Photoreceptor action

The repair of dimer lesions, induced with the aid of light of relatively long wavelength that is not absorbed by the dimer sites (2 300-400 nm), is based on photoreceptor action, as dealt with in Section 8.2.2.3 above. It occurs if DNA photolyases, i.e. structure-specific (not sequence-specific) enzymes are present in the system during the irradiation [6]. Photolyases are proteins of 450-550 amino acids containing two non-covalently bound chromophore cofactors (see Chart 8.6). 

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. 

Phycocymes shows a positive UV-induced light-growth-response (Fig. 3 3) as well as a negative phototropic curvature, as depicted in Fig. 5 1. The exceptionally good fit of this action spectrum and auxin absorption (Fig. 5 2) might indicate that auxin is the UV-photoreceptor. 

It has been suggested that the photoreceptor might be the 15—15-cis-isomer of j3-carotenoid, which — in contrast to the trans-isomer — shows a UV-peak, as demonstrated in Fig.6 2 and 3,183,194). However, its position exhibits ahypsochromic shift of 30 to 40 nm compared to known action spectra. Moreover, in the only case investigated, that of Phycomyces, no cis-0-carotenoid has been found138). 

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). 

Summarizing, even a close match of any action and absorption spectra does not allow a definite identification of the photoreceptor. Additional information is required. 

Therefore, the action spectrum for phototropism does not simply reflect the absorption spectrum of the active photoreceptor pigment itself, but instead, its absorption spectrum somehow modified by shading pigments. However, on this basis Thimann and Curry failed to calculate a curve fitting the experimental action spec-... 

And only for that (extrapolated) case, the near UV-peak of the action spectrum vanishes, from which the authors conclude that the active photoreceptor is probably carotenoid in nature (cf., Fig. 6 3). 

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. 

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 ... 

Only for the (extrapolated) limit of zero bending the action spectrum reflects the absorption spectrum of the photoreceptor for phototropism (avena coleoptile). Just for this limit the UV-peak of the action spectrum disappears 162). 

Although action spectra for these photoresponses have not been established the reactions were very helpful in establishing a formal ordering of the phototropic defective mutants (see Sect. D) and in demonstrating that the same photoreceptor of phototropism is also very likely active in the mycelium. 

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. 

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