Identifying the lowest electronic states of the chlorophylls in the CP47 core antenna protein of photosystem II

CP47 is a pigment-protein complex in the core of photosystem II that tranfers excitation energy to the reaction center. Here we report on a spectroscopic investigation of the isolated CP47 complex. By deconvoluting the 77 K absorption and linear dichroism, red-most states at 683 and 690 nm have been identified with oscillator strengths corresponding to approximately 3 and approximately 1 chlorophyll, respectively. Both states contribute to the 4 K emission, and the Stark spectrum shows that they have a large value for the difference polarizability between their ground and excited states. From site-selective polarized triplet-minus-singlet spectra, an excitonic origin for the 683 nm state was found. The red shift of the 690 nm state is most probably due to strong hydrogen bonding to a protein ligand, as follows from the position of the stretch frequency of the chlorophyll 13(1) keto group (1633 cm(-)(1)) in the fluorescence line narrowing spectrum at 4 K upon red-most excitation. We discuss how the 683 and 690 nm states may be linked to specific chlorophylls in the crystal structure [Zouni, A., Witt, H.-T., Kern, J., Fromme, P., Krauss, N., Saenger, W., and Orth, P. (2001) Nature 409, 739-743].     

Evidence for two spectroscopically different dimers of light-harvesting complex I from green plants

A preparation consisting of isolated dimeric peripheral antenna complexes from green plant photosystem I (light-harvesting complex I or LHCI) has been characterized by means of (polarized) steady-state absorption and fluorescence spectroscopy at low temperatures. We show that this preparation can be described reasonably well by a mixture of two types of dimers. In the first dimer about 10% of all Q(y)() absorption of the chlorophylls arises from two chlorophylls with absorption and emission maxima at about 711 and 733 nm, respectively, whereas in the second about 10% of the absorption arises from two chlorophylls with absorption and emission maxima at about 693 and 702 nm, respectively. The remaining chlorophylls show spectroscopic properties comparable to those of the related peripheral antenna complexes of photosystem II. We attribute the first dimer to a heterodimer of the Lhca1 and Lhca4 proteins and the second to a hetero- or homodimer of the Lhca2 and/or Lhca3 proteins. We suggest that the chlorophylls responsible for the 733 nm emission (F-730) and 702 nm emission (F-702) are excitonically coupled dimers and that F-730 originates from one of the strongest coupled pair of chlorophylls observed in nature.     

Photophysical behavior and assignment of the low-energy chlorophyll states in the CP43 proximal antenna protein of higher plant photosystem II

We have employed absorption, circular dichroism (CD), and persistent spectral hole-burning measurements at 1.7 K to study the photoconversion properties and exciton coupling of low-energy chlorophylls (Chls) in the CP43 proximal antenna light-harvesting subunit of photosystem II (PSII) isolated from spinach. These approximately 683 nm states act as traps for excitation energy in isolated CP43. They "bleach" at 683 nm upon illumination and photoconvert to a form absorbing in the range approximately 660-680 nm. We present new data that show the changes in the CD spectrum due to the photoconversion process. These changes occur in parallel with those in absorption, providing evidence that the feature undergoing the apparent bleach is a component of a weakly exciton-coupled system. From our photoconversion difference spectra, we assign four states in the Chl long-wavelength region of CP43, two of which are the known trap states and are both highly localized on single Chls. The other two states are associated with weak exciton coupling (maximally approximately 50 cm(-)(1)) to one of these traps. We propose a mechanism for photoconversion that involves Chl-protein hydrogen bonding. New hole-burning data are presented that indicate this mechanism is distinct to that for narrow-band spectral hole burning in CP43. We discuss the photophysical behavior of the Chl trap states in isolated CP43 compared to their behavior in intact PSII preparations. The latter represent a more intact, physiological complex, and we find no clear evidence that they exhibit the photoconversion process reported here.     

