Phototaxis and photophobic responses in Stentor coeruleus. Action spectrum and role of Ca2+ fluxes

Negative phototactic orientation, step-up photophobic responses and light-induced action potentials have been studied in the ciliate Stentor coeruleus. A resolved action spectrum, based on fluence rate-response curves, is consistent with stentorin as the photoreceptor. Calcium flux blockers prolong the response time for ciliary stop and reversal and inhibit step-up photophobic responses. Drugs believed to affect the membrane-bound calcium pump likewise inhibit phobic responses. On the other hand, alpha-phosphatidic acid promotes Ca2+-influx and enhances the photophobic sensitivity of the organism, thus providing an unambiguous evidence for the role of Ca2+ influx. A change in the response time decreases the degree of phototactic orientation, indicating that negative phototaxis in this organism is brought about by subsequent phobic responses of individual rows of cilia as the associated photoreceptor granules experience an increase in light intensity when the organism rotates during forward locomotion in lateral light.

Tetranitromethane oxidation of phytochrome chromophore as a function of spectral form and molecular weight

Tetranitromethane bleaches Avena phytochrome. The phytochrome (far-red absorbing form; Pfr) chromophore of 124 kilodalton (kD) phytochrome is oxidized 8 times more rapidly than the red absorbing form (Pr). Proteolysis of the 124 kD molecule to the extensively studied mixture of 118 and 114 kD polypeptides increases the rate of oxidation of Pfr 5-fold without affecting the rate of Pr oxidation. As a result, the Pfr form of 118/114 kD preparations is oxidized at a rate 40 times greater than the Pr form. Further proteolytic degradation of the chromoprotein to 60 kD results in an additional increase in the oxidation rates of both Pr and Pfr. These differences in reactivity to tetranitromethane indicate that the chromophore of Pfr is either intrinsically more chemically reactive and/or physically more accessible than the Pr chromophore and that the reactivity/accessibility of both spectral forms is increased by proteolysis. The enhanced reactivity of the Pfr chromophore after proteolytic cleavage of the 6 to 10 kD polypeptide segment(s) from the 124 kD species is further evidence that these segment(s) affect the environment of the native photoreceptor.

The photobinding of 5,7-dimethoxycoumarin to adenovirus type-2 DNA. A method for in vitro mutagenesis

The photobinding of 5,7-dimethoxycoumarin to isolated adenovirus-type 2 DNA has been investigated with respect to the influence of the ionic environment, and varying molar ratios of DNA(p): 5,7-dimethoxycoumarin. In particular, the ultraviolet radiation-induced covalent addition of 5,7-dimethoxycoumarin to adenovirus DNA was increased by reducing the concentration of Na+. The maximum photobinding of 5,7-[3H]dimethoxycoumarin to adenovirus DNA under the given ionic condition was one 5,7-dimethoxycoumarin per 101 nucleotides. Moreover, restriction enzyme analysis of the 5,7-dimethoxycoumarin-DNA photoadduct versus unmodified viral DNA, suggested that the sequence d(A-T) is the preferential site for intercalation and subsequent photobinding of 5,7-dimethoxycoumarin. This susceptibility of d(A-T) sequences to 5,7-dimethoxycoumarin interaction has a corresponding influence on the survival of adenovirus because of the A-T-rich sequences that occur in some of the early gene regions of the adenovirus genome. Specifically, 5,7-dimethoxycoumarin per 800 nucleotides in adenovirus DNA reduced the surviving fraction of adenovirus to a value of 0.1 after DNA infectivity (transfection) into human 293 cells. Results suggest that 5,7-dimethoxycoumarin may be used for generating a limited 'library' of mutations in each of the five early gene regions of the adenovirus genome.

