Reconstitution of Escherichia coli DNA photolyase with various folate derivatives

DNA photolyase from Escherichia coli contains both flavin and pterin. However, the isolated enzyme is depleted with respect to the pterin chromophore (0.5 mol of pterin/mol of flavin). The extinction coefficient of the pterin chromophore at 360 nm is underestimated by a method used in earlier studies which assumes stoichiometric amounts of pterin and flavin. The extinction coefficient of the pterin chromophore, determined on the basis of its (p-aminobenzoyl)polyglutamate content (epsilon 360 = 25.7 x 10(3) M-1 cm-1), is in good agreement with that expected for a 5,10-methenyltetrahydrofolate derivative. Also consistent with this structure, the pterin chromophore could be reversibly hydrolyzed to yield a 10-formyltetrahydrofolate derivative or reduced to yield a 5-methyltetrahydrofolate derivative. The isolated enzyme could be reconstituted with various folate derivatives to yield enzyme that contained equimolar amounts of pterin and flavin. Similar results were obtained in reconstitution studies with the natural pterin chromophore, with 5,10-methenyltetrahydrofolate, and with 10-formyltetrahydrofolate. The results show that the polyglutamate moiety, previously identified in the natural chromophore, is not critical for binding. Reconstitution with the natural pterin chromophore did not affect catalytic activity. The latter is consistent with our previous studies which show that, although the pterin chromophore acts as a sensitizer in native enzyme, it is not essential for dimer repair which can occur at the same rate under saturating light with flavin (1,5-dihydro-FAD) as the only chromophore.     

The folate cofactor of Escherichia coli DNA photolyase acts catalytically

Escherichia coli DNA photolyase catalyzes the light-driven (300-500 nm) repair of pyrimidine dimers formed between adjacent pyrimidine bases in DNA exposed to UV light (200-300 nm). The light-driven repair process is facilitated by two enzyme-bound cofactors, FADH2 and 5,10-methenyltetrahydrofolate. The function of the folate has been characterized in greater detail in this series of experiments. Investigations of the relative binding affinities of photolyase for the monoglutamate and polyglutamate forms of 5,10-methenyltetrahydrofolate show that the enzyme has a greater affinity for the naturally occurring polyglutamate forms of the folate and that the exogenously added monoglutamate derivative is less tightly associated with the protein. Multiple turnover experiments reveal that the folate remains bound to photolyase even after 10 turnovers of the enzyme. Examination of the rates of repair by photolyase containing stoichiometric folate in the presence or absence of free folate under multiple turnover conditions and at micromolar concentrations of enzyme also demonstrates that the folate acts catalytically. The stimulation of turnover by exogenous folate seen at low concentrations of photolyase is shown to be due to the lower affinity of photolyase for the monoglutamate derivative used in reconstitution procedures. These results demonstrate that the folate of E. coli DNA photolyase is a bona fide cofactor and does not decompose or dissociate during multiple turnovers of the enzyme.     

Chromophore function and interaction in Escherichia coli DNA photolyase: reconstitution of the apoenzyme with pterin and/or flavin derivatives

Native DNA photolyase, as isolated from Escherichia coli, contains a neutral flavin radical (FADH.) plus a pterin chromophore (5,10-methenyltetrahydropteroylpolyglutamate) and can be converted to its physiologically significant form by reduction of FADH. to fully reduced flavin (FADH2) with dithionite or by photoreduction. Either FADH2 or the pterin chromophore in dithionite-reduced native enzyme can function as a sensitizer in catalysis. Various enzyme forms (EFADox, EFADH., EFADH2, EPteFADox, EPteFADH., EPteFADH2, EPte) containing stoichiometric amounts of FAD in either of its three oxidation states and/or 5,10-methenyltetrahydrofolate (Pte) have been prepared in reconstitution experiments. Studies with EFADox and EPte showed that these preparations retained the ability to bind the missing chromophore. The results suggest that there could be considerable flexibility in the biological assembly of holoenzyme since the order of binding of the enzyme's chromophores is apparently unimportant, the binding of FAD is unaffected by its redox state, and enzyme preparations containing only one chromophore are reasonably stable. The same catalytic properties are observed with dithionite-reduced native enzyme or EFADH2. These preparations do not exhibit a lag in catalytic assays whereas lags are observed with preparations containing FADox or FADH. in the presence or absence of pterin. Photochemical studies show that these lags can be attributed to enzyme activation under assay conditions in a reaction involving photoreduction of enzyme-bound FADox or FADH. to FADH2. EPte is catalytically inactive, but catalytic activity is restored upon reconstitution of EPte with FADox. The results show that pterin is not required for dimer repair when FADH2 acts as the sensitizer but that FADH2 is required when dimer repair is initiated by excitation of the pterin chromophore. The relative intensity of pterin fluorescence in EPte, EPteFADH., EPteFADox, or EPteFADH2 has been used to estimate the efficiency of pterin singlet quenching by FADH. (93%), FADox (90%), or FADH2 (58%). Energy transfer from the excited pterin to flavin is energetically feasible and may account for the observed quenching of pterin fluorescence and also explain why photoreduction of FADox or FADH. is accelerated by the pterin chromophore. An irreversible photobleaching of the pterin chromophore is accelerated by FADH2 in a reaction that is accompanied by a transient oxidation of FADH2 to FADH.. Both pterin bleaching and FADH2 oxidation are inhibited by substrate.     

