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• W1993552306 abstract "Cryptochromes are blue light-activated photoreceptors found in multiple organisms with significant similarity to photolyases, a class of light-dependent DNA repair enzymes. Unlike photolyases, cryptochromes do not repair DNA and instead mediate blue light-dependent developmental, growth, and/or circadian responses by an as yet unknown mechanism of action. It has recently been shown that Arabidopsis cryptochrome-1 retains photolyase-like photoreduction of its flavin cofactor FAD by intraprotein electron transfer from tryptophan and tyrosine residues. Here we demonstrate that substitution of two conserved tryptophans that are constituents of the flavin-reducing electron transfer chain in Escherichia coli photolyase impairs light-induced electron transfer in the Arabidopsis cryptochrome-1 photoreceptor in vitro. Furthermore, we show that these substitutions result in marked reduction of light-activated autophosphorylation of cryptochrome-1 in vitro and of its photoreceptor function in vivo, consistent with biological relevance of the electron transfer reaction. These data support the possibility that light-induced flavin reduction via the tryptophan chain is the primary step in the signaling pathway of plant cryptochrome. Cryptochromes are blue light-activated photoreceptors found in multiple organisms with significant similarity to photolyases, a class of light-dependent DNA repair enzymes. Unlike photolyases, cryptochromes do not repair DNA and instead mediate blue light-dependent developmental, growth, and/or circadian responses by an as yet unknown mechanism of action. It has recently been shown that Arabidopsis cryptochrome-1 retains photolyase-like photoreduction of its flavin cofactor FAD by intraprotein electron transfer from tryptophan and tyrosine residues. Here we demonstrate that substitution of two conserved tryptophans that are constituents of the flavin-reducing electron transfer chain in Escherichia coli photolyase impairs light-induced electron transfer in the Arabidopsis cryptochrome-1 photoreceptor in vitro. Furthermore, we show that these substitutions result in marked reduction of light-activated autophosphorylation of cryptochrome-1 in vitro and of its photoreceptor function in vivo, consistent with biological relevance of the electron transfer reaction. These data support the possibility that light-induced flavin reduction via the tryptophan chain is the primary step in the signaling pathway of plant cryptochrome. Cryptochromes are found in plants, animals, and microbial systems, where they mediate numerous blue light-dependent developmental, growth, and/or circadian reponses (1Green C.B. Curr. Biol. 2004; 14: R847-R849Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 2Lin C. Shalitin D. Annu. Rev. Plant Biol. 2003; 54: 469-496Crossref PubMed Scopus (383) Google Scholar, 3Christie J.M. Briggs W.R. J. Biol. Chem. 2001; 276: 11457-11460Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar, 4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar). Cryptochrome-type photoreceptors are distinguished by their significant similarity to photolyases, a class of DNA repair enzymes (4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar) that removes lesions in UV-damaged DNA via a light-activated electron transfer mechanism. Despite their similarity to photolyases, and the fact that they bind the same flavin cofactor, FAD, the cryptochromes do not repair DNA and appear to function by interaction with downstream cellular signaling intermediates of the various response pathways (1Green C.B. Curr. Biol. 2004; 14: R847-R849Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 2Lin C. Shalitin D. Annu. Rev. Plant Biol. 2003; 54: 469-496Crossref PubMed Scopus (383) Google Scholar). The mechanism whereby light activates the cryptochrome photoreceptors, and the significance of their marked structural similarity to photolyases (5Park H.W. Kim S.T. Sancar A. Deisenhofer J. Science. 1995; 268: 1866-1872Crossref PubMed Scopus (502) Google Scholar, 6Brudler R. Hitomi K. Daiyasu H. Toh H. Kucho K. Ishiura M. Kanehisa M. Roberts V.A. Todo T. Tainer J.A. Getzoff E.D. Mol. Cell. 2003; 11: 59-67Abstract Full Text Full Text PDF PubMed Scopus (271) Google Scholar, 7Brautigam C.A. Smith B.S. Ma Z. Palnitkar M. Tomchick D.R. Machius M. Deisenhofer J. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12142-12147Crossref PubMed Scopus (247) Google Scholar), is currently unknown. Photolyases can undergo two distinct light-induced electron transfer reactions upon excitation of their FAD cofactor (4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar, 8Carell T. Burgdorf L.T. Kundu L.M. Cichon M. Curr. Opin. Chem. Biol. 2001; 5: 491-498Crossref PubMed Scopus (147) Google Scholar, 9Byrdin M. Sartor V. Eker A.P. Vos M.H. Aubert C. Brettel K. Mathis P. Biochim. Biophys. Acta. 2004; 1655: 64-70Crossref PubMed Scopus (75) Google Scholar). The first reaction initiates DNA repair and requires the flavin in its fully reduced form. In the second reaction, known as photoactivation, the semi-reduced flavin is converted to the fully reduced form by an electron ultimately provided by an extrinsic reductant. An intraprotein electron transfer pathway connecting the buried flavin to the protein surface has been derived for this photoactivation reaction in Escherichia coli photolyase based on crystallographic structural information and on a combination of site-directed mutagenesis and spectroscopy (10Cheung M.S. Daizadeh I. Stuchebrukhov A.A. Heelis P.F. Biophys. J. 1999; 76: 1241-1249Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar, 11Aubert C. Vos M.H. Mathis P. Eker A.P.M. Brettel K. Nature. 2000; 405: 586-590Crossref PubMed Scopus (358) Google Scholar, 12Byrdin M. Eker A.P.M. Vos M.H. Brettel K. Proc. Natl. Acad. Sci. U. S. A. 2003; 100: 8676-8681Crossref PubMed Scopus (97) Google Scholar). This pathway comprises a chain of three tryptophan residues (Trp382-Trp359-Trp306) that are conserved throughout the photolyase/cryptochrome family. Recently, a study with purified Arabidopsis cryptochrome-1 (cry1) 1The abbreviation used is: cry1, Arabidopsis cryptochrome-1. demonstrated occurrence of a similar photoreaction, starting from the fully oxidized form of the flavin and involving tryptophan and tyrosine residues as intrinsic electron donors (13Giovani B. Byrdin M. Ahmad M. Brettel K. Nat. Struct. Biol. 2003; 10: 489-490Crossref PubMed Scopus (242) Google Scholar). To explore the possible functional relevance of this reaction to cryptochrome photoreceptor activity, we have substituted redox inactive phenylalanines for two tryptophan residues, Trp400 and Trp324, which are found in the cry1 sequence and crystal structure (7Brautigam C.A. Smith B.S. Ma Z. Palnitkar M. Tomchick D.R. Machius M. Deisenhofer J. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12142-12147Crossref PubMed Scopus (247) Google Scholar) at the homologous positions to Trp382 and Trp306 of E. coli photolyase, respectively. The mutant proteins (W400F and W324F) thereby lack the predicted electron donor proximal to the flavin (Trp400) or exposed to the protein surface (Trp324) and should hence be impaired in electron transfer. If electron transfer is relevant for photoreceptor function, these amino acid substitutions should result in impairment of cry1-dependent blue light responses in vivo. Preparation of Mutant Photoreceptor Proteins—By means of oligonucleotide-directed mutagenesis, single amino acid substitutions (tryptophan to phenylalanine) were introduced into the coding sequence of cry1 at positions Trp324 and Trp400 by using the Altered Sites II in vitro mutagenesis system from Promega (Madison, WI). Mutations were confirmed by sequence analysis. Mutant and wild-type proteins were expressed in a baculovirus expression system and purified by nickel column affinity chromatography as described previously (13Giovani B. Byrdin M. Ahmad M. Brettel K. Nat. Struct. Biol. 2003; 10: 489-490Crossref PubMed Scopus (242) Google Scholar, 14Lin C. Robertson D.E. Ahmad M. Raibekas A.A. Jorns M.S. Dutton P.L. Cashmore A.R. Science. 1995; 269: 968-970Crossref PubMed Scopus (377) Google Scholar). Flavin Photoreduction Assay—Flavin photoreduction in isolated cry1 was performed by irradiation under anaerobic conditions with white light in the presence of β-mercaptoethanol essentially as described (14Lin C. Robertson D.E. Ahmad M. Raibekas A.A. Jorns M.S. Dutton P.L. Cashmore A.R. Science. 