Electronic spectra of PS I mutants: the peripheral subunits do not bind red chlorophylls in Synechocystis sp. PCC 6803

Steady-state fluorescence and absorption spectra have been obtained in the Qy spectral region (690-780 nm and 600-750 nm, respectively) for several subunit-deficient photosystem I mutants from the cyanobacterium Synechocystis sp. PCC 6803. The 77 K fluorescence spectra of the wild-type and subunit-deficient mutant photosystem I particles are all very similar, peaking at approximately 720 nm with essentially the same excitation spectrum. Because emission from far-red chlorophylls absorbing near 708 nm dominates low-temperature fluorescence in Synechocystis sp., these pigments are not coordinated to any the subunits PsaF, Psa I, PsaJ, PsaK, PsaL, or psaM. The room temperature (wild-type-mutant) absorption difference spectra for trimeric mutants lacking the PsaF/J, PsaK, and PsaM subunits suggest that these mutants are deficient in core antenna chlorophylls (Chls) absorbing near 685, 670, 675, and 700 nm, respectively. The absorption difference spectrum for the PsaF/J/I/L-deficient photosystem I complexes at 5 K reveals considerably more structure than the room-temperature spectrum. The integrated absorbance difference spectra (when normalized to the total PS I Qy spectral area) are comparable to the fractions of Chls bound by the respective (groups of) subunits, according to the 4-A density map of PS I from Synechococcus elongatus. The spectrum of the monomeric PsaL-deficient mutant suggests that this subunit may bind pigments absorbing near 700 nm.     

Pathways for energy transfer in the core light-harvesting complexes CP43 and CP47 of photosystem II

The pigment-protein complexes CP43 and CP47 transfer excitation energy from the peripheral antenna of photosystem II toward the photochemical reaction center. We measured the excitation dynamics of the chlorophylls in isolated CP43 and CP47 complexes at 77 K by time-resolved absorbance-difference and fluorescence spectroscopy. The spectral relaxation appeared to occur with rates of 0.2-0.4 ps and 2-3 ps in both complexes, whereas an additional relaxation of 17 ps was observed only in CP47. Using the 3.8-A crystal structure of the photosystem II core complex from Synechococcus elongatus (A. Zouni, H.-T. Witt, J. Kern, P. Fromme, N. Krauss, W. Saenger, and P. Orth, 2001, Nature, 409:739-743), excitation energy transfer kinetics were calculated and a Monte Carlo simulation of the absorption spectra was performed. In both complexes, the rate of 0.2-0.4 ps can be ascribed to excitation energy transfer within a layer of chlorophylls near the stromal side of the membrane, and the slower 2-3-ps process to excitation energy transfer to the calculated lowest excitonic state. We conclude that excitation energy transfer within CP43 and CP47 is fast and does not contribute significantly to the well-known slow trapping of excitation energy in photosystem II.     

The Origin of the Low-Energy Form of Photosystem I Light-Harvesting Complex Lhca4: Mixing of the Lowest Exciton with a Charge-Transfer State

The peripheral light-harvesting complex of photosystem I contains red chlorophylls (Chls) that, unlike the typical antenna Chls, absorb at lower energy than the primary electron donor P700. It has been shown that the red-most absorption band arises from two excitonically coupled Chls, although this interaction alone cannot explain the extreme red-shifted emission (25 nm, ∼480 cm⁻¹ for Lhca4 at 4 K) that the red Chls present. Here, we report the electric field-induced absorption changes (Stark effect) on the Q_y region of the Lhca4 complex. Two spectral forms, centered around 690 nm and 710 nm, were necessary to describe the absorption and Stark spectra. The analysis of the lowest energy transition yields a high value for the change in dipole moment, Δμ_710nm ≈ 8 Df⁻¹, between the ground and excited states as compared with monomeric, Δμ = 1 D, or dimeric, Δμ = 5 D, Chl a in solution. The high value of the Δμ demonstrates that the origin of the red-shifted emission is the mixing of the lowest exciton state with a charge-transfer state of the dimer. This energetic configuration, an excited state with charge-transfer character, is very favorable for the trapping and dissipation of excitations and could be involved in the photoprotective mechanism(s) of the photosystem I complex.     

