A PHYTOCHROME STUDY USING TWO-LASER/TWO-COLOR FLASH PHOTOLYSIS: I700IS A MANDATORY INTERMEDIATE IN THE PrPfrPHOTOTRANSFORMATION

Kinetic and thermodynamic analysis of the light-induced processes in plant and cyanobacterial phytochromes

The light-induced processes of the biological photoreceptor phytochrome (recombinant phyA of oat and recombinant CphA from the cyanobacterium Tolypothrix PCC7601) have been investigated in a time-resolved manner in the temperature range from 0 to 30°C. Both proteins were heterologously expressed and assembled in vitro with phycocyanobilin. The Pr state of plant phytochrome phyA is converted to the Pfr state after formation of four intermediates with an overall quantum yield of ~18%. The reversal reaction (Pfr-to-Pr) shows several intermediates, all of which, even the first detectable one, exhibit already all spectral features of the Pr state. The canonical phytochrome CphA from Tolypothrix showed a similar intermediate sequence as its plant ortholog. Whereas the kinetics for the forward reaction (Pr-to-Pfr) was nearly identical for both proteins, the reverse process (Pr formation) in the cyanobacterial phytochrome was slower by a factor of three. As found for the Pfr-to-Pr intermediates in the plant protein, also in CphA all detectable intermediates showed the spectral features of the Pr form. For both phytochromes, activation parameters for both the forward and the backward reaction pathways were determined.

Femtosecond kinetics of photoconversion of the higher plant photoreceptor phytochrome carrying native and modified chromophores

The photoprocesses of native (phyA of oat), and of C-terminally truncated recombinant phytochromes, assembled instead of the native phytochromobilin with phycocyanobilin (PCB-65 kDa-phy) and iso-phycocyanobilin (iso-PCB-65 kDa-phy) chromophores, have been studied by femtosecond transient absorption spectroscopy in both their red absorbing phytochrome (P(r)) and far-red absorbing phytochrome (P(fr)) forms. Native P(r) phytochrome shows an excitation wavelength dependence of the kinetics with three main picosecond components. The formation kinetics of the first ground-state intermediate I(700), absorbing at approximately 690 nm, is mainly described by 28 ps or 40 ps components in native and PCB phytochrome, respectively, whereas additional approximately 15 and 50 ps components describe conformational dynamics and equilibria among different local minima on the excited-state hypersurface. No significant amount of I(700) formation can be observed on our timescale for iso-PCB phytochrome. We suggest that iso-PCB-65 kDa-phy either interacts with the protein differently leading to a more twisted and/or less protonated configuration, or undergoes P(r) to P(fr) isomerization primarily via a different configurational pathway, largely circumventing I(700) as an intermediate. The isomerization process is accompanied by strong coherent oscillations due to wavepacket motion on the excited-state surface for both phytochrome forms. The femto- to (sub-)nanosecond kinetics of the P(fr) forms is again quite similar for the native and the PCB phytochromes. After an ultrafast excited-state relaxation within approximately 150 fs, the chromophores return to the first ground-state intermediate in 400-800 fs followed by two additional ground-state intermediates which are formed with 2-3 ps and approximately 400 ps lifetimes. We call the first ground-state intermediate in native phytochrome I(fr 750), due to its pronounced absorption at that wavelength. The other intermediates are termed I(fr 675) and pseudo-P(r). The absorption spectrum of the latter already closely resembles the absorption of the P(r) chromophore. PCB-65 kDa-phy shows a very similar kinetics, although many of the detailed spectral features in the transients seen in native phy are blurred, presumably due to wider inhomogeneous distribution of the chromophore conformation. Iso-PCB-65 kDa-phy shows similar features to the PCB-65 kDa-phy, with some additional blue-shift of the transient spectra of approximately 10 nm. The sub-200 fs component is, however, absent, and the picosecond lifetimes are somewhat longer than in 124 kDa phytochrome or in PCB-65 kDa-phy. We interpret the data within the framework of two- and three-dimensional potential energy surface diagrams for the photoisomerization processes and the ground-state intermediates involved in the two photoconversions.

Ultrafast dynamics of phytochrome from the cyanobacterium synechocystis, reconstituted with phycocyanobilin and phycoerythrobilin