1995; 269: 968-970Crossref PubMed Scopus (377) Google Scholar). Samples contained between 3 and 24 μm protein, 10 mm β-mercaptoethanol, 0.5 m NaCl, 0.3 m imidazole, 50 mm Tris, pH 7.5, and were kept at 12 °C during the experiments. Absorption spectra were measured in a Beckmann DU7400 spectrophotometer. Transient Absorption Spectroscopy—Transient absorption changes were measured with a time resolution of 100 μs as described (13Giovani B. Byrdin M. Ahmad M. Brettel K. Nat. Struct. Biol. 2003; 10: 489-490Crossref PubMed Scopus (242) Google Scholar), using excitation flashes at 355 nm, 5-ns duration, ∼5 mJ energy per cm2, repetition rate 1.983 Hz. Samples contained ∼10 μm protein, 0.5 m NaCl, and 50 mm Tris, pH 7.5, and were kept at 12 °C during the experiments. 32 signals were averaged for each sample. Autophosphorylation Assay—Isolated cryptochrome was radiolabeled as described (15Bouly J.P. Giovani B. Djamei A. Mueller M. Zeugner A. Dudkin E.A. Batschauer A. Ahmad M. Eur. J. Biochem. 2003; 270: 2921-2928Crossref PubMed Scopus (95) Google Scholar). 2 μg of protein were incubated with [γ-32P]ATP for 10 min at 22 °C, either in darkness or under broad band blue light (400–500 nm, 30 μmol m-2 s-1), resolved on polyacrylamide gels and stained with Coomassie dye to visualize the protein bands. Gels were subsequently dried and subjected to autoradiography. Phenotypic Analysis of Arabidopsis Mutant Seedlings—Mutant cry1 coding sequences were cloned behind the strong 35S promoter of the binary vector pKYLX6 and transformed into cryptochrome-deficient (cry1cry2 mutant) Arabidopsis plants via agrobacterium-mediated transformation as described (16Ahmad M. Jarillo J. Cashmore A.R. Plant Cell. 1998; 10: 197-208Crossref PubMed Scopus (142) Google Scholar). Arabidopsis seedlings expressing wild-type or mutant cry1 were analyzed after 5 days of growth in continuous blue light (30 μmol m-2 s-1) according to methods and procedures described and/or referenced (16Ahmad M. Jarillo J. Cashmore A.R. Plant Cell. 1998; 10: 197-208Crossref PubMed Scopus (142) Google Scholar, 17Ahmad M. Lin C. Cashmore A.R. Plant J. 1995; 8: 653-658Crossref PubMed Scopus (179) Google Scholar). Anthocyanin accumulation was quantified by extraction of pigments from 30 seedlings in 1 ml of acidified methanol and measurement of the absorbance at 530 nm, corrected for the contribution of chlorophyll as described (16Ahmad M. Jarillo J. Cashmore A.R. Plant Cell. 1998; 10: 197-208Crossref PubMed Scopus (142) Google Scholar, 17Ahmad M. Lin C. Cashmore A.R. Plant J. 1995; 8: 653-658Crossref PubMed Scopus (179) Google Scholar). Wild-type and mutant cry1 proteins expressed in a baculovirus expression system were checked for pigment content by steady state absorption spectroscopy. As reported previously for wild type cry1 (14Lin C. Robertson D.E. Ahmad M. Raibekas A.A. Jorns M.S. Dutton P.L. Cashmore A.R. Science. 1995; 269: 968-970Crossref PubMed Scopus (377) Google Scholar), both W324F and W400F mutant proteins bound the FAD cofactor in fully oxidized form as evidenced by its characteristic absorption spectrum in the 400–500 nm region (Fig. 1). We applied two different methods to determine whether the mutations affected light-induced electron transfer in the cry1 photoreceptor in vitro, (i) a global assay of flavin photoreduction (Fig. 1) and (ii) a time-resolved study of intraprotein electron transfer (Fig. 2). In the photoreduction assay, wild-type and mutant proteins were subjected to continuous illumination in the presence of the reductant β-mercaptoethanol, and flavin reduction was monitored by absorption changes in the 400–600 nm region. Wild-type protein showed progressive flavin reduction during the course of a half-hour illumination as described previously (14Lin C. Robertson D.E. Ahmad M. Raibekas A.A. Jorns M.S. Dutton P.L. Cashmore A.R. Science. 1995; 269: 968-970Crossref PubMed Scopus (377) Google Scholar), very little reduction was seen either in the W400F or W324F mutant protein (Fig. 1 inset), consistent with interruption of the electron transfer pathway needed for flavin photoreduction. As a specific test for the occurence of intraprotein electron transfer, photoreactions were studied in the absence of external reductant on a much more rapid time scale. The fully oxidized flavin cofactor was excited by a short laser flash and the resulting photoreactions were monitored by transient absorption spectroscopy with 100-μs time resolution. As shown previously (13Giovani B. Byrdin M. Ahmad M. Brettel K. Nat. Struct. Biol. 2003; 10: 489-490Crossref PubMed Scopus (242) Google Scholar), the rapid