Two-photon excited fluorescence from higher electronic states of chlorophylls in photosynthetic antenna complexes: a new approach to detect strong excitonic chlorophyll a/b coupling

Stepwise two-photon excitation of chlorophyll a and b in the higher plant main light-harvesting complex (LHC II) and the minor complex CP29 (as well as in organic solution) with 100-fs pulses in the Q(y) region results in a weak blue fluorescence. The dependence of the spectral shape of the blue fluorescence on excitation wavelength offers a new approach to elucidate the long-standing problem of the origin of spectral "chlorophyll forms" in pigment-protein complexes, in particular the characterization of chlorophyll a/b-heterodimers. As a first result we present evidence for the existence of strong chlorophyll a/b-interactions (excitonically coupled transitions at 650 and 680 nm) in LHC II at ambient temperature. In comparison with LHC II, the experiments with CP29 provide further evidence that the lowest energy chlorophyll a transition (at approximately 680 nm) is not excitonically coupled to chlorophyll b.     

Origin of the F685 and F695 fluorescence in Photosystem II

The emission spectra of CP47-RC and core complexes of Photosystem II (PS II) were measured at different temperatures and excitation wavelengths in order to establish the origin of the emission and the role of the core antenna in the energy transfer and charge separation processes in PS II. Both types of particles reveal strong dependences of spectral shape and yield on temperature. The results indicate that the well-known F-695 emission at 77 K arises from excitations that are trapped on a red-absorbing CP47 chlorophyll, whereas the F-685 nm emission at 77 K arises from excitations that are transferred slowly from 683 nm states in CP47 and CP43 to the RC, where they are trapped by charge separation. We conclude that F-695 at 77 K originates from the low-energy part of the inhomogeneous distribution of the 690 nm absorbing chlorophyll of CP47, while at 4 K the fluorescence originates from the complete distribution of the 690 nm chlorophyll of CP47 and from the low-energy part of the inhomogeneous distribution of one or more CP43 chlorophylls.     

Spectroscopic properties of PSI-IsiA supercomplexes from the cyanobacterium Synechococcus PCC 7942

The cyanobacterium Synechococcus PCC 7942 grown under iron starvation assembles a supercomplex consisting of a trimeric Photosystem I (PSI) complex encircled by a ring of 18 CP43' or IsiA light-harvesting complexes [Nature 412 (2001) 745]. Here we present a spectroscopic characterization by temperature-dependent absorption and fluorescence spectroscopy, site-selective fluorescence spectroscopy at 5 K, and circular dichroism of isolated PSI-IsiA, PSI and IsiA complexes from this cyanobacterium grown under iron starvation. The results suggest that the IsiA ring increases the absorption cross-section of PSI by about 100%. Each IsiA subunit binds about 16-17 chlorophyll a (Chl a) molecules and serves as an efficient antenna for PSI. Each of the monomers of the trimeric PSI complex contains two red chlorophylls, which presumably give rise to one exciton-coupled dimer and at 5 K absorb and fluoresce at 703 and 713 nm, respectively. The spectral properties of these C-703 chlorophylls are not affected by the presence of the IsiA antenna ring. The spectroscopic properties of the purified IsiA complexes are similar to those of the related CP43 complex from plants, except that the characteristic narrow absorption band of CP43 at 682.5 nm is missing in IsiA.     

Deletion Mutagenesis of the Cytochrome b559 Protein Inactivates the Reaction Center of Photosystem II

In green plant-like photosynthesis, oxygen evolution is catalyzed by a thylakoid membrane-bound protein complex, photosystem II. Cytochrome b559, a protein component of the reaction center of this complex, is absent in a genetically engineered mutant of the cyanobacterium, Synechocystis 6803 [Pakrasi, H.B., Williams, J.G.K., and Arntzen, C.J. (1988). EMBO J. 7, 325-332]. In this mutant, the genes psbE and psbF, encoding cytochrome b559, were deleted by targeted mutagenesis. Two other protein components, D1 and D2 of the photosystem II reaction center, are also absent in this mutant. However, two chlorophyll-binding proteins, CP47 and CP43, as well as a manganese-stabilizing extrinsic protein component of photosystem II are stably assembled in the thylakoids of this mutant. Thus, this deletion mutation destabilizes the reaction center of photosystem II only. The mutant also lacks a fluorescence maximum peak at 695 nm (at 77 K) even though the CP47 protein, considered to be the origin of this fluorescence peak, is present in this mutant. We propose that the fluorescence at 695 nm originates from an interaction between the reaction center of photosystem II and CP47. The deletion mutant shows the absence of variable fluorescence at room temperature, indicating that its photosystem II complex is photochemically inactive. Also, photoreduction of QA, the primary acceptor quinone in photosystem II, could not be detected in the mutant. We conclude that cytochrome b559 plays at least an essential structural role in the reaction center of photosystem II.     