Reconstitution of Escherichia coli photolyase with flavins and flavin analogues

Escherichia coli DNA photolyase contains two chromophore cofactors, 1,5-dihydroflavin adenine dinucleotide (FADH2) and (5,10-methenyltetrahydrofolyl)polyglutamate (5,10-MTHF). A procedure was developed for reversible resolution of apophotolyase and its chromophores. To investigate the structures important for the binding of FAD to apophotolyase and of photolyase to DNA, reconstitution experiments with FAD, FMN, riboflavin, 1-deazaFAD, 5-deazaFAD, and F420 were attempted. Only FAD and 5-deazaFAD showed high-affinity binding to apophotolyase. The apoenzyme had no affinity to DNA but did regain its specific binding to thymine dimer containing DNA upon binding stoichiometrically to FAD or 5-deazaFAD. Successful reduction of enzyme-bound FAD with dithionite resulted in complete recovery of photocatalytic activity.     

DNA photorepair: chromophore composition and function in two classes of DNA photolyases

DNA photolyase catalyzes the repair of pyrimidine dimers in UV-damaged DNA in a reaction which requires visible light. Class I photolyases (Escherichia coli, yeast) contain 1,5-dihydroFAD (FADH2) plus a pterin derivative (5,10-methenyltetrahydropteroylpolyglutamate). In class II photolyases (Streptomyces griseus, Scenedesmus acutus, Anacystis nidulans, Methanobacterium thermoautotrophicum) the pterin chromophore is replaced by an 8-hydroxy-5-deazaflavin derivative. The two classes of enzymes exhibit a high degree of amino acid sequence homology, suggesting similarities in protein structure. Action spectra studies show that both chromophores in each enzyme tested act as sensitizers in catalysis. Studies with E. coli photolyase show that the pterin chromophore is not required when FADH2 acts as the sensitizer but that FADH2 is required when the pterin chromophore acts as sensitizer. FADH2 is probably the chromophore that directly interacts with substrate in a reaction which may be initiated by electron transfer from the excited singlet state (1FADH2*) to form a flavin radical plus an unstable pyrimidine dimer radical. Pterin, the major chromophore in E. coli photolyase, may act as an antenna to harvest light energy which is then transferred to FADH2.     

Action mechanism of Escherichia coli DNA photolyase. II. Role of the chromophores in catalysis

DNA photolyase repairs pyrimidine dimers in DNA in a reaction that requires visible light. Photolyase from Escherichia coli is normally isolated as a blue protein and contains 2 chromophores: a blue FAD radical plus a second chromophore that exhibits an absorption maximum at 360 nm when free in solution. Oxidation of the FAD radical is accompanied by a reversible loss of activity which is proportional to the fraction of the enzyme flavin converted to FADox. Quantitative reduction of the radical to fully reduced FAD causes a 3-fold increase in activity. The results show that a reduced flavin is required for activity and suggest that flavin may act as an electron donor in catalysis. Comparison of the absorption spectrum calculated for the protein-bound second chromophore (lambda max = 390 nm) with fluorescence data and with the relative action spectrum for dimer repair indicates that the second chromophore is the fluorophore in photolyase and that it does act as a sensitizer in catalysis. On the other hand, enzyme preparations containing diminished amounts of the second chromophore do not exhibit correspondingly lower activity. This suggests that reduced flavin may also act as a sensitizer in catalysis. The blue color of the enzyme is lost upon reduction of the FAD radical. The fully reduced E. coli enzyme exhibits absorption and fluorescence properties very similar to yeast photolyase. This indicates that the two enzymes probably contain similar chromophores but are isolated in different forms with respect to the redox state of the flavin.     

