174>117)) again suggests a flavin photoreceptor. These authorsIS4) assume an effective decrease of the lifetime of the lowest triplet state by quick triplet-triplet (Tj Tn) turnovers, thereby inhibiting photochemistry (i.e. initiation of phototropism).

Among the many sensory reactions Phycomyces displays, the study of the photoreceptor and adaptation deserves maximal attention, since Phycomyces shares these two attributes with a variety of other blue light sensitive organisms. Action-spectroscopy indicates a flavin as the photoreceptor of Phycomyces. /3-carotene was positively ruled out as a possible receptor, since mutants with no trace amounts of )3-carotene are phototropical normal. The photoreceptor has not yet been isolated. As in other systems the difficulty consists in distinguishing the flavin photoreceptor from the bulk flavoproteins in the cell. One therefore needs unambiguous criteria for the identification of the photoreceptor. The most promising approach for an isolation would be a photoreceptor mutant and we described the properties those mutants should have. Until now there is no firm evidence that the photomutants, madA or madB are defective in the photoreceptor. 

In Euglena both step-down and step-up photophobic reactions occur at different irradiances, separated by an indifferent irradiance range with no evident reactions (12). The action spectrum for the step-down photophobic responses is characteristic of a flavin photoreceptor (17) which has recently been... 

Doughty MJ, Diehn B (1980) Flavins as Photoreceptor Pigments for Behavioral Responses. 41 45-70... 

Crosson, S. and K. Moffat (2001). Structure of a flavin-binding plant photoreceptor domain Insights into light-mediated signal transduction. Proc Natl Acad Sci 98 2995-3000. 

The defenders of the carotenoid-photoreceptor-hypothesis have always understood the shape of these action spectra in the blue to mean that the bluelight receptor is a carotenoid. Indeed, in Fig. 6 3 it can be observed, that the three-peak absorption spectrum of trans-0-carotenoid (in hexene) agrees well with the observed action spectrum of the avena coleoptile (Fig. 3 5). However, there remains one loose end which has been the crucial point of controversy in this field, ever since Galston and Baker66 suggested in 1949 that the photoreceptor for phototropism might be a flavin Flavin absorbs in the near UV, /3-carotenoid does not. 

Nevertheless, the avena coleoptile exhibits a curvature to unilateral UV-illumina-tion with a satisfactory log-linear response/time relationship38) (the bending mode is similar to that observed for the second positive curvature which develops from the coleoptile base cf. 2.2). Fig. 5 338) shows that the double-peaked action spectrum does not match neither flavin (Fig. 5 5,16S)) nor carotenoid absorption (Fig. 5 4,183)), most likely excluding both as photoreceptors. The growth hormone auxin (cf. 2.4 and Scheme 1) has been discussed to be a possible photoreceptor. However, in this case, this is not supported by the action spectrum either. 

Composite action spectra characteristics of carotenoid (Fig. 8 2,169)) and flavin (Fig. 8 1,49)), imitated by the low temperature absorption spectra, are compared with the avena action spectrum (Fig. 8 3). Song and Moore pointed out on this basis, that the carotenoid is a rather unlikely photoreceptor, whereas the flavin is169). 

In order to be photochemically active, light must be absorbed by a specific pigment molecule. In the case for physiological bluelight-action, the most favorable photoreceptor candidate is a flavin, as discussed so far. Sun et al.172) assign a it -> rr character to all major flavin transitions (S0 St 450nm Sj—-> S0 530nm ... 

As discussed throughout this article, there is strong evidence that the bluelight photoreceptor is a flavin (flavoprotein) anistropically bound to a membrane moiety. All potential sensory transduction mechanisms require such a secondary component. 

Nevertheless, there is good evidence that in all purely bluelight sensitive organisms, the photoreceptor is a flavin (flavoprotein) (Table 2), which appears to be bound to membranes (plasmalemma) in a highly dichroic manner. The mechanism of sensory transduction is probably correlated with light-induced redox reactions mediated by a flavin. This observation is consistent with the fact that nearly all physiolog-... 

Several papers in the last two years attempted to separate the photoreceptor-flavin (and/or the associated b-type cytochrome) from the overwhelming amount of bulk-flavin which is involved in metabolism rather than photoreception. However, a clear-cut assay for the bluelight receptor has not yet been found. Specific photoaffinity labeling and the considerable selectivity in the photoreduction of a b-type cytochrome, as mediated by suitable dyes, are the most promising attempts. 

Of the protozoa in which flavins have been proposed to serve as photoreceptor pigments for behavioral responses, Euglena has been the most intensely studied. In this contribution a survey is given of the historical studies of photomovement in this organism, followed by a critical discussion of the experimental conditions under which such studies might be carried out. 

Different photoreceptor pigments may reasonably be expected to undergo different primary photoprocesses upon light absorption, aside from possessing different spectral characteristics. Specifically, light absorption in flavins fairly easily leads to electronic excitation of the molecule to the triplet state, while this process does not readily occur in carotenoids87). 

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