Selective Formation of Photosystem 1 under Illumination at Low Intensity

Analyses of chlorophylls a and b and P700 in the wheat leaves grown for 8 days under illumination with white light at different intensities suggested selective formation of photosystem 1 of the photosynthesis at low light intensities. This was confirmed for the two types of chloroplasts isolated from leaves grown at light intensities of 1.1 and 240 μ W/cm², respectively, by measuring their pigment compositions, activities of photosystems 1 and 2, and absorption and fluorescence spectra. The chloroplasts developed at the low intensity showed properties only of photosystem 1 while those developed at the high intensity showed properties of both photosystems 1 and 2. Only photosystem 1 particles were obtained by fractionation of low intensity chloroplasts by treatment with digitonin followed by centrifugation, while high intensity chloroplasts could be fractionated into photosystem-1 and photosystem-2 particles. When the leaves grown at low light intensity were illuminated with strong light, photosystem 2 was developed. The fluorescence emission spectrum of low intensity chloroplasts at 77°K showed two peaks at 685 and 734 nm, and the spectrum of high intensity chloroplasts showed three peaks at 685, 697 and 740 nm.     

Analyses of absorption and fluorescence spectra of water-soluble chlorophyll proteins, pigment system II particles and chlorophyll a in diethylether solution by the curve-fitting method.

Absorption and fluorescence spectra in the red region of water-soluble chlorophyll proteins, Lepidium CP661, CP663 and Brassica CP673, pigment System II particles of spinach chloroplasts and chlorophyll a in diethylether solution at 25 degrees C were analyzed by the curve-fitting method (French, C.S., Brown, J.S. and Lawrence, M.C. (1972) Plant Physiol 49, 421--429). It was found that each of the chlorophyll forms of the chlorophyll proteins and the pigment System II particles had a corresponding fluorescence band with the Stokes shift ranging from 0.6 to 4.0 nm. The absorption spectrum of chlorophyll a in diethylether solution was analyzed to one major band with a peak at 660.5 nm and some minor bands, while the fluorescence spectrum was analyzed to one major band with a peak at 664.9 nm and some minor bands. A mirror image was clearly demonstrated between the resolved spectra of absorption and fluorescence. The absorption spectrum of Lepidium CP661 was composed of a chlorophyll b form with a peak at 652.8 nm and two chlorophyll a forms with peaks at 662.6 and 671.9 nm. The fluorescence spectrum was analyzed to five component bands. Three of them with peaks at 654.8, 664.6 and 674.6 nm were attributed to emissions of the three chlorophyll forms with the Stokes shift of 2.0--2.7 nm. The absorption spectrum of Brassica CP673 had a chlorophyll b form with a peak at 653.7 nm and four chlorophyll a forms with peaks at 662.7, 671.3, 676.9 and 684.2 nm. The fluorescence spectrum was resolved into seven component bands. Four of them with peaks at 666.7, 673.1, 677.5 and 686.2 nm corresponded to the four chlorophyll a forms with the Stokes shift of 0.6--4.0 nm. The absorption spectrum of the pigment System II particles had a chlorophyll b form with a peak at 652.4 nm and three chlorophyll a forms with peaks at 662.9, 672.1 and 681.6 nm. The fluorescence spectrum was analyzed to four major component bands with peaks at 674.1, 682.8, 692.0 and 706.7 nm and some minor bands. The former two bands corresponded to the chlorophyll a forms with peaks at 672.1 and 681.6 nm with the Stokes shift of 2.0 and 1.2 nm, respectively. Absorption spectra at 25 degrees C and at --196 degrees C of the water-soluble chlorophyll proteins were compared by the curve-fitting methods. The component bands at --196 degrees C were blue-shifted by 0.8--4.1 nm and narrower in half widths as compared to those at 25 degrees C.     

