Blue photoreceptors

The amino acid sequences of the cone photoreceptors have been compared with each other and with rhodopsin. The result is striking. Each of the cone photoreceptors is approximately 40% identical in sequence with rhodopsin. Similarly, the blue photoreceptor is 40% identical with each of the green and red photoreceptors. The green and red photoreceptors, however, are > 95% identical with each other, differing in only 15 of 364 positions (Figure 32.27). 

Yuan, H., and Bauer, C. E. (2008) PixE promotes dark oligomerization of the BLUE photoreceptor PixD. Proc. Natl. Acad. Sci. USA 105, 11715-11719. 

Feiler R., Bjornson R., Kirschfeld K., Mismer D., Rubin C.M., Smith D.R, Socolich M., and Zuker C.S. 1992. Ectopic expression of ultraviolet-rhodopsins in the blue photoreceptor cells of Drosophila Visual physiology and photochemistry of transgenic animals. /. Neurosci. 12 3862-3868. 

Amorphous (vitreous) selenium, vacuum-deposited on an aluminum substrate such as a dmm or a plate, was the first photoconductor commercially used in xerography (6). It is highly photosensitive, but only to blue light (2). Its light absorption falls off rather rapidly above 550 nm. Because of the lack of photoresponse in the red or near infrared regions, selenium photoreceptors caimot be used in laser printers having He—Ne lasers (632.8 nm), or soHd-state lasers (680—830 nm). 

Partch, C. L. et al., Postranslational regulation of mammalian circadian clock by cryptochrome and proteinphosphatase 5, Proc Aarf Acad. Sci. USA, 103, 10467, 2006. Briggs, W.R., Christie, J.M., and Salomon, M., Phototropins a new family of flavinbinding blue light receptors in plants, Antioxid. Redox Signal, 3, 775, 2001. Briggs,W.R. et al.. The phototropin family of photoreceptors. Plant Cell, 13, 993, 2001. 

Jung, A., J. Reinstein, T. Domratcheva, R. L. Shoeman, and I. Schlichting (2006). Crystal structures of the AppA BLUF domain photoreceptor provide insights into blue light-mediated signal transduction. J Mol Biol 362 717-732. 

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. 

Fig. 19. Hypothetical orientation of the SO - SI transition dipoles (blue band) of the bluelight photoreceptor in the Phycomyces sporangiophore as concluded from polarized light experiments96). Horizontally polarized light is about 20% more effective than vertically polarized light, as sketched...

To date, analyses of the Phycomyces light responses have remained at a rather formal level whereas the ultimate goal must be a detailed description of the chemical steps in reception, transmission and reaction to the stimulus of light. The first of these steps is excitation of the photoreceptor by light and it is therefore not unexpected that many attemps have been made to identify and isolate this compound. In spite of much effort during the past years, not only wi th Phycomyces, the blue light photo-... 

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. 

The molecular basis of most of these processes is not yet clear. Most of what we know concerns the absorption of quanta by the photoreceptor pigments, while stimulus transformation and signal transmission have as yet been investigated only in part and in a very few organisms. In blue-green algae, not even the movement mechanism is clear, so that any model of the motor response is highly speculative. 

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