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.     

Conformational change accompanies redox reactions of the tetraheme cytochrome c-554 of Nitrosomonas europaea

Cytochrome c-554 of the ammonia-oxidizing chemolithoautotropic bacteria is thought to mediate electron transfer from hydroxylamine oxidoreductase to a terminal oxidase and/or to ammonia monooxygenase. The cytochrome has four c hemes which interact magnetically and have the same redox potential. We report that the kinetics of reduction of ferric cytochrome c-554 by dithionite or the oxidation of ferrous cytochrome c-554 by O2 or H2O2 are complex and multiphasic. Transient rapid-scan difference spectra indicate discrete maxima at approximately 418 nm, 425 nm and 432 nm. Absorbance changes at all three difference maxima appear to occur in all kinetic phases, although not in equal amounts for each wavelength. Reduction by 20 mM dithionite was biphasic. At pH 7.5 the first phase, which involved approximately 50% of the total absorbance change, had a rate constant (20 degrees C) of 140 s-1 and energy of activation of 20 kJ X mol-1. The slow phase had a rate constant 0.43 s-1 and a relatively high energy of activation, 87 kJ X mol-1, suggesting that a change in protein configuration accompanied the reaction. As the pH of the solution increased, the rate constant for both phases decreased and the fraction of absorbance change in the rapid phase increased. Oxidation of ferrous cytochrome c-554 by O2 involved a discrete rapid phase with a rate constant of 14 s-1, accounting for 6% of the absorbance. The remainder of the reaction was multiphasic with rate constants in the range 0.1-0.01 s-1. With H2O2 as the oxidant, the rapid phase involved 39% of the change in absorbance with a rate constant of 19 s-1. The remainder of the reoxidation was multiphasic with rate constants ranging over 0.4-0.01 s-1.     

The influence of light quality on the phytochrome content of light grown Sinapis alba L. and Phaseolus aureus Roxb.

The effect on the phytochrome system of light regimes establishing a range of photoequilibria was studied in two light grown dicotyledonous plants, both of which were treated with the herbicide SAN 9789 to prevent chlorophyll accumulation. In Sinapis alba L. cotyledons the results are comparable with phytochrome behaviour in etiolated mustard seedlings; the level of Pfr becomes independent of wave-length whereas the total phytochrome level is wave-length dependent. Contrasting properties are exhibited in Phaseolus aureus Roxb. leaves in which total phytochrome is unaffected by light quality; consequently the Pfr level is dependent on wavelength. Nevertheless, the amount of phytochrome in mung leaves increased after transfer to darkness suggesting that light still has a profound influence on the phytochrome system, even though light quality during the light period and prior to darkness does not.Abbreviations FR far-red light - WL white light - PAR photosynthetically active radiation - Pfr far-red light absorbing form of phytochrome - Pr red light absorbing form of phytochrome - Ptot total phytochrome level (=Pr+Pfr) - Pfr/Pfr+Pr - SAN 9789 4-chloro-5-(methylamino) 2(,, trifluoro-m tolyl)-3(2H)-pyridazinone     

Phytochrome from the green alga Mesotaenium caldariorum. Purification and preliminary characterization

A method to purify the phytochrome photoreceptor from the unicellular green alga Mesotaenium caldariorum is presented. Preparative scale formation of algal protoplasts and controlled osmotic cell lysis have permitted separation of intact organelles from the phytochrome-enriched soluble protein fraction. We have utilized the observation that red light-absorbing (Pr) and far-red light-absorbing (Pfr) forms of phytochrome are differentially retained on an anion exchange matrix to purify M. caldariorum phytochrome to apparent homogeneity. M. caldariorum phytochrome preparations with A650/A280 ratios greater than 0.78 exhibit a single 120-kDa band on silver-stained sodium dodecyl sulfate-polyacrylamide gels. Immunoblot analyses using a cross-reactive pea phytochrome monoclonal antibody reveal that 1) the 120-kDa band represents the full-length polypeptide, 2) phytochrome is predominantly localized in the algal cytoplasm, and 3) there are 150,000-250,000 phytochrome molecules/cell. Steric exclusion high pressure liquid chromatography analysis under nondenaturing conditions indicates that M. caldariorum phytochrome has an apparent mass of 355 kDa. The absorption maxima for Pr and Pfr are 650 and 722 nm, respectively. Both are blue-shifted compared with those of phytochromes from dark-grown angiosperm tissue. The molar absorption coefficient for Pr at 650 nm is 86,800 +/- 2800 liter mol-1 cm-1, which is lower than that of higher plant phytochromes. The significance of the similarities and differences of the molecular properties of phytochromes from M. caldariorum and higher plant sources is discussed.     