Energy transfer of aromatic amino acids in photosystem 2 core antenna complexes CP43 and CP47

Energy transfer of aromatic amino acids in photosystem 2 (PS2) core antenna complexes CP43 and CP47 was studied using absorption spectroscopy, fluorescence spectroscopy, and the 0.35 nm crystal structure of PS2 core complex. The energy of tyrosines (Tyrs) was not effectively transferred to tryptophans (Trps) in CP43 and CP47. The fluorescence emission spectrum of CP43 and CP47 by excitation at 280 nm should be a superposition of the Tyr and Trp fluorescence emission spectra. The aromatic amino acids in CP43 and CP47 could transfer their energy to chlorophyll (Chl) a molecules by the Dexter mechanism and the Föster mechanism, and the energy transfer efficiency in CP47 was much higher than that in CP43. In CP47 the Föster mechanism must be the dominant energy transfer mechanism between aromatic amino acids and Chl a molecules, whereas in CP43 the Dexter mechanism must be the dominant one. Hence solar ultraviolet radiation brings not only damages but also benefits to plants.     

Stepwise two-photon excited fluorescence from higher excited states of chlorophylls in photosynthetic antenna complexes

Stepwise two-photon excited fluorescence (TPEF) spectra of the photosynthetic antenna complexes PCP, CP47, CP29, and light-harvesting complex II (LHC II) were measured. TPEF emitted from higher excited states of chlorophyll (Chl) a and b was elicited via consecutive absorption of two photons in the Chl a/b Qy range induced by tunable 100-fs laser pulses. Global analyses of the TPEF line shapes with a model function for monomeric Chl a in a proteinaceous environment allow distinction between contributions from monomeric Chls a and b, strongly excitonically coupled Chls a, and Chl a/b heterodimers/-oligomers. The analyses indicate that the longest wavelength-absorbing Chl species in the Qy region of LHC II is a Chl a homodimer with additional contributions from adjacent Chl b. Likewise, in CP47 a spectral form at approximately 680 nm (that is, however, not the red-most species) is also due to strongly coupled Chls a. In contrast to LHC II, the red-most Chl subband of CP29 is due to a monomeric Chl a. The two Chls b in CP29 exhibit marked differences: a Chl b absorbing at approximately 650 nm is not excitonically coupled to other Chls. Based on this finding, the refractive index of its microenvironment can be determined to be 1.48. The second Chl b in CP29 (absorbing at approximately 640 nm) is strongly coupled to Chl a. Implications of the findings with respect to excitation energy transfer pathways and rates are discussed. Moreover, the results will be related to most recent structural analyses.     

Analyses of absorption and fluorescence spectra of water-soluble chlorophyll proteins,pigment System II particles and chlorophyll a in diethylether solution by the curve-fitting method

Absorption and fluorescence spectra in the red region of water-soluble chlorophyll proteins, Lepidium CP661, CP663 and Brassica CP673, pigment System II particles of spinach chloroplasts and chlorophyll a in diethylether solution at 25°C were analyzed by the curve-fitting method (French, C.S., Brown, J.S. and Lawrence, M.C. (1972) Plant Physiol. 49, 421–429). It was found that each of the chlorophyll forms of the chlorophyll proteins and the pigment System II particles had a corresponding fluorescence band with the Stokes shift ranging from 0.6 to 4.0 nm.The absorption spectrum of chlorophyll a in diethylether solution was analyzed to one major band with a peak at 660.5 nm and some minor bands, while the fluorescence spectrum was analyzed to one major band with a peak at 664.9 nm and some minor bands. A mirror image was clearly demonstrated between the resolved spectra of absorption and fluorescence. The absorption spectrum of Lepidium CP661 was composed of a chlorophyll b form with a peak at 652.8 nm and two chlorophyll a forms with peaks at 662.6 and 671.9 nm. The fluorescence spectrum was analyzed to five component bands. Three of them with peaks at 654.8, 664.6 and 674.6 nm were attributed to emissions of the three chlorophyll forms with the Stokes shift of 2.0–2.7 nm. The absorption spectrum of Brassica CP673 had a chlorophyll b form with a peak at 653.7 nm and four chlorophyll a forms with peaks at 662.7, 671.3, 676.9 and 684.2 nm. The fluorescence spectrum was resolved into seven component bands. Four of them with peaks at 666.7, 673.1, 677.5 and 686.2 nm corresponded to the four chlorophyll a forms with the Stokes shift of 0.6–4.0 nm. The absorption spectrum of the pigment System II particles had a chlorophyll b form with a peak at 652.4 nm and three chlorophyll a forms with peaks at 662.9, 672.1 and 681.6 nm. The fluorescence spectrum was analyzed to four major component bands with peaks at 674.1, 682.8, 692.0 and 706.7 nm and some minor bands. The former two bands corresponded to the chlorophyll a forms with peaks at 672.1 and 681.6 nm with the Stokes shift of 2.0 and 1.2 nm, respectively.Absorption spectra at 25°C and at ?196°C of the water-soluble chlorophyll proteins were compared by the curve-fitting method. The component bands at ?196°C were blue-shifted by 0.8–4.1 nm and narrower in half widths as compared to those at 25°C.     

