Changes in Stoichiometry among PSI, PSII and Cyt b6-f Complexes in Response to Chromatic Light for Cell Growth Observed with the Red Alga Porphyra yezoensis

Stoichiometry among 3 thylakoid components, PSI and PSII andCyt b6-f complexes, was determined with the red alga Porphyrayezoensis with special reference to the regulation of PSI/PSIIstoichiometry in response to light regime. The ratio of PSIto PSII abundance was four times greater in thalli grown underorange light which excites mainly phycobilisome, thus PSII,than that under red light which excites preferentially Chl a,thus PSI. Cyt b₆-f abundance remained almost constant. The PSIand PSII content was regulated separately under the two growthlight conditions as was also observed with the red alga Porphyridiumcruentum by Cunningham et al. [(1990) Plant Physiol. 93: 888].This differs from the cyanophyte Synechocystis PCC 6714 whereadjustment occurs only in the PSI content [(1987) Plant CellPhysiol. 28: 1547]. However, results on the marine cyanophyteSynechococcus NIBB 1071 indicate that changes in the PSI/PSIIsoichiometry is similar to red algae. In this species, as inthe red algae, more than one PSII is associated with each phycobilisome.The light regime also induced changes in the phycobiliproteincomposition in Porphyra yezoensis. Under PSII light, phycoerythrinincreased, and phycocyanin decreased, while under PSI lightthe response was reversed. The change suggests an occurrenceof complementary chromatic adaptation. (Received April 8, 1994; Accepted June 1, 1994)     

Cyanobacterial Acclimation to Photosystem I or Photosystem II Light

The organization and function of the photochemical apparatus of Synechococcus 6301 was investigated in cells grown under yellow and red light regimes. Broadband yellow illumination is absorbed preferentially by the phycobilisome (PBS) whereas red light is absorbed primarily by the chlorophyll (Chl) pigment beds. Since PBSs are associated exclusively with photosystem II (PSII) and most of the Chl with photosystem I (PSI), it follows that yellow and red light regimes will create an imbalance of light absorption by the two photosystems. The cause and effect relationship between light quality and photosystem stoichiometry in Synechococcus was investigated. Cells grown under red light compensated for the excitation imbalance by synthesis/assembly of more PBS-PSII complexes resulting in high PSII/PSI = 0.71 and high bilin/Chl = 1.30. The adjustment of the photosystem stoichiometry in red light-grown cells was necessary and sufficient to establish an overall balanced absorption of red light by PSII and PSI. Cells grown under yellow light compensated for this excitation imbalance by assembly of more PSI complexes, resulting in low PSII/PSI = 0.27 and low bilin/Chl = 0.42. This adjustment of the photosystem stoichiometry in yellow light-grown cells was necessary but not quite sufficient to balance the absorption of yellow light by the PBS and the Chl pigment beds. A novel excitation quenching process was identified in yellow light-grown cells which dissipated approximately 40% of the PBS excitation, thus preventing over-excitation of PSII under yellow light conditions. It is hypothesized that State transitions in O₂ evolving photosynthetic organisms may serve as the signal for change in the stoichiometry of photochemical complexes in response to light quality conditions.     

The Accumulation of Protochlorophyllide in Cells of Synechocystis PCC 6714 with a Low PSI/PSII Stoichiometry