Light quantity and quality interactions in the control of elongation growth in light-grown Chenopodium rubrum L. seedlings

The spectral control of hypocotyl elongation in light-grown Chenopodium rubrum L. seedlings has been studied. The results showed that although the seedlings responded to changes in the quantity of combined red and far-red radiation, they were also very sensitive to changes in the quantity of blue radiation reaching the plant. Altering the proportion of red: far-red radiation in broad waveband white light caused marked differences in hypocotyl extension. Comparison of the responses of green and chlorophyll-free seedlings indicated no qualitative difference in the response to any of the light sources used, although photosynthetically incompetent plants were more sensitive to all wavelengths. Blue light was found to act primarily of a photoreceptor which is different from phytochrome. It is concluded that hypocotyl extension rate in vegetation shade is photoregulated by the quantity of blue light and the proportion of red: far-red radiation. In neutral shade, such as that caused by stones or overlying soil, hypocotyl extension appears to be regulated primarily by the quantity of light in the blue waveband and secondarily by the quantity of light in the red and far-red wavebands.Abbreviations B blue - FR far-red - k ₁, k ₂ rate constants for photoconverison of Pr to Pfr and Pfr to Pr, respective - k ₁/k ₁ +k ₂= phytochrome photoequilibrium - k ₁ +k ₂= phytochrome cycling rate - Pr=R absorbing form of phytochrome - Pfr=FR absorbing form of phytochrome - P_tot Pr+Pfr - PAR photosynthetically active radiation = 400–700 nm - R red - WL white light     

Modifications of Sulfhydryl Groups on Phytochrome and Their Influence on Physicochemical Differences between the Red- and Far-Red-Absorbing Forms

Phytochrome extracted from shoots of dark-grown rye (Secale cereale cv Rymin) and oat (Avena sativa cv Garry) as the far-red-form (Pfr) and/or under conditions conducive to oxidation exhibited a blue shift in the visible absorption maximum of its red-light-absorbing form (Pr) relative to that measured in vivo. This spectral alteration could not be reversed but could be prevented by inclusion of 10 millimolar diethyldithiocarbamate and 140 millimolar 2-mercaptoethanol in homogenization buffers. Similar blue shifts were induced in purified rye phytochrome by addition of the sulfhydryl-modifying reagent, 5,5′-dithiobis-(2-nitrobenzoic acid) (DTNB). In spectrally normal phytochrome (i.e., no detectable blue shift), Pfr had three to four more sulfhydryls available for rapid reaction with DTNB than did Pr. This difference was maintained over a 2.5-hour time course. Phytochrome purified under conditions resulting in a blue-shifted Pr absorption maximum exhibited a decreased short-term reactivity of Pfr to DTNB. Comparison of the binding and elution of altered and unaltered phytochrome from agarose-immobilized Cibacron blue 3GA confirmed that the Pfr form of spectrally normal phytochrome had a greater affinity for the dye than did the Pr form but that spectral alteration of phytochrome was accompanied by a loss of this difference as evidenced by an increased binding of Pr to the dye. It was concluded that phytochrome has highly reactive sulfhydryl residues located on the portion of the protein that undergoes conformational changes on interconversion of Pr and Pfr and that these residues require rigorous protection in order to extract the native form of the protein from plant tissue.     

Degradation of phytochrome A and the high irradiance response in Arabidopsis: a kinetic analysis

The ability to respond to far‐red‐rich light is essential for seedlings germinating below dense canopies. Physiological and genetic studies have demonstrated that phytochrome A is the only photoreceptor mediating responses to far‐red light. However, all phytochromes including phytochrome A are believed to be activated by red light and to be inactivated by far‐red light. To address the fundamental question of why phytochrome A has its highest physiological activity at presumably inactivating wavelengths, we analysed light‐induced degradation of phytochrome A in Arabidopsis. Rate constants were obtained for all reaction events in a two‐step model of degradation. Based on biochemical data, the model includes a tagging mechanism preceding degradation. The parameterized model describes Pr accumulation, wavelength dependencies of degradation kinetics and steady‐state levels as well as Pfr‐induced Pr degradation. Subsequently, experimentally derived fluence rate response curves, action spectrum and response curves to dichromatic irradiation were compared to simulations based on the model of degradation. Two kinetically defined phytochrome subspecies, untagged Pfr and tagged Pr, have steady‐state levels closely matching the physiological response curves. Therefore, sensing of far‐red light by phytochrome A can be quantitatively explained based exclusively on regulated protein degradation.     