Differential changes in degradation of chlorophyll-protein complexes of photosystem I and photosystem II during flag leaf senescence of rice.

The stability of chlorophyll-protein complexes of photosystem I (PSI) and photosystem II (PSII) was investigated by chlorophyll (Chl) fluorescence spectroscopy, absorption spectra and native green gel separation system during flag leaf senescence of two rice varieties (IIyou 129 and Shanyou 63) grown under outdoor conditions. During leaf senescence, photosynthetic CO(2) assimilation rate, carboxylase activity of Rubisco, chlorophyll and carotenoids contents, and the chlorophyll a/b ratio decreased significantly. The 77 K Chl fluorescence emission spectra of thylakoid membranes from mature leaves had two peaks at around 685 and 735 nm emitting mainly from PSII and PSI, respectively. The total Chl fluorescence yields of PSI and PSII decreased significantly with senescence progressing. However, the decrease in the Chl fluorescence yield of PSI was greater than in the yield of PSII, suggesting that the rate of degradation in chlorophyll-protein complexes of PSI was greater than in chlorophyll-protein complexes of PSII. The fluorescence yields for all chlorophyll-protein complexes decreased significantly with leaf senescence in two rice varieties but the extents of their decrease were significantly different. The greatest decrease in the Chl fluorescence yield was in PSI core, followed by LHCI, CP47, CP43, and LHCII. These results indicate that the rate of degradation for each chlorophyll-protein complex was different and the order for the stability of chlorophyll-protein complexes during leaf senescence was: LHCII>CP43>CP47>LHCI>PSI core, which was partly supported by the green gel electrophoresis of the chlorophyll-protein complexes.     

Heat stress induces an inhibition of excitation energy transfer from phycobilisomes to photosystem II but not to photosystem I in a cyanobacterium Spirulina platensis.

The effects of high temperature (30-52.5 degrees C) on excitation energy transfer from phycobilisomes (PBS) to photosystem I (PSI) and photosystem II (PSII) in a cyanobacterium Spirulina platensis grown at 30 degrees C were studied by measuring 77 K chlorophyll (Chl) fluorescence emission spectra. Heat stress had a significant effect on 77 K Chl fluorescence emission spectra excited either at 436 or 580 nm. In order to reveal what parts of the photosynthetic apparatus were responsible for the changes in the related Chl fluorescence emission peaks, we fitted the emission spectra by Gaussian components according to the assignments of emission bands to different components of the photosynthetic apparatus. The 643 and 664 nm emissions originate from C-phycocyanin (CPC) and allophycocyanin (APC), respectively. The 685 and 695 nm emissions originate mainly from the core antenna complexes of PSII, CP43 and CP47, respectively. The 725 and 751 nm band is most effectively produced by PSI. There was no significant change in F725 and F751 during heat stress, suggesting that heat stress had no effects on excitation energy transfer from PBS to PSI. On the other hand, heat stress induced an increase in the ratio of Chl fluorescence yield of PBS to PSII, indicating that heat stress inhibits excitation energy transfer from PBS to PSII. However, this inhibition was not associated with an inhibition of excitation energy transfer from CPC to APC since no significant changes in F643 occurred at high temperatures. A dramatic enhancement of F664 occurring at 52.5 degrees C indicates that excitation energy transfer from APC to the PSII core complexes is suppressed at this temperature, possibly due to the structural changes within the PBS core but not to a detachment of PBS from PSII, resulting in an inhibition of excitation energy transfer from APC to PSII core complexes (CP47 + CP43). A decrease in F685 and F695 in heat-stressed cells with excitation at 436 nm seems to suggest that heat stress did not inhibit excitation energy transfer from the Chl a binding proteins CP47 and CP43 to the PSII reaction center and the decreased Chl fluorescence yields from CP43 and CP47 could be explained by the inhibition of the energy transfer from APC to PSII core complexes (CP47 + CP43).     

Chlorophyll-protein composition of the thylakoid membrane from Prochlorothrix hollandica, a prokaryote containing chlorophyll b

The chlorophyll-protein complexes of the thylakoid membrane from Prochlorothrix hollandica were identified following electrophoresis under nondenaturing conditions. Five complexes, CP1-CP5, were resolved and these green bands were analyzed by spectroscopic and immunological methods. CP1 contains the photosystem I (PSI) reaction center, as this complex quenched fluorescence at room temperature, and had a 77 K fluorescence emission peak at 717 nm. CP4 contains the major chlorophyll-a-binding proteins of the photosystem II (PSII) core, because this complex contained polypeptides which cross-reacted to antibodies raised against Chlamydomonas PSII proteins 5 and 6. Furthermore, fluorescence excitation studies at 77 K indicated that only a Chl a is bound to CP4. Complexes CP2, CP3 and CP5 contained functionally bound Chl a and b as judged by absorption spectroscopy at 20 degrees C and fluorescence excitation spectra at 77 K. CP2, CP3 and CP5 all contain polypeptides of 30-33 kDa which are immunologically distinct from the LHC-II complex of higher plant thylakoids.     

Effects of light quality on chlorophyll-forms Ca 684, Ca 690 and Ca 699 of the diatom Phaeodactylum tricornutum

Colored light modifies the relative concentration of chlorophyll-forms of the diatom Phaeodactylum tricornutum compared to white-light control. No change in the ratio carotenoids/chlorophylls was observed after 4 days exposure to green light (max: 530 nm), blue light (max: 470 nm) or red light ( > 650 nm) of same intensity.However, the absorption spectra were modified, the content in Ca 684, Ca 690, Ca 699 forms increased in red and green light cultures and photosynthetic unit size of PS II decreased by 30% in green and blue light cultures.Fluorescence emission and fluorescence excitation spectra according to the Butler and Kitajima method (1975) were carried out for each culture. Ca 669 form was predominant in the two photosystems. The newly appeared far red forms fluoresce at 715 nm like PS I forms.We conclude that these new forms originated in a rearrangement of PS II forms. They do not transmit excitation energy to reaction center of PS I and are disconnected from the other chlorophyll-forms of the photosynthetic antennae.Abbreviations ABS absorption - Ca chlorophyll-complex - chla chlorophyll a - chl c chlorophyll c - chl t total chlorophylls - D.C.M.U. 3-(3, 4 dichlorophenyl) 1-diméthyl-urea - d^v division - F fluorescence - PS I and PS II photosystem I and photosystem II     

The highly abundant chlorophyll-protein complex of iron-deficient Synechococcus sp. PCC7942 (CP43') is encoded by the isiA gene.

A chlorophyll (Chl)-protein complex designated CPVI-4 becomes the major pigment-protein complex in Synechococcus sp. PCC7942 cells grown under conditions of iron limitation. Work by Laudenbach et al. (J Bacteriol [1988] 170: 5018-5026) has identified an iron-repressible operon, designated isiAB, containing the flavodoxin gene and a gene predicted to encode a Chl-binding protein resembling CP43 of photosystem II. To test the hypothesis that the CP43-like protein is a component of the CPVI-4 complex, we have inactivated the isiAB operon in Synechococcus sp. PCC7942 using directed insertional mutagenesis. Mutant cells grown under conditions of iron limitation exhibit pronounced changes in their spectroscopic and photosynthetic properties relative to similarly grown wild-type cells. Notably, the strong 77 K fluorescence emission at 685 nm, which dominates the spectrum of iron-deficient wild-type cells, is dramatically reduced in the mutant. The loss of this emission appears to unmask the otherwise obscured photosystem II emissions at 685 and 695 nm. Most importantly, mildly denaturing gel electrophoresis shows that mutant cells no longer express the CPVI-4 complex, indicating that the isiA gene encodes a component of this abundant Chl-protein complex.