absorption increase at 520 nm in wild-type cry1 (Fig. 2, upper trace) reflects concomitant formation of the semireduced neutral flavin radical and of a neutral tryptophanyl radical due to tryptophan-to-flavin electron transfer. The subsequent polyphasic absorption decay has been attributed to reduction of the tryptophanyl radical by a tyrosine residue and to electron backtransfer from the flavin radical to the tryptophanyl and tyrosyl radicals. For both the W324F and W400F mutant proteins, we observed absorbance changes that were approximately five times weaker than in the wild type protein (Fig. 2), strongly suggesting an important role of these two tryptophan residues for flavin photoreduction. During the transient absorption studies, we noticed a strong enhancement in flash-induced fluorescence for the W400F mutant protein (visible in Fig. 2 as a negative spike). According to separate fluorescence measurements under identical excitation conditions, the fluorescence yield ratios were ∼8:2:1 for W400F, W324F, and wild type proteins, respectively (data not shown). The strongly enhanced fluorescence in the W400F mutant protein suggests that W400 is the primary electron donor to the flavin in wild-type cry1 and that this nearby tryptophan quenches fluorescence of the fully oxidized flavin due to fast electron transfer, as suggested for other flavoproteins (19Zhong D. Zewail A.H. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 11867-11872Crossref PubMed Scopus (215) Google Scholar, 20Mataga N. Chosrowjan H. Taniguchi S. Tanaka F. Kido N. Kitamura M. J. Phys. Chem. B. 2002; 106: 8917-8920Crossref Scopus (102) Google Scholar). To determine a possible functional role for cry1 of the light-induced electron transfer, we examined the effect of the same amino acid substitutions on an in vitro autophosphorylation reaction found in purified preparations of cry1 (15Bouly J.P. Giovani B. Djamei A. Mueller M. Zeugner A. Dudkin E.A. Batschauer A. Ahmad M. Eur. J. Biochem. 2003; 270: 2921-2928Crossref PubMed Scopus (95) Google Scholar, 21Shalitin D. Yu X. Maymon M. Mockler T. Lin C. Plant Cell. 2003; 15: 2421-2429Crossref PubMed Scopus (141) Google Scholar), This autophosphorylation activity is both light- and redox-sensitive, requires the presence of the flavin cofactor within the apoprotein, and has been proposed as an early step in light activation of the photoreceptor (15Bouly J.P. Giovani B. Djamei A. Mueller M. Zeugner A. Dudkin E.A. Batschauer A. Ahmad M. Eur. J. Biochem. 2003; 270: 2921-2928Crossref PubMed Scopus (95) Google Scholar). Both W324F and W400F mutant proteins showed basal levels of phosphorylation in the dark, reduced in comparison with wild-type protein at the same concentration (Fig. 3). Significantly, there was no increase in autophosphorylation of mutant protein samples due to illumination by blue light as compared with the dark controls. This was in marked contrast to the wild-type protein, where significant stimulation by blue light occurred. To explore the functional relevance of the electron transfer reaction in vivo (Fig. 4), W324F and W400F mutated coding sequences were cloned into a plant expression vector and were transformed into cryptochrome-deficient Arabidopsis plants. Transgenic seedlings were evaluated by Western blot analysis with anti-cry1 antibody to verify synthesis of the mutant photoreceptor proteins (Fig. 4b). As functional assays for cry1-specific activity in vivo, transgenic seedlings were evaluated for blue light-dependent inhibition of hypocotyl elongation (Fig. 4, a and d) and for blue light-dependent anthocyanin accumulation (16Ahmad M. Jarillo J. Cashmore A.R. Plant Cell. 1998; 10: 197-208Crossref PubMed Scopus (142) Google Scholar, 17Ahmad M. Lin C. Cashmore A.R. Plant J. 1995; 8: 653-658Crossref PubMed Scopus (179) Google Scholar) (Fig. 4c), in comparison with seedlings containing wild-type cry1 protein at a similar concentration. By both these functional assays, activity was shown to be significantly reduced in the two transgenic lines containing a mutant cry1 photoreceptor (W324F and W400F) as compared with seedlings containing wild type cry1 (Fig. 4, c and d). The present study documents a strong correlation between a primary photochemical reaction (electron transfer) and cry1 photoreceptor function, both in vitro and in vivo. We show that substitution of tryptophan residues that are homologous to those that participate in the photoactivation reaction of E. coli photolyase result in marked impairment in electron transfer of isolated cry1 photoreceptors. This impairment is unlikely to be caused by major structural perturbations, as the efficient FAD binding observed in the mutants implies proper contacts with multiple amino acid residues throughout the protein (7Brautigam C.A. Smith B.S. Ma Z. Palnitkar M. Tomchick D.R. Machius M. Deisenhofer J. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12142-12147Crossref PubMed Scopus (247) Google Scholar). Furthermore, amino acid substitutions at the homologous positions in E. coli photolyase do not impair DNA repair activity (18Li Y.F. Heelis P.F. Sancar A. Biochemistry. 1991; 30: 6322-6329Crossref PubMed Scopus (151) Google Scholar). Hence, although we cannot exclude the possibility of minor structural alterations in the protein resulting from these mutations, we attribute the impairment in electron transfer in the mutants to most likely result from the elimination of essential members of the electron transfer chain. Our results thus suggest that functional electron transfer along the triple tryptophan chain is conserved between plant cryptochromes and their photolyase homologues, revealing a functional significance of the marked amino acid similarity between cryptochromes and photolyases. Although greatly reduced, there was residual electron transfer in the mutant proteins (Fig. 2). This may be due to low yield side pathways involving other tryptophan and/or tyrosine residues that are close to the flavin. According to the crystal structure of the photolyase-like domain of wild-type cry1 (7Brautigam C.A. Smith B.S. Ma Z. Palnitkar M. Tomchick D.R. Machius M. Deisenhofer J. Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 12142-12147Crossref PubMed Scopus (247) Google Scholar), the best candidates are Trp385 (at 7.1 Å edge-to-edge distance from the flavin), Trp356 (7.1 Å), Trp334 (9.2 Å), Trp377 (9.7 Å), and Trp402 (6.9 Å). For comparison, Trp400 is at 4.5 Å from the flavin. Based on an excited state lifetime of several nanoseconds for the fully oxidized flavin (22Heelis P.F. Chem. Soc. Rev. 1982; 11: 15-39Crossref Google Scholar) and on the distance dependence of intraprotein electron transfer rates (23Page C.C. Moser C.C. Chen X. Dutton P.L. Nature. 1999; 402: 47-52Crossref PubMed Scopus (1531) Google Scholar), oxidizable residues within ∼10 Å could well reduce the photoexcited flavin with non-negligible yields. A light-dependent autophosphorylation reaction has been speculated to be an early step in activation of the cryptochrome photoreceptor (15Bouly J.P. Giovani B. Djamei A. Mueller M. Zeugner A. Dudkin E.A. Batschauer A. Ahmad M. Eur. J. Biochem. 2003; 270: 2921-2928Crossref PubMed Scopus (95) Google Scholar, 21Shalitin D. Yu X. Maymon M. Mockler T. Lin C. Plant Cell. 2003; 15: 2421-2429Crossref PubMed Scopus (141) Google Scholar). In our in vitro phosphorylation assay (Fig. 3), both the W400F and W324F mutant proteins retained basal (light-independent) autophosphorylation, indicating that they retain the capacity for undergoing the autophosphorylation reaction, although somewhat reduced in comparison with wild type protein. The prominent stimulation of autophosphorylation by blue light observed in wild type was, however, completely suppressed in both the W400F and W324F mutant proteins, indicating that intraprotein electron transfer is necessary for light stimulation of this autophosphorylation response. In vivo photoreceptor function of cryptochrome was markedly reduced in both subsititutions (W324F and W400F), as measured both by reduced anthocyanin accumulation and reduced hypocotyl growth inhibition under blue light in transgenic plants (Fig. 4). We detected some residual activity in the W324F and W400F mutants. This activity may have resulted from residual electron transfer activity via alternate (and less efficient) electron transfer pathways, as discussed above. Summarizing, our data suggest that light-induced electron transfer provides the trigger for light responsivity of the cry1 photoreceptor. In photolyases, photoactivation converts the semi-reduced flavin with absorption bands in the 500–650 nm region to the fully reduced form that can participate in the DNA repair reaction and has no pronounced absorption bands in the visible. This photoactivation reaction is normally not essential for function as photolyases are typically found in the fully reduced form in living cells and proteins deficient in photoactivation can still function in repair (4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar, 18Li Y.F. Heelis P.F. Sancar A. Biochemistry. 