A monoclonal antibody specific for the red-absorbing form of phytochrome

Characterisation of a new monoclonal antibody (mAb), designated LAS 41, directed against 124-kilodalton (kDa) etiolated-oat (Avena sativa L.) phytochrome, indicates that it recognises an epitope unique to the red-light-absorbing form, Pr. In a solid-phase enzyme-linked immunosorbent assay (ELISA), LAS 41 exhibits a seven- to eight-fold higher affinity for Pr than for the far-red-light-absorbing form of phytochrome, Pfr. In addition, in immunoprecipitation assays LAS 41 effectively precipitates 100% of phytochrome presented as Pr but only precipitates a maximum of 24.5% of phytochrome presented as Pfr. These values are indicative of binding exclusively to Pr. Peptide-mapping studies show that LAS 41 recognises and epitope located within a region 6–10 kDa from the aminoterminus of the phytochrome molecule. Since binding of LAS 41 to Pr induces alterations in the spectral properties of Pr, this indicates that at least part of the 4 kDa domain to which the antibody binds is essential for protein-chromophore interaction. Subsequent photoconversion of LAS 41-Pr complexes produces native Pfr spectra, with concomitant production of free antibody and antigen, as shown by a modified ELISA. The specificity of LAS 41 for Pr has facilitated the purification of Pfr which is free of contaminating Pr. This has enabled direct determination of the mole fraction of Pfr established by red light to be 0.874.Abbreviations ELISA enzyme-linked immunsorbent assay - kDa kilodalton - mAb monoclonal antibody - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - SDS-PAGE sodium dodecyl sulphate polyacrylamide gel electrophoresis - (A) difference in absorbance (A ₆₆₅ ^Pr –A ₇₃₀ ^Pr )-(A ₆₆₅ ^Pfr –A ₇₃₀ ^Pfr ) - Ar/Afr spectral change ratio (SCR) - max mole fraction of Pfr following saturating red light     

A photoreversible conformational change in 124 kDa Avena phytochrome

Tryptophan (Trp) fluorescence quenching of phytochrome has been studied using anionic, cationic and neutral quenchers, I⁻, Cs⁺ and acrylamide, respectively, in an effort to understand the molecular differences between the Pr and Pfr forms. The data have been analyzed using both Stern-Volmer and modified Stern-Volmer kinetic treatments. The anionic quencher, I⁻, was proven to be an ineffective quencher with Stern-Volmer constants, K_sv, of 0.60 and 0.63 M⁻¹, respectively, for the Pr and Pfr forms of phytochrome. The cationic quencher, Cs⁺, showed about a 2-fold difference in the K_sv of Pr and Pfr, indicating a significant change in the fluorescent Trp environments during the Pr to Pfr phototransformation. However, only 25–37% of the fluorescent Trp residues were accessible to the cationic quencher. Most of the fluorescent Trp residues were accessible to acrylamide, but the quenching by acrylamide was indistinguishable for the Pr and Pfr forms. An additional quenching by acrylamide after a saturated quenching with Cs⁺ showed more than 40% increase in the K_sv of Pfr over Pr. These observations, along with the finding of two distinct components in the Trp fluorescence lifetime, indicate the existence of Trp residues in at least two different sets of environments in the phytochrome protein. The two components of the fluorescence had lifetimes of 1.1 ns (major) and 4.7 ns (minor) for Pr and 0.9 ns (major) and 4.6 ns (minor) for Pfr. Fluorescence quenching was found to be both static and dynamic as the Stern-Volmer constants for the steady-state fluorescence quenching were higher than for the dynamic fluorescence quenching. Based on the quenching results, in combination with the location of Trp residues in the primary structure, we conclude that the Pr to Pfr phototransformation involves a significant conformation change in the phytochrome molecule, preferentially in the 74 kDa chromophore-bearing domain.     