The presence and distribution of reduced folates in Escherichia coli dihydrofolate reductase mutants

Escherichia coli DNA photolyase was overproduced and purified from each of two mutant E. coli strains lacking dihydrofolate reductase. The extent of over-production in the mutants was comparable to that seen in the wild type strain. Examination of the isolated photolyase from these strains revealed that the folate cofactor, 5,10-methenyltetrahydrofolate, was present in these proteins at a level of 60-80% compared to that purified from the wild type strain. Further examination of the dihydrofolate reductase-deficient strains revealed the presence of other tetrahydrofolate derivatives. These findings demonstrate that dihydrofolate reductase is not essential for the production of tetrahydrofolates in E. coli.     

Identification of chromophore binding domains of yeast DNA photolyase.

Photolyases contain two chromophores, flavin plus either methenyltetrahydrofolate (MTHF) or 8-OH-5-deazaflavin (HDF). Amino acid sequence comparison reveals that all photolyases sequenced to date have extensive sequence homology in the carboxyl-terminal half; in the amino-terminal region the folate and deazaflavin class enzymes are more homologous to other members of the same class. This modular arrangement of sequence homologies suggests that the amino-terminal half of photolyase is involved in MTHF or HDF binding whereas the carboxyl-terminal half carries the flavin binding site. In this study we attempted to identify such structural domains of yeast photolyase by partial proteolysis and gene fusion techniques. Partial digestion with chymotrypsin yielded an amino-terminal 34-kDa fragment containing tightly bound MTHF and a carboxyl-terminal 20-kDa polypeptide which lacked chromophore or DNA binding activity. However, a fusion protein carrying the carboxyl-terminal 275 amino acids of yeast photolyase bound specifically to FAD but not to MTHF or DNA. We conclude that the amino-terminal half of yeast photolyase constitutes the folate binding domain and that the carboxyl-terminal half carries the flavin binding site.     

Effect of 5-deazaflavin on energy transduction during catalysis by Escherichia coli DNA photolyase.

DNA photolyase from Escherichia coli contains 1,5-dihydroFAD (FADH2) plus 5,10-methenyltetrahydropteroylpolyglutamate. The action spectrum observed for apoenzyme reconstituted with 5-deazaFADH2 (EdFADH2) matched its absorption spectrum after correction for the presence of a small amount of inactive 5-deazaFADox. The quantum yield for dimer repair with EdFADH2 (phi EdFADH2 = 0.110) was 6-fold lower than that observed with apoenzyme reconstituted with FADH2. Excited-state redox potential calculations indicate that 5-deazaFADH2 singlet is a better one-electron donor (E = -3.5 V) than FADH2 singlet (E = -2.7 V). Other studies indicate that the quantum yield for electron transfer from reduced flavin singlet to pyrimidine dimer (0.88) is unaffected when FADH2 is replaced by 5-deazaFADH2. Enhanced back electron transfer from pyrimidine dimer radical to flavin radical may account for the decreased quantum yield observed with EdFADH2 since, in the ground state, 5-deazaFADH. is a better oxidant than FADH.. The action spectrum observed for apoenzyme reconstituted with 5-deazaFADH2 plus 5,10-CH(+)-H4folate (EPtedFADH2) matched the absorption spectrum determined for enzyme-bound 5-deazaFADH2, indicating that the pterin chromophore was inactive as a sensitizer. This differs from results obtained with native enzyme, where pterin acts as a sensitizer via efficient singlet-singlet energy transfer to FADH2. The quantum yield for dimer repair by 5-deazaFADH2 bound to EPtedFADH2 (phi EPtedFADH2 = 0.0318) was 28.9% of that observed for EdFADH2. Spectroscopic studies indicate that singlet-singlet energy transfer in EPtedFADH2 is very efficient but only occurs in the "wrong" direction, i.e., from excited 5-deazaFADH2 to pterin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Methylenetetrahydrofolate reductase from Escherichia coli: elucidation of the kinetic mechanism by steady-state and rapid-reaction studies

The flavoprotein methylenetetrahydrofolate reductase (MTHFR) from Escherichia coli catalyzes the reduction of 5,10-methylenetetrahydrofolate (CH(2)-H(4)folate) to 5-methyltetrahydrofolate (CH(3)-H(4)folate) using NADH as the source of reducing equivalents. The enzyme also catalyzes the transfer of reducing equivalents from NADH or CH(3)-H(4)folate to menadione, an artificial electron acceptor. Here, we have determined the midpoint potential of the enzyme-bound flavin to be -237 mV. We have examined the individual reductive and oxidative half-reactions constituting the enzyme's activities. In an anaerobic stopped-flow spectrophotometer, we have measured the rate constants of flavin reduction and oxidation occurring in each half-reaction and have compared these with the observed catalytic turnover numbers measured under steady-state conditions. We have shown that, in all cases, the half-reactions proceed at rates sufficiently fast to account for overall turnover, establishing that the enzyme is kinetically competent to catalyze these oxidoreductions by a ping-pong Bi-Bi mechanism. Reoxidation of the reduced flavin by CH(2)-H(4)folate is substantially rate limiting in the physiological NADH-CH(2)-H(4)folate oxidoreductase reaction. In the NADH-menadione oxidoreductase reaction, the reduction of the flavin by NADH is rate limiting as is the reduction of flavin by CH(3)-H(4)folate in the CH(3)-H(4)folate-menadione oxidoreductase reaction. We conclude that studies of individual half-reactions catalyzed by E. coli MTHFR may be used to probe mechanistic questions relevant to the overall oxidoreductase reactions.     

Photochemical properties of Escherichia coli DNA photolyase: selective photodecomposition of the second chromophore

Escherichia coli DNA photolyase contains a stable flavin radical and a second chromophore (SC) of unknown structure. The effects of flash (both conventional and laser) excitation of either the radical alone or both the radical and the second chromophore have been investigated by variation of the excitation wavelengths. Radical excitation leads to an electron abstraction by the lowest excited doublet state of the radical from an amino acid residue, probably a cysteine or tyrosine. On a longer time scale, a back-reaction occurs that can be prevented by the presence of certain electron donors, e.g., thiols, NADH, or tyrosine, but not pyrimidine dimers. Excitation of the second chromophore leads to electronic energy transfer from second chromophore excited states to the ground-state flavin radical doublet state, thus increasing the population of the lowest excited doublet state. Repetitive excitation of the enzyme with white light leads to photodecomposition of the second chromophore but not of the flavin adenine dinucleotide cofactor. Enzyme with photodecomposed SC retains full activity.     

Active site of Escherichia coli DNA photolyase: mutations at Trp277 alter the selectivity of the enzyme without affecting the quantum yield of photorepair

Escherichia coli DNA photolyase repairs pyrimidine dimers by a photoinduced electron-transfer reaction. The enzyme binds to UV-damaged DNA independent of light (the dark reaction) and upon absorbing a 300-500-nm photon breaks the cyclobutane ring of the dimer (the light reaction) and thus restores the DNA. No structural information on the enzyme is available at present. However, comparison of the sequences of photolyases from five different organisms has identified highly conserved regions of homology. These regions are presumably involved in chromophore (flavin and folate) and substrate binding or catalysis. Trp277 (W277) in E. coli photolyase is conserved in all photolyases sequenced to date. We replaced this residue with Arg, Glu, Gln, His, and Phe by site-specific mutagenesis. Properties of the mutant proteins indicate that W277 is involved in binding to DNA but not in chromophore binding or catalysis. Of particular significance is the finding that compared to wild type W277R and W277E mutants have about 300- and 1000-fold lower affinity, respectively, for substrate but were indistinguishable from wild-type enzyme in their photochemical and photocatalytic properties.     

Photochemical properties of Escherichia coli DNA photolyase: a flash photolysis study

Escherichia coli DNA photolyase contains a stable flavin neutral blue radical that is involved in photosensitized repair of pyrimidine dimers in DNA. We have investigated the effect of illumination on the radical using light of lambda greater than 520 nm from either a camera flash or laser. We find that both types of irradiations result in the photoreduction of the flavin radical with a quantum yield of 0.10 +/- 0.02. While photoreduction with the camera flash is minimal in the absence of an electron donor (dithiothreitol), laser flash photolysis at 532 nm reduces the flavin to the same extent in the presence or absence or an electron donor. Thus, it is concluded that the primary step in photoreduction involves an electron donor that is a constituent of the enzyme itself. Laser flash photolysis produces a transient absorption band at 420 nm that probably represents the absorption of the lowest excited doublet state (2(1)IIII*) of the radical and decays with first-order kinetics with k1 = 0.8 X 10(6) s-1. The photoreduction data combined with the results of recent studies on the activity of dithionite-reduced enzyme suggest that electron donation by excited states of E-FADH2 is the mechanism of flavin photosensitized dimer repair by E. coli DNA photolyase.     