1991; 30: 6322-6329Crossref PubMed Scopus (151) Google Scholar). In the case of plant cryptochromes, action spectroscopy has indicated that there is no significant response to wavelengths of light above 500 nm and that the response maximum is around 450 nm, suggestive of flavin in the oxidized form in vivo (24Ahmad M. Grancher N. Heil M. Black R.C. Giovani B. Galland P. Lardemer D. Plant Physiol. 2002; 129: 774-785Crossref PubMed Scopus (172) Google Scholar). Therefore, a primary light reaction for cryptochrome whereby the oxidized flavin undergoes light-dependent photoreduction would appear a reasonable mechanism, which may then trigger subsequent conformational or other biochemical changes that have been suggested to be the basis for signaling (25Yang H.Q. Wu Y.J. Tang R.H. Liu D. Liu Y. Cashmore A.R. Cell. 2000; 103: 815-827Abstract Full Text Full Text PDF PubMed Scopus (332) Google Scholar). Cryptochrome-like photoreceptors have been identified in a number of other organisms including animals, where they have been shown to play an important role in the circadian clock (1Green C.B. Curr. Biol. 2004; 14: R847-R849Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar, 2Lin C. Shalitin D. Annu. Rev. Plant Biol. 2003; 54: 469-496Crossref PubMed Scopus (383) Google Scholar, 3Christie J.M. Briggs W.R. J. Biol. Chem. 2001; 276: 11457-11460Abstract Full Text Full Text PDF PubMed Scopus (163) Google Scholar, 4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar). There are significant structural similarities between animal and plant cryptochromes, including conservation of the three tryptophan residues that have been implicated in the electron transfer chain of E. coli photolyase and which we have here demonstrated to be implicated in light-dependent electron transfer in plant cry1. Substitution of some of these tryptophan residues by tyrosine has been reported to reduce activity of Xenopus cryptochromes in an in vitro transfection assay (26Zhu H. Green C.B. Curr. Biol. 2001; 11: 1945-1949Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar). However, since the effect of cryptochrome on reporter gene expression is independent of light in the transfection assay, it cannot be concluded from this experiment that electron transfer is involved in animal cryptochrome activity. On the other hand, mutations of the homologous tryptophans to phenylalanines in Drosophila cryptochrome (27Froy O. Chang D.C. Reppert S.M. Curr. Biol. 2002; 12: 147-152Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar) did not abolish its light-dependent activation either in transfection assays or at the level of protein stability. Therefore it would appear unlikely, in Drosophila cryptochromes, that light activation depends on electron transfer via the same pathway functioning in the plant cry1. Since plant and animal cryptochromes apparently evolved independently from different photolyase ancestors (4Sancar A. Chem. Rev. 2003; 103: 2203-2237Crossref PubMed Scopus (1042) Google Scholar), they may have a different primary mechanism of action. Alternatively, electron transfer in animal-type cryptochromes may still be important for light activation but may occur via an alternative pathway to that found in plant cryptochrome. Indeed, the type 6-4 photolyases to which the animal cryptochromes are most closely related may utilize an electron transfer pathway involving primarily tyrosine (28Weber S. Kay C.W. Mogling H. Möbius K. Hitomi K. Todo T. Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 1319-1322Crossref PubMed Scopus (76) Google Scholar). It is therefore intriguing to speculate that the electron transfer activity present in photolyases contains latent signaling potential that has been co-opted for a role in blue-light photoreceptors multiple times throughout the course of evolution." @default.
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• W1993552306 title "Light-induced Electron Transfer in Arabidopsis Cryptochrome-1 Correlates with in Vivo Function" @default.
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