Evidence for phytochrome involvement in light-mediated stomatal movement in Phaseolus vulgaris L.

Observations made with primary leaves of Phaseolus vulgaris L. demonstrated that phytochrome modulates light-induced stomatal movement. Removal of the far-red-absorbing form of the pigment (Pfr) with far-red (FR) radiation decreased the time required by the stomata to reach maximal opening following a dark-to-light transition; this effect of FR was fully reversible with red. Removal of Pfr with FR also decreased the time required to reach maximal closure following a light-to-dark transition, and the rate of closure was dependent on the final irradiation treatment before darkness. No evidence was found for phytochrome involvement in determining stomatal aperture under constant conditions of either darkness of light.Abbreviations and symbols Chl chlorophyll - D darkness - FR far-red - phytochrome photostationary state - Pfr, Pr FR- and R-absorbing forms of phytochrome, respectively - R red     

Evidence for catalytic dismutation of superoxide by cobalt(II) derivatives of bovine superoxide dismutase in aqueous solution as studied by pulse radiolysis.

By using the technique of pulse radiolysis to generate O2-., it is demonstrated that Co(II) derivatives of bovine superoxide dismutase in which the copper alone and both the copper and zinc of the enzyme have been substituted by Co(II), resulting in (Co,Zn)- and (Co,Co)-proteins, are capable of catalytically dismutating O2-. with 'turnover' rate constants of 4.8 X 10(6) dm3.s-1.mol-1 and 3.1 X 10(6) dm3.s-1.mol-1 respectively. The activities of the proteins are independent of the pH (7.4-9.4) and are about three orders of magnitude less than that of the native (Cu,Zn)-protein. The rate constants for the initial interaction of O2-. with the Co-proteins were determined to be (1.5-1.6) X 10(9) dm3.s-1.mol-1; however, in the presence of phosphate, partial inhibition is apparent [k approximately (1.9-2.3) X 10(8) dm3.s-1.mol-1]. To account for the experimental observations, two reaction schemes are presented, involving initially either complex-formation or redox reactions between O2-. and Co(II). This is the first demonstration that substitution of a metal into the vacant copper site of (Cu,Zn)-protein results in proteins that retain superoxide dismutase activity.     

Discrimination between the red- and far-red-absorbing forms of phytochrome from Avena sativa L. by monoclonal antibodies

A set of rat monoclonal antibodies (ARC MAC 48 to 52 and 54 to 56), raised to phytochrome from dark-grown seedlings of Avena sativa L. was tested for the ability to discriminate between the red-absorbing (Pr) and far-red-absorbing (Pfr) forms of phytochrome by indirect enzyme-linked immunosorbent assay. MAC 50 bound more strongly to Pfr and MAC 49 and 52 showed preferential binding to Pr from extracts of dark-grown Avena seedlings; MAC 50 also bound more strongly to Pfr from brushite-purified phytochrome. The remainder of the monoclonal antibodies and a rabbit polyclonal antiphytochrome preparation did not discriminate between Pr and Pfr. The results provide evidence for conformational changes in defined regions of the phytochrome apoprotein upon photoconversion.Abbreviations ELISA enzyme-linked immunosorbent assay - FR far-red light - McAb monoclonal antibody(ies) - PBS phosphate-buffered saline - Pfr far-red-absorbing form of phytochrome - Pr red-absorbing form of phytochrome - R red light - PMSF phenylmethylsulphonylfluoride     

Laser flash photolysis studies of the kinetics of electron-transfer reactions of Saccharomyces flavocytochrome b2: evidence for conformational gating of intramolecular electron transfer induced by pyruvate binding