Femtosecond time-resolved transient absorption spectroscopy was employed to characterize for the first time the primary photoisomerization dynamics of a bacterial phytochrome system in the two thermally stable states of the photocycle. The 85-kDa phytochrome Cph1 from the cyanobacterium Synechocystis PCC 6803 expressed in Escherichia coli was reconstituted with phycocyanobilin (Cph1-PCB) and phycoerythrobilin (Cph1-PEB). The red-light-absorbing form Pr of Cph1-PCB shows an approximately 150 fs relaxation in the S(1) state after photoexcitation at 650 nm. The subsequent Z-E isomerization between rings C and D of the linear tetrapyrrole-chromophore is best described by a distribution of rate constants with the first moment at (16 ps)(-1). Excitation at 615 nm leads to a slightly broadened distribution. The reverse E-Z isomerization, starting from the far-red-absorbing form Pfr, is characterized by two shorter time constants of 0.54 and 3.2 ps. In the case of Cph1-PEB, double-bond isomerization does not take place, and the excited-state lifetime extends into the nanosecond regime. Besides a stimulated emission rise time between 40 and 150 fs, no fast relaxation processes are observed. This suggests that the chromophore-protein interaction along rings A, B, and C does not contribute much to the picosecond dynamics observed in Cph1-PCB but rather the region around ring D near the isomerizing C(15) [double bond] C(16) double bond. The primary reaction dynamics of Cph1-PCB at ambient temperature is found to exhibit very similar features as those described for plant type A phytochrome, i.e., a relatively slow Pr, and a fast Pfr, photoreaction. This suggests that the initial reactions were established already before evolution of plant phytochromes began.

Time-resolved thermodynamic analysis of the oat phytochrome A phototransformation. A photothermal beam deflection study

The time-resolved enthalpy and the structural volume changes after excitation of native oat phytochrome A were studied in the micro- to milliseconds range by photothermal beam deflection (PBD), a technique that follows the time-resolved refractive index changes upon decay of the excited species. The first set of intermediates, I700(1) and I700(2), stores ca 83% of the energy of the first excited state, in agreement with previous optoacoustic data, whereas the second set stores only ca 18%. The temperature dependence of the amplitudes ratio for the optical absorbances of the (I700(1) + I700(2)) intermediates set is explained on the basis of the thermochromic equilibrium between Pr,657 and Pr,672, which also is in line with the present PBD data. These data were best fitted with a parallel mechanism (with equal yield in each branch) for the production of the first set of intermediates, I700(1) and I700(2), as well as the second set of intermediates, Ibl1 and Ibl2. Thus, the final steps toward Pfr should be largely driven by positive entropic changes brought about by protein movements, in line with previous resonance Raman data. For the production of the first set of intermediates (I700(1) and I700(2)) an expansion of 18 +/- 13 mL mol-1 was determined, and a further expansion > or = 7 mL mol-1 was estimated for the decay from I700(1) to the set of Ibl intermediates, indicating that the far red-absorbing form of phytochrome (Pfr) has a larger volume than the red-absorbing form of phytochrome. This is in agreement with previous chromatographic and circular dichroism data according to which Pfr shows a larger volume and the chromophore shows a higher accessibility, respectively, in the Pfr state.

Differential effects of mutations in the chromophore pocket of recombinant phytochrome on chromoprotein assembly and Pr-to-Pfr photoconversion

Site-directed mutagenesis was performed with the chromophore-bearing N-terminal domain of oat phytochrome A apoprotein (amino acid residues 1-595). Except for Trp366, which was replaced by Phe (W366F), all the residues exchanged are in close proximity to the chromophore-binding Cys321 (i.e. P318A, P318K, H319L, S320K, H322L and the double mutant L323R/Q324D). The mutants were characterized by their absorption maxima, and the kinetics of chromophore-binding and the Pr-->Pfr conversion. The strongest effect of mutation on the chromoprotein assembly, leading to an almost complete loss of the chromophore binding capability, was found for the exchanges of His322 by Leu (H322L) and Pro318 by Lys (P318K), whereas a corresponding alanine mutant (P318A) showed wild-type behavior. The second histidine (H319) is also involved in chromophore fixation, as indicated by a slower assembly rate upon mutation (H319L). For the other mutants, an assembly process very similar to that of the wild-type protein was found. The light-induced Pr-->Pfr conversion kinetics is altered in the mutations H319L and S320K and in the double mutant L323R/Q324D, all of which exhibited a significantly faster I700 decay and accelerated Pfr formation. P318 is also involved in the Pr-->Pfr conversion, the millisecond steps (formation of Pfr) being significantly slower for P318A. Lacking sufficient amounts of W366F, assembly kinetics could not be determined in this case, while the fully assembled mutant underwent the Pr-->Pfr conversion with kinetics similar to wild-type protein.

Recombinant type A and B phytochromes from potato. Transient absorption spectroscopy