Escherichia coli DNA photolyase is a flavoprotein

Escherichia coli DNA photolyase (photoreactivating enzyme) was purified to homogeneity from a strain that greatly overproduces the protein. The purified enzyme has absorption peaks at 280 and 380 nm, a fluorescence emission peak at 480 nm and, upon denaturation, releases a chromophore that has the spectroscopic properties of flavin adenine dinucleotide (FAD), indicating that FAD is an intrinsic chromophore of the enzyme.     

A photolyase-like protein from Agrobacterium tumefaciens with an iron-sulfur cluster

Photolyases and cryptochromes are evolutionarily related flavoproteins with distinct functions. While photolyases can repair UV-induced DNA lesions in a light-dependent manner, cryptochromes regulate growth, development and the circadian clock in plants and animals. Here we report about two photolyase-related proteins, named PhrA and PhrB, found in the phytopathogen Agrobacterium tumefaciens. PhrA belongs to the class III cyclobutane pyrimidine dimer (CPD) photolyases, the sister class of plant cryptochromes, while PhrB belongs to a new class represented in at least 350 bacterial organisms. Both proteins contain flavin adenine dinucleotide (FAD) as a primary catalytic cofactor, which is photoreduceable by blue light. Spectral analysis of PhrA confirmed the presence of 5,10-methenyltetrahydrofolate (MTHF) as antenna cofactor. PhrB comprises also an additional chromophore, absorbing in the short wavelength region but its spectrum is distinct from known antenna cofactors in other photolyases. Homology modeling suggests that PhrB contains an Fe-S cluster as cofactor which was confirmed by elemental analysis and EPR spectroscopy. According to protein sequence alignments the classical tryptophan photoreduction pathway is present in PhrA but absent in PhrB. Although PhrB is clearly distinguished from other photolyases including PhrA it is, like PhrA, required for in vivo photoreactivation. Moreover, PhrA can repair UV-induced DNA lesions in vitro. Thus, A. tumefaciens contains two photolyase homologs of which PhrB represents the first member of the cryptochrome/photolyase family (CPF) that contains an iron-sulfur cluster.     

The Class III Cyclobutane Pyrimidine Dimer Photolyase Structure Reveals a New Antenna Chromophore Binding Site and Alternative Photoreduction Pathways

Photolyases are proteins with an FAD chromophore that repair UV-induced pyrimidine dimers on the DNA in a light-dependent manner. The cyclobutane pyrimidine dimer class III photolyases are structurally unknown but closely related to plant cryptochromes, which serve as blue-light photoreceptors. Here we present the crystal structure of a class III photolyase termed photolyase-related protein A (PhrA) of Agrobacterium tumefaciens at 1.67-Å resolution. PhrA contains 5,10-methenyltetrahydrofolate (MTHF) as an antenna chromophore with a unique binding site and mode. Two Trp residues play pivotal roles for stabilizing MTHF by a double π-stacking sandwich. Plant cryptochrome I forms a pocket at the same site that could accommodate MTHF or a similar molecule. The PhrA structure and mutant studies showed that electrons flow during FAD photoreduction proceeds via two Trp triads. The structural studies on PhrA give a clearer picture on the evolutionary transition from photolyase to photoreceptor.     

Cloning, sequencing, expression and characterization of DNA photolyase from Salmonella typhimurium.

We have cloned the phr gene that encodes DNA photolyase from Salmonella typhimurium by in vivo complementation of Escherichia coli phr gene defect. The S.typhimurium phr gene is 1419 base pairs long and the deduced amino acid sequence has 80% identity with that of E. coli photolyase. We expressed the S.typhimurium phr gene in E.coli by ligating the E.coli trc promoter 5' to the gene, and purified the enzyme to near homogeneity. The apparent molecular weight of S.typhimurium photolyase is 54,000 dalton as determined by SDS-polyacrylamide gel electrophoresis, which is consistent with the calculated molecular weight of 53,932 dalton from the deduced phr gene product. S.typhimurium photolyase is purple-blue in color with near UV-visible absorption peaks at 384, 480, 580, and 625 nm and a fluorescence peak at 470 nm. From the characteristic absorption and fluorescence spectra and reconstitution experiments, S.typhimurium photolyase appears to contain flavin and methenyltetrahydrofolate as chromophore-cofactors as do the E.coli and yeast photolyases. Thus, S.typhimurium protein is the third folate class photolyase to be cloned and characterized to date. The binding constant of S.typhimurium photolyase to thymine dimer in DNA is kD = 1.6 x 10(-9) M, and the quantum yield of photorepair at 384 nm is 0.5.     