The kinetics of reduction of the flavocytochrome from Saccharomyces cerevisiae by exogenous deazaflavin semiquinones have been investigated by using laser flash photolysis. Direct reduction by deazaflavin semiquinone of both the b2 heme and the FMN cofactor occurred via second-order kinetics with similar rate constants (9 x 10(8) M-1 s-1). A slower, monoexponential, phase of FMN reoxidation was also observed, concurrent with a slow phase of heme reduction. The latter accounted for approximately 20-25% of the total heme absorbance change. Both of these slow phases were protein concentration dependent, yielding identical second-order rate constants (1.1 x 10(7) M-1 s-1), and were interpreted as resulting from intermolecular electron transfer from the FMN semiquinone on one protein molecule to an oxidized heme on a second molecule. Consistent with this conclusion, no slow phase of heme reduction was observed with deflavo-flavocytochrome b2. Upon the addition of pyruvate (but not D-lactate or oxalate), the second-order rate constant for heme reduction was unaffected, but direct reduction of the FMN cofactor was no longer observed. Reduction of the heme cofactor was followed by a slower partial reoxidation, which occurred concomitantly with a monoexponential phase of FMN reduction. Both processes were protein concentration independent and were interpreted as the result of intramolecular electron transfer from reduced b2 heme to oxidized FMN. Potentiometric titrations of the flavocytochrome in the absence and presence of pyruvate demonstrated that the thermodynamic driving force for electron transfer from FMN to heme is much greater in the absence of pyruvate. Despite this, intramolecular electron transfer was only observed in the presence of pyruvate. This result is interpreted in terms of a conformational change induced by pyruvate binding which permits electron transfer between the cofactors. The rate constant for intramolecular electron transfer in the presence of pyruvate was dependent on ionic strength, suggesting the occurrence of electrostatic effects which influence this process.     

The effect of red light in the high irradiance reaction of phytochrome:

Abstract. A proposal has recently been made (Johnson & Tasker, 1979) that phytochrome acts through the multiplicative effect of two components of the phytochrome system, only one of them being Pfr and the other a product of the photoconversion reaction Pr → Pfr . It has been shown previously that HIR action spectra based on this assumption show a good resemblance to the observed responses with the possible exception of the red region of the spectrum. There, a greater response is predicted than is often observed and fluence rate dependency is predicted, on which matter there is disagreement in the literature.
The experiments reported here show that, because chlorophyll is formed in red light but not in far-red, chlorophyll screening lowers the effective fluence rate in red light with respect to phytochrome. When such screening is eliminated the observed responses show strong fluence rate dependency, as predicted. Evidence is also presented to support the conclusion that the two proposed effectors must act together multiplicatively.     

Internal electron transfer in cytochrome c oxidase: evidence for a rapid equilibrium between cytochrome a and the bimetallic site.

Internal electron-transfer reactions in cytochrome oxidase following flash photolysis of the CO compounds of the enzyme reduced to different degrees (2-4 electron equiv) have been followed at 445, 605, and 830 nm. Apart from CO dissociation and recombination, two kinetic phases are seen both at 445 and at 605 nm with rate constants of 2 x 10(5) and 1.3 x 10(4) s-1, respectively; at 605 nm, an additional phase with a rate constant of 400 s-1 is resolved. At 830 nm, only the second reaction phase (rate constant of 1.3 x 10(4) s-1) is observed. The amplitude of the first phase is largest with the two-electron-reduced enzyme, whereas that of the second phase is maximal at the three-electron-reduction level. Neither phase shows any marked pH dependence. The reaction in the first phase has a free energy of activation of 41 kJ mol-1 and an entropy of activation of -14 JK-1 mol-1. Analysis suggests that the two rapid reaction phases represent internal electron redistributions between the bimetallic site and cytochrome a, and between cytochrome a and CuA, respectively. The slow phase (400 s-1) probably involves a structural rearrangement.     

Effect of malformin on phytochrome reactions in Vigna and Avena

The rate of destruction of the far red absorbing form of phytochrome(Pfr) in green or etiolated cuttings of Vigna radiata was slowerin the presence of malformin than in its absence. Malforminhad no effect on the accumulation of total phytochrome in thedark, or on the reaccumulation of phytochrome after destructionin red light. The amount of photoconversion of the red absorbingform of phytochrome (Pr) to Pfr or Pfr to Pr by given dosesof red or far red radiation was slightly but consistently lessin malformin-treated cuttings of V. radiata than in controls.Malformin had no effect on the rate of destruction or photoconversionof phytochrome in etiolated shoots of Avena sativa. The decreasein destruction rate of Pfr by malformin in V. radiata may contributeto the inhibition of dark abscission by malformin after lighttreatment. (Received October 3, 1979; )     

Temperature effects on the MgATP-induced electron transfer between the nitrogenase proteins from Azotobacter vinelandii.

The temperature dependence of the pre-steady-state MgATP-dependent electron transfer from the MoFe protein to the Fe protein of the nitrogenase from Azotobacter vinelandii has been investigated between 6 degrees C and 31 degrees C by stopped-flow spectrophotometry. Below 14 degrees C, the data are consistent with a model in which interaction of MgATP with nitrogenase is fast and irreversible, and is followed by reversible electron transfer. From the extent and from the rate of the absorbance change, the rate constants for electron transfer from Fe protein to MoFe protein and of the reverse reaction were calculated. The direct rate constant increases with temperature (6-14 degrees C) from about 1 s-1 to about 26 s-1. The rate constant for the reverse reaction was found to be approximately 4 s-1 and invariant with the reaction temperature. Analysis of the data obtained in the temperature range between 6 degrees C and 12 degrees C within the framework of the transition-state theory show that electron transfer from the Fe protein to the MoFe protein occurs via a highly disordered transition state with activation parameters delta H(0) ++ = 289 kJ.mol-1 and delta S(0) ++ = 792 J.K-1.mol-1. The Eyring plot of the stopped-flow data displays an inflection point around 14 degrees C. From the stopped-flow data obtained between 18 degrees and 27 degrees C the activation parameters delta H(0) ++ and delta S(0) ++ for the reduction of the MoFe protein by Fe protein are calculated to be 90 kJ.mol-1 and 99 J.K-1.mol-1 respectively. A second inflection point in the Eyring plot could exist around 28 degrees C.     

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.     

Spectroscopic properties and chromophore conformations of the photomorphogenic receptor: phytochrome.

Fluorescence lifetimes of 'large (mol. wt. 120,000) and 'small' (mol. wt. 60,000) phytochromes isolated from oat and rye seedlings grown in the dark have been measured at 199 K and 298 K. Phytochrome model compounds have also been studied by phase modulation fluorometrically at 77 K for comparison with lifetime data for phytochrome. It was found that the fluorescence lifetime of 'large' phytochrome was significantly shorter than that of 'small' phytochrome and its chromophore models. The phytochrome chromophore of Pr form has been analyzed by fluorescence polarization, CD, and molecular orbital methods. The fluorescence excitation polarization of 'small' phytochrome and the chromophore model in buffer/glycerol mixture (3 : 1, v/v) at 77 K is very hight (0.4) at the main absorption band and is negative (--0.1) and close to 0 in the near ultraviolet band, respectively. Analyses of the spectroscopic data suggest that the chromophore conformation of Pr and Pfr forms of phytochrome are essentially identical. The induced ellipticity of 'large' rye phytochrome in the blue and near ultraviolet regions was found to be significantly higher than that of 'small' phytochrome, indicating that the binding interaction between the phytochrome chromophore and apoprotein is much tighter in the former than in the latter. In addition, the excitation energy transfer does occur from Trp residue(s) to the chromophore in 'large' phytochrome but not in 'small' Pr. This illustrates one feature of the role played by the large molecular weight apoprotein in the binding site interactions and primary photoprocesses of Pr. Finally, a plausible model for the primary photoprocesses and the mechanism of phytochrome interactions triggered by the Pr leads to Pfr phototransformation have been proposed on the basis of the above results.     

Integral association of phytochrome with a membranous fraction fromAvena shoots: in vivo characterization and physiological significance

Phytochrome in the far-red light absorbing form (Pfr) was observed to disappear in vivo more rapidly from the non-cation-requiring pelletable phytochrome population than from the supernantant phytochrome population of oat seedlings given an increasing dark incubation after red irradiation. The amount of pelletable phytochrome in the red light absorbing form (Pr) remained relatively stable while supernatant Pr was lost. These observations indicated that supernant Pfr was subject to loss during the incubation, while pelletable Pfr was subject to both dark reversion and loss.During the incubation, the ability of far-red irradiation to reverse the red-induced increase in phytochrome pelletability was lost, with kinetics similar to those of the loss of pelletable Pfr.Far-red reversibility of the red-induced increase in coleoptile elongation correlated with the change intotal Pfr in both supernatant and pelletable phytochrome populations, but with the change in the ratio of Pfr to total phytochrome only in the pelletable phytochrome population.The possible significance of these results is discussed with reference to the action of phytochrome in the photocontrol of physiological growth responses.Abbreviations Pfr phytochrome in the far-red light absorbing form - Pr phytochrome in the red absorbing form - Ptot total phytochrome