The cDNAs encoding full-length type A and B phytochromes (phyA and phyB, respectively) from potato were expressed in inducible yeast systems (Saccharomyces cerevisiae and Pichia pastoris). In addition, a deletion mutant of phyB (delta 1-74) was expressed. The apoproteins were reconstituted into chromoproteins by incorporation of the native chromophore, phytochromobilin (P phi B), and of phycocyanobilin (PCB). The incorporation of P phi B yielded chromoproteins with difference absorptions lambda max at 660 and 712 nm (Pr and Pfr, respectively) for phyA, and at 665 and 723 nm for phyB. All difference maxima of PCB phytochromes are blue-shifted by several nanometers with respect to those obtained with the P phi B chromophore. The deletion construct with PCB shows difference absorption maxima at 652 and 705 nm with the Pfr absorbance considerably reduced. Time-resolved kinetic analysis of a phyB-type phytochrome by nanosecond flash photolysis was performed for the first time. Recombinant full-length phyB afforded transient absorbance changes similar (but not identical) to those of phyA from Avena, whereas the kinetic behavior of these intermediates was very different. Contrary to phyA from Avena, the I700 intermediate from phyB reconstituted with either PCB or P phi B decayed following single exponential kinetics with a lifetime of 87 or 84 microseconds, respectively, at 10 degrees C. The formation of Pfr of PCB-containing recombinant phyB (phyB-PCB) could be fitted with three lifetimes of 9, 127, and 728 ms. The corresponding lifetimes of phyB-P phi B are 22.5, 343, and 2083 ms. Whereas for phyB-PCB all three millisecond lifetimes are related to the formation of Pfr, the 2 s component of phyB-P phi B is concomitant with a rapid recovery of Pr. For recombinant potato phyA and delta 1-74 phyB, no time-resolved data could be obtained due to the limited quantities available. As described for phytochromes of other dicotelydons, the Pfr forms of full-length phyA and PhyB of potato underwent rapid dark conversion to Pr.

Fourier-transform infrared spectroscopy of phytochrome: difference spectra of the intermediates of the photoreactions

The photocycle of 124 kDa phytochrome A from Avena sativa was studied by Fourier-transform infrared spectroscopy at low temperatures. Difference spectra between the parent state Pr and the intermediates of the Pr-->Pfr pathway, i.e. lumi-R, meta-Ra, and meta-Rc, and between Pfr and the intermediates of the Pfr-->Pr pathway, lumi-F and meta-F, were obtained in 1H2O and 2H2O for the first time. Each spectrum shows characteristic spectral features which allow a clear distinction between the different intermediates. A general feature is that greater changes occur with increasing temperature, i.e. at the later steps of the photoreactions. Nevertheless, the changes in the spectral regions of the protein (amide I and amide II) were found to be surprisingly small, excluding larger conformational changes of the protein. All spectra of the intermediates are characterized by a strong negative band around 1700 cm-1. This band is tentatively assigned to the C = O stretch of ring D of the chromophore. Since it is not observed in the difference spectra between the parent states, it is concluded that ring D is located in a similar molecular environment in Pr and Pfr. In the photoproducts lumi-R and lumi-F, this band undergoes an upshift to 1720 cm-1. The high frequencies suggest that the chromophore is protonated in these intermediates as well as in Pr and Pfr.

Chromophore-protein interaction controls the complexity of the phytochrome photocycle

A new protocol for the preparation of recombinant phytochromes results in significantly higher yields which, for the first time, have made kinetic studies possible. Flash photolysis with nanosecond laser excitation reveals that, in recombinant and native phytochromes, the decay kinetics of the primary photoproducts I700i and the kinetics of the formation of the Pfr form are similar. Phycocyanobilin-containing recombinant phytochrome, however, shows only a monoexponential decay of the I700 intermediate with a time constant of approximately 90 microseconds, and a biexponential formation of the Pfr form, albeit with time constants (approximately 13 and 100 ms) somewhat shorter than those from native phytochrome. Thus the seemingly small structural modification of the chromophore (substitution of the native vinyl for an ethyl group) has a profound influence on the availability of protein conformational rearrangement pathways. The result is therefore of general interest in chromoprotein dynamics.

Effects of bound monoclonal antibodies on the decay of the phototransformation intermediates I700(1,2) from native Avena phytochrome

The kinetics of the microsecond phototransformation intermediates of 124 kDa Avena phytochrome (I700(1,2) were studied in the presence of bound monoclonal antibodies at various temperatures. A global analysis was applied to the decays at all wavelengths at each temperature in order to derive the rate constants and the decay-associated spectra of the three decay components. Monoclonal antibodies bound to specific epitopes altered the Arrhenius parameters of both I700(1,2) decay components. The strongest influence on these parameters was observed with OAT 8 (epitope between residues 624 and 686), which decreased by more than 50% the activation parameters of both components. This decrease is interpreted to result from an increased flexibility induced by this antibody in the ground state or in the transition state of bonds changing during the decay of both I700 transients. Thus, the OAT 8 epitope appears to be functionally important during the decay of the I700(1,2) intermediates. For the case of I700(1), bound OAT 23 and OAT 25 (epitopes between residues 1 and 66) reduced even further the relatively small flexibility of these bonds in the red light-absorbing form of phytochrome (Pr) without antibodies, as reflected by the high preexponential factors for its decay. This resulted also in higher activation energies for this decay in the presence of the antibodies. Thus, the amino-terminus should act as a rigid spacer of the chromophore cavity without affecting it during the microsecond transformation, because the Arrhenius parameters for these decays are similar to those for small phytochrome. The possible implications of the influence of the various antibodies on the bleaching remaining after the decay of I700(1,2) are discussed.