Photocarcinogenicity of 8-methoxypsoralen and aflatoxin B1 with longwave ultraviolet light

The carcinogenicity of 8-methoxypsoralen (8-MOP) and aflatoxin B1 (AFB1) with longwave ultraviolet light (UVA) to hairless mouse skin was investigated. Skin tumors were induced efficiently by 8-MOP + UVA, and the time to 50% tumor incidence was about 24 weeks. Histopathologically, some tumors were squamous cell carcinomas. AFB1 did not show any phototoxic and photocarcinogenic effects on mouse skin in this study. Although the structure and the photoreactivity of AFB1 to DNA were similar to those of 8-MOP, the photocarcinogenic response of these compounds to mouse skin was quite different.

Nature of phototransformation of phytochrome As probed by intrinsic tryptophan residues

The phototransformation of the photomorphogenic photoreceptor phytochrome was probed by the intrinsic luminescence of the tryptophan (Trp) residues. The red light absorbing form of phytochrome (Pr) showed a decreased tryptophan phosphorescence intensity, compared to that of the far-red light absorbing form of phytochrome (Pfr), and a delayed fluorescence from the chromophore upon excitation of the tryptophan residues with 290-nm light. The tryptophan phosphorescence in both Pr and Pfr showed decreased lifetimes (0.29 and 1.84 s, respectively) compared to that of the free tryptophan (6.00 s). In addition, the decay kinetics of the delayed fluorescence in Pr showed a short-lifetime component (0.24 s), which is similar to the tryptophan phosphorescence lifetime value. This is due to an efficient triplet-singlet (3Trp-1Pr) energy transfer in the Pr from. The increases in the tryptophan phosphorescence quantum yield and lifetime in the Pfr form have been interpreted on the basis of chromophore reorientation on the protein surface as a result of the Pr lead to Pfr phototransformation. The Stern-Volmer plot of the quenching data further confirms preferential exposure of the tryptophan residues in the Pfr form (46% "exposed' tryptophan residues in the Pr form as compared to 72% in the Pfr form). These results provide strong support for the hydrophobic model of Pfr [Hahn, T. R., & Song, P. S. (1981) Biochemistry 20, 2602-2609).

Molecular topography of phytochrome as deduced from the tritium-exchange method

The hydrogen-tritium-exchange measurements on phytochrome have been performed to detect the conformational differences between the red-absorbing (Pr) and the far-red absorbing (Pfr) forms of phytochrome. The large and small Pfr molecules revealed more exchangeable protons that did the corresponding Pr molecules by 96 and 70 protons, respectively. These results suggest that the Pr leads to Pfr phototransformation is accompanied by an additional exposure of the peptide chains in the Pfr molecule. Of 1682 theoretically exchangeable hydrogens in undegraded phytochrome, only 442 (26%) and 346 (21%) protons were found to be exchangeable (excluding instantaneously exchangeable protons that cannot be determined by the present method). Thus, the phytochrome protein appears to be compact and highly folded. The kinetic analyses of the tritium exchange-out curves indicate that two kinetically different groups are responsible for the conformational differences between the Pr and Pfr forms of phytochrome. These components are due to (1) the exposure of hydrogen-bonded peptide segments (alpha helix and/or beta-pleated sheet) in the chromophore vicinity of Pfr and (2) the exposure of hydrogen-bonded peptide segments on the chromophore peptide domain as well as on the chromophore-free tryptic domain of undegraded phytochrome.

Binding of phytochrome to liposomes and protoplasts

The physiologically active form of oat phytochrome, Pfr, increases its binding to egg lecithin unilamellar liposomes with increasing ionic strength of the medium while the binding of Pr is almost constant. The preferential binding of Pfr is as much as twice that of Pr at KCl concentrations of above 0.2 M, in 0.1 M phosphate buffer (pH 7.2) at 27 degrees C. The binding of phytochrome to liposomes is also enhanced by approximately 80% at 27 degrees C compared to that at 3 degrees C. Thus, it appears that the binding between Pfr phytochrome and liposomes is hydrophobic in nature whereas the binding of Pr is not predominantly through hydrophobic interactions. The binding of both Pfr and Pr to multilamellar liposomes increases with increasing cholesterol content in the liposomes. The extent of phytochrome's binding is higher in the neutral pH region than above pH 7.5. It takes several hours to reach an equilibrium of binding. The photoreversion of liposome-bound Pfr is inhibited by 40% compared to that of free Pfr, while the phototransformation of liposome-bound Pr to Pfr is promoted by 30%. The rate of dithionite-accelerated dark reversion of liposome-bound Pfr is lower by 50% than that of the free form. These results are consistent with the proposal that the hydrophobic binding site involved results from a vacancy produced by the reorientation or displacement of the Pfr chromophore from the protein. Upon binding to phytochrome, unilamellar liposomes undergo fusion to form larger diameter liposomes. No preferential binding of the Pfr form was found with intact oat protoplasts in vitro.

Proton release from Stentor photoreceptors in the excited states

Steady-state and picosecond pulse excitations of the photophobic-phototactic receptors isolated from Stentor coeruleus produced anionic species predominantly in the excited singlet state, although neutral photoreceptors in the ground state were exclusively excited. The same photoreceptor in vivo also emits fluorescence from the excited state of its anionic species, with an excitation spectrum identical to the absorption spectrum of the neutral species in the ground state. The excited state dissociation of protons from the photoreceptor chromophore (stentorin; hypericin covalently linked to protein) efficiently occurs in less than 10 ps. A possible role of the transient-proton release from the photoreceptor, in the signal transduction photoresponse of Stentor, is briefly discussed.

Phototransformation and dark reversion of phytochrome in deuterium oxide

The photostationary equilibrium between the Pr and Pfr forms of phytochrome shows a strong solvent deuterium isotope effect. Phytochrome transformation from the Pr to the Pfr form exhibits a small deuterium isotope effect, in Tris-D2O upon irradiation with red light, only after a photocycling of the phytochrome. In contrast, both the photoreversion and dark reversion of Pfr show an enhanced rate in D2O. In addition to the shift in the photostationary equilibrium in D2O, another pronounced effect of D2O on phytochrome is reflected in a significant enhancement of the fluorescence quantum yield of phytochrome (Pr). This result is interpreted in terms of the primary reaction involving an intramolecular proton transfer and its consequence in the phototransformation of phytochrome. It is further proposed that a tyrosyl residue acts as a general acid catalyst in the Pr to Pfr phototransformation, which is slower in D2O than in H2O. The D2O solvent isotope effect on the photoreversion and dark reversion of Pfr is explained on the basis of acid catalysis, probably a specific acid catalysis by deuteronium ion.

Hydrophobic properties of phytochrome as probed by 8-anilinonaphthalene-1-sulfonate fluorescence

8-Anilinonaphthalene-1-sulfonate (ANS) complexes with phytochrome, exhibiting a higher affinity for the Pfr form of phytochrome than for the Pr form. ANS fluorescence is enhanced by the additional binding of ANS to Pfr upon transformation of phytochrome from Pr to Pfr. The specific site of ANS binding appears to be the hydrophobic surface area of the protein, which becomes at least partially exposed in the Pfr form. An exposed, hydrophobic surface area in the Pfr phytochrome has been confirmed by the effects of ANS on the phototransformation of phytochrome. ANS accelerates the Pr leads to Pfr phototransformation, and it inhibits Pfr leads to Pr photoreversion and dark reversion. These effects are interpretable in terms of competitive binding of ANS to the chromophore binding site. Binding of ANS results in a drastic bleaching of the chromophore's absorption bands at 660 and 730 nm, particularly of the latter. This can be attributed to the exposed chromophore, which tends to resume a cyclic conformation with concomitant blue shift and hypochromism of the Qy bands. Sodium dithionite counteracts the inhibitory effects of ANS on the dark reversion of Pfr to Pr, and its effect on the biphasic kinetics of the reversion has been discussed in terms of the Pfr model proposed.

Primary photoprocesses of undegraded phytochrome excited with red and blue light at 77 K

1. Red light irradiation of phytochrome (Pr) at 77 K produces an intermediate absorbing at 696 nm. The photostationary state concentration of this intermediate is rapidly established with that of Pr as the result of spectral overlap between the Qy band of Pr and the Qx band of the intermediate. 2. The 696 nm intermediate reverts back to Pr preferentially without yielding a substantial amount of Pfr upon thawing the 77 K sample to higher temperatures. 3. Blue light irradiation of Pr with or without exogenous FMN at 77 K results in the formation of two intermediates absorbing at 684 nm and 696 nm. The 684 nm intermediate is photochemically converted to the 696 nm intermediate at 77 K. Possibilities for the preferential formation of the 684 nm intermediate with blue light are discussed. 4. At 277 K, blue light irradiation of phytochrome (Pr) containing exogenous FMN increases the rate of phototransformation from Pr to Pfr three times over Pr having no FMN. On the other hand, exogenous FMN has no effect on the rate of transformation of Pr to Pfr by red light. 5. Energy transfer occurs from FMN to Pr at 77 K, initiating the photoprocesses of the Pr. The energy transfer apparently occurs within flavin-phytochrome complexes.

The pH dependence of photosensory responses in Stentor coeruleus and model system

1. Live Stentor coeruleus exhibits a substantially red-shifted fluorescence maximum, corresponding to the anionic species of the photoreceptor chromophore. No change was observed in either the absorption or fluorescence excitation spectrum, indicating an efficient deprotonation of the photoreceptor pigment upon excitation by light. 2 Changes in external pH exhibit a dramatic effect on the pulmonary response of Stentor. Phototaxis is specifically inhibited at pH less than 6, with loss of photosensory perception which is restored when the pH is returned to pH greater than 6. 3. Fluorescence changes of 9-aminoacridine in suspensions of live Stentor indicate the generation of a pH gradient upon irradiation with light. Both pH gradient and phototaxis were inhibited by the addition of nigericin and p-tri-fluoromethoxy carbonyl cyanide phenylhydrazone (FCCP). 4. Incorporation of the Stentor photoreceptor protein in to artificial liposomes demonstrates the ability of the system to generate pH gradients across model membranes as monitored by the quenching of 9-aminoacridine fluorescence. The effect of external pH on net proton movement in the model system is strikingly similar to the pH dependent of the liver Stentor, thus lending support for transient proton flux being an important mode of light signal processing for photosensory transduction.

Spectroscopic characterization of beta-lactoglobulin-retinol complex

1. The absorption spectrum of retinol when bound to beta-lactoglobulin is vibrationally resolved. The circular dichroism spectrum exhibits the same structure, as does the fluorescence excitation spectrum. 2. Two molecules of retinol are bound per protein dimer, with a binding constant (Kd) of 2 x 10(-8) M. Also, by fluorescence titration it was found that the monomer binds one molecule of retinol with essentially the same Kd. 2. Energy transfer occurs from tryptophan (donor) to retinol (acceptor) with a rate constant, k, of 4.4 x 10(8) s-1. The distance between the centers of mass of the transition is 34 A, corresponding to the energy transfer efficiency of 44%. 4. The fluoresence lifetime of retinol increases dramatically on binding to beta-lactoglobulin, from approx. 2 to approx. 10 ns, as does the fluorescence quantum yield. 5. The retinol binding to beta-lactoglobulin does not show a pH dependence and the binding site is hydrophobic. 6. On the Sephadex G-100 column, retinol is chemically modified to a retro derivative which binds even more strongly to beta-lactoglobulin than does retinol. 7. The beta-lactoglobulin-retinol complex rotates anisotropically in solution with a fast (3 ns) and a slower (12 ns) component. This may be attributed to retinol being found at a flexible region of the protein, where only segmental flexibility is observed, weighted by its proximity to one of the major axis rotational times.

Effect of hydrogen bonding on electronic spectra and reactivity of flavins

Riboflavin tetrabutyrate undergoes characteristic spectral changes, in both the first and second absorption band regions, upon hydrogen bonding with trichloroacetic acid of trifluoroacetic acid. On the basis of the calculated electron densities, hydrogen bonding at the heteroatoms of the isoalloxazine nucleus is considered to occur with increasing concentrations of the proton donor, first at N(1), then at O(12), O(14), and N(3)H, and finally at N(5). The idea that the major effect of the hydrogen bonding at the N(1), N(3)H, and oxygen atoms of the flavin nucleus is to facilitate the electrophilicity of the N(5) position, which was predicted by molecular orbital calculations, was supported by the observation that the hydrogen-bonded flavin in its triplet state abstracts hydrogen from the donor N-benzyl-n,n'-dimethylethylenediamine at a faster rate than do the non-hydrogen-bonded species in CCI4. The implications of the present study in the spectroscopic and catalytic properties of flavoproteins are briefly discussed.

Photochemically induced binding of aflatoxins to DNA and its effects on template activity

The covalent binding of photoactivated aflatoxin B1 ((AFB1) to DNA and its effect on template activity have been investigated. AFB1 in its ground state complexes more preferentially with denatured DNA than with native DNA. The covalent linkage between 3H-AFB1 and DNA under near-ultraviolet light irradiation shows 1 AFB1 per 529 Pi of calf thymus DNA. The photoaddition also induces a conformational change of the DNA, as detected by circular dichroism. Because the photobinding of AFB2 to DNA is negligible (1 AFB2 per 5485 Pi), we suggest that the 8,9-C=C bond is the major binding site of AFB1. The AFB1-DNA adduct shows a substantial inhibition of its template activity for DNA synthesis (by 52%) and RNA transcription (by 74%) in vitro. Since both AFB1 and AFB2 themselves had little inhibitory effect on the activities of DNA and RNA polymerases, the alteration of DNA by photoactivated AFB1 and AFB2 is responsible for the dramatic reduction of template activity in DNA and RNA synthesis. Short-chain polynucleotides retained by type VS membrane filter (0.025-micrometers pore size) indicate that premature chain termination occurred in transcribing the AFB1-DNA and AFB2-DNA adducts as the result of disrupted movement of the enzymes along the DNA chain.

Spectroscopic characterization of the Stentor photoreceptor

1. On the basis of chromatographic and spectroscopic (absorption, fluorescence and its polarization, fluorescence lifetime, circular dichroism) characterization of the Stentor photoreceptor (stentorin) for photophobic response, the photoreceptor chromophore released from mild acid hydrolysis has been identified as hypericin. 2. The native chromophore is apparently linked to a protein (65 K) containing Lys and several hydrophobic residues, which is soluble in acetone and n-pentane. The peptide-linked stentorin (I) chromophore exhibits circular dichroism in the visible region due to the induced optical activity provided by the peptide. 3. The sodium dodecyl sulfate polyacrylamide gel electrophoresis of a 38% fraction of the sucrose density centrifugation has resolved stentorin II proteins having molecular weights of 13 000, 16 000, 65 000 and 130 000. These proteins, as well as the acetone-soluble peptide, have been spectroscopically characterized with particular emphasis on their primary photoreactivity as the photophobic receptor of Stentor coeruleus. 4. Irradiation of whole living Stentor in dilute buffer solutions induces a decrease in the pH of the medium. A strong dependence upon pH in the fluorescence spectra of both synthetic and native chromophores is also evident, showing a significant drop in the pKa of one or more hydroxyl groups in the excited state. A mechanism for the photophobic response, based on this lowering of the pKa as the primary photoprocess, has been discussed.