Changes in intracellular levels of Chl a precursors were examinedin relation to changes in the PSI/PSII stoichiometry in thecyanophyte Synechocystis PCC 6714. Protochlorophyllide (Pchlide)accumulated markedly in cells with a low PSI/PSII stoichiometrygrown under light that is absorbed by Chl a (PSI light) whereasno accumulation occurred in cells with a high PSI/PSII stoichiometrygrown under light absorbed by phycobilisomes (PSII light). Levelsof Pchlide in cells grown under PSI light decreased rapidlyupon a shift to PSII light. The rapid decrease in Pchlide accompanieda transient increase in chlorophyllide a, indicating that reductionof Pchlide was enhanced by shift to PSII light. The action spectrumindicated that the Pchlide decrease upon the shift to PSII lightdepended on excitation of Pchlide, suggesting that the accumulationof Pchllide was due to limited excitation of Pchlide, so thatPchlide photoreduction, under PSI light. However, comparisonof levels of Pchlide and the photosystem complexes in wild-typePlectonema boryanum with those in a mutant that lacked the darkPchlide reductase (YFC 1004) indicated that dark reduction compensatedfor the limited photoreduction under PSI light. Similar compensationby dark reduction was confirmed with Synechocystis PCC 6714.In cultures of Synechocystis under conditions where Pchlidecould not be photoreduced, accumulation of Pchlide and low PSI/PSIIstoichiometry occurred only when cells were illuminated withlight that preferentially excited PSI. The results indicatethat the low PSI/PSII stoichiometry in cells grown under PSIlight is not a result of inefficient synthesis of Chl a witha reduced rate of Pchlide photoreduction. They suggest furtherthat accumulation of Pchlide under PSI light results from retardationof the Chl a synthesis due to suppression of PSI synthesis. ¹Present address: Tsurukawa 5-15-11, Machida, Tokyo, 195 Japan.     

Effects of Chromatic Adaptation on the Photochemical Apparatus of Photosynthesis in Porphyridium cruentum

Cells of Porphyridium cruentum were grown in different colors of light which would be absorbed primarily by chlorophyll (Chl) (red and blue light) or by the phycobilisomes (green or two intensities of cool-white fluorescent light), and samples of these cells were frozen to −196 C for measurements of absorption and fluorescence emission spectra. Cells grown in the high intensity white light had least of all of the photosynthetic pigments, a higher ratio of carotenoid/Chl, but essentially the same ratio of phycobilin to Chl as cells grown in the low intensity white light. The ratio of photosystem II (PSII) to photosystem I (PSI) pigments was affected by light quality; the ratios of phycobilin to Chl and of short wavelength (PSII) Chl to long wavelength (PSI) Chl were both greater in the cells grown in red or blue light.     

Changes in Photosystem Stoichiometry in Response to Environmental Conditions for Cell Growth Observed with the Cyanophyte Synechocystis PCC 6714

Changes in photosystem stoichiometry in response to shift ofenvironments for cell growth other than light regime were studiedwith the cyanophyte Synechocystis PCC 6714 in relation to thechange induced by light-quality shift. Following two environment-shiftswere examined: the shift of molecular form of inorganic carbonsource for photosynthesis from CO₂ to HCO₃^– (CO₂ stress)and the increase in salinity of the medium with NaCl (0.5 M)(Na⁺ stress). Both CO₂ and Na⁺ stresses induced the increasein PSI abundance resulting in a higher PSI/PSII stoichiometry.CO₂ stress was found to elevate simultaneously Cyt c oxidaseactivity (V_max). The feature was the same as that caused bylight-quality shift from preferential excitation of PSI to PSII(light stress) though the enhancement by either stress was smallerthan that by light stress. Under our experimental conditions,PSI/PSII stoichiometry appeared to increase at a fairly constantrate to the basal level even when the basal level had been differentlydetermined by the light-quality. Enhancing rates for PSI/PSIIstoichiometry and for Cyt c oxidase activity were also similarto each other. Since the two stresses affect the thylakoid electrontransport similarly to the shift of light-quality, we interpretedour results as follows: three environmental stresses, CO₂, Na⁺,and light stresses, cause changes in electron turnover capacityof PSI and Cyt c oxidase under a similar, probably a common,mechanism for monitoring redox state of thylakoid electron transportsystem. ¹On leave from Department of Biology, College of Natural Science,Kyngpook National University, Taegu 702-701, Korea. ₂Present address: Department of Marine Bioscience, Fukui Pre-fecturalUniversity, Obama, Fukui, 917 Japan.     