Purification from baker's yeast of an activator of DNA photolyase.

The activity of purified DNA photolyase from Baker's yeast is enhanced by a compound (Activator (III)) obtained from yeast by chloroform extraction ion exchange chromatography and gel filtration. Thin layer chromatography and spectral data indicate that the compound is homogeneous. Activator III emits at 350 and 440 nm when excited at 290 nm, and emits at 440 nm when excited at 358 nm. After acid hydrolysis, emission at 440 nm is produced only by excitation at 358 nm, indicating that activator (III) contains two separate chromophoric moieties. The chromophore excited by 358 nm light has a pK of 9-11, while the other chromophore has a pK of 4-5, and possibly of 9-11. The enhancement of photolytic activity by activator (III) at a concentration equimolar with that of the enzyme and the similarity of the fluorescent spectra of the activator with that of heat-denatured photolyase, suggests that the activator may be the chromophore associated with the enzyme.     

Action mechanism of Escherichia coli DNA photolyase. III. Photolysis of the enzyme-substrate complex and the absolute action spectrum

The absolute action spectrum of Escherichia coli DNA photolyase was determined in vitro. In vivo the photoreactivation cross-section (epsilon phi) is 2.4 X 10(4) M-1 cm-1 suggesting that the quantum yield (phi) is about 1.0 if one assumes that the enzyme has the same spectral properties (e.g. epsilon 384 = 1.8 X 10(4) M-1 cm-1) in vivo as those of the enzyme purified to homogeneity. The relative action spectrum of the pure enzyme (blue enzyme that contains FAD neutral semiquinone radical) agrees with the relative action spectrum for photoreactivation of E. coli, having lambda max = 384 nm. However, the absolute action spectrum of the blue enzyme yields a photoreactivation cross-section (epsilon phi = 1.2 X 10(3) at 384 nm) that is 20-fold lower than the in vivo values indicative of an apparent lower quantum yield (phi approximately equal to 0.07) in vitro. Reducing the enzyme with dithionite results in reduction of the flavin semiquinone and a concomitant 12-15-fold increase in the quantum yield. These results suggest that the flavin cofactor of the enzyme is fully reduced in vivo and that, upon absorption of a single photon in the 300-500 nm range, the photolyase chromophore (which consists of reduced FAD plus the second chromophore) donates an electron to the pyrimidine dimer causing its reversal to two pyrimidines. The reduced chromophore is regenerated at the end of the photochemical step thus enabling the enzyme to act catalytically.+     

Excited-state properties of Escherichia coli DNA photolyase in the picosecond to millisecond time scale

Escherichia coli DNA photolyase contains a stable flavin radical that is readily photoreduced in the presence of added electron donors. Picosecond, nanosecond, and conventional flash photolysis technique have been employed to investigate the events leading to photoreduction from 40 ps to tens of milliseconds following flash excitation. Direct light absorption by the flavin radical produces the first excited doublet state which undergoes rapid (within 100 ps) intersystem crossing to yield the lowest excited quartet (n pi*) state. In contrast, light absorption by the folate chromophore produces a new intermediate state via interaction of the folate excited singlet state with the ground-state flavin radical, leading to an enhanced yield of the excited radical doublet state and hence quartet state. Subsequent reaction of the excited quartet state involves hydrogen atom abstraction from a tryptophan residue. Secondary electron transfer from added electron donors occurs to the oxidized tryptophan radical with rate constants ranging from 10(4) (dithiothreitol) to 4 x 10(6) M-1 s-1 (n-propyl gallate). The low value of the latter rate compared to reduction of the tryptophan radical in lysozyme suggests that the reactive tryptophan is highly buried in photolyase. A redox potential diagram has been constructed for the ground and excited states involved. It is concluded that the one-electron reduction potential of the excited quartet state of the flavin radical must be at least 1.23 V more positive than the ground state, in agreement with the value of delta E greater than 1.77 V calculated from spectroscopic data.