Effect of Chloramphenicol on Etiochloroplasts of Wild and Chlorophyll b-Less Barley Genotypes

Greening of etiolated seedlings of wild and Chl b-less barley(Hordeum vulgare L.) genotypes in the presence of D-threochloramphenicol(CAP) led to macrogranal arrangements accompanying the inhibitionof Chl synthesis and an enhancement of the total protein contentin differentiated etiochloroplasts. In treated mutant plastids,protein/Chl ratio reached up to 100. No light-dependent O₂ evolution was detected in CAP-treatedplastids which had deficiency in polypeptides belonging to thephotosystem II (PSII) centres. On the other hand, plastids displayeda high photosystem I (PSI) activity despite the absence of the92 kDa polypeptide linked to the PSI centre. The accumulationof polypeptides ranging from 16 to 20 kDa suggest that theycould originate from primary complexes consisting of few Chlmolecules, but they were sufficient to allow the activity ofthe reaction centres. No accumulation of the 25–27 kDapolypeptides linked to the PSII antenna was detected. The increase in the proportion of trans-₃hexadecenoic acid (16:1tr) in phosphatidylglycerol (PG) of etiochloroplasts from bothtypes after CAP treatment could indicate an alteration of theregulation process of 16:1 tr biosynthesis occurring in plastids.The formation of macrograna could optimize the energy transferin altered thylakoid membranes. The accumulation of PG-16:1tr molecules could be related to the formation of active primarycomplexes in thylakoid when Chl synthesis is altered. (Received March 30, 1988; Accepted June 1, 1988)     

Regulation of Stoichiometry between PSI and PSII in Response to Light Regime for Photosynthesis Observed with Synechocystis PCC 6714: Relationship between Redox State of Cyt b6-f Complex and Regulation of PSI Formation

The effect of the Cyt b₆-f redox state on the PSI formationwas examined with the cyanophyte Synechocystis PCC 6714 by usinga Q-cycle inhibitor, HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide).HQNO inhibited the rapid reduction of flash-oxidized Cyt f,the reaction correlating with the stimulation of PSI formation,on one hand, and accumulated reduced Cyt b₆, on the other, indicatingthat the electron flow in the Q-cycle correlates with regulationof PSI synthesis. HQNO also inhibited the stimulation of PSIformation under PSII light, resulting in a low PSI/PSII ratioeven under PSII light, while the PSI formation under PSI lightwas not suppressed by HQNO. Simultaneous inhibition of Cyt b₆oxidation through the Q-cycle and the stimulated PSI formationby HQNO suggests that an HQNO-sensitive Cyt b₆ oxidation isinvolved in the mechanism of monitoring the state of electrontransport system for regulation of PSI formation. (Received March 3, 1993; Accepted August 9, 1993)     

Chromatic Regulation in Chlamydomonas reinhardtii: Time Course of Photosystem Stoichiometry Adjustment Following a Shift in Growth Light Quality

Photosystem stoichiometry adjustments in Chlamydomonas reinhardtiiwere induced upon a sudden shift in the light quality duringcell growth. Reversible changes in the PSI/PSII ratio were acompensation response to changes in the balance of light absorptionby the two photosystems. Quantitations of PSII, Cyt b₆-f complexand PSI revealed a constancy in the cellular content of PSIIand the Cyt b₆-f complex, and variable amounts of PSI in C.reinhardtii. These results strengthen the notion that PSI isthe thyla-koid component subject to chromatic regulation andresponsible for the adjustment and optimization of the PSI/PSII ratio in the thylakoid of oxygenic photosynthesis. Additionalresults, obtained upon the use of protein biosynthesis translationinhibitors (chloramphenicol and cyclohex-imide), suggested thata chromatically-induced lowering of the PSI/PSII ratio in C.reinhardtii occurs by suppression of de novo biosynthesis ofPSI components and, therefore, by dilution of the PSI complexin the thylakoid membrane, rather than by active degradationof assembled PSI in chlo-roplasts. (Received November 8, 1996; Accepted December 6, 1996)     

Properties of Chlamydomonas Photosystem II Core Complex with a His-Tag at the C-Terminus of the D2 Protein

A His-tagged PSII core complex was purified from recombinantChlamydomonas reinhardtii D2-H thylakoids by single-step Ni²⁺-affinitycolumn chromatography and its properties were partially characterizedin terms of their PSII functions and chemical compositions.The PSII core complex that has a His-tag extension at the C-terminusof the D2 protein evolved oxygen at a high rate of 2,400 µmol(mg Chl)^–1h^–1 at the optimum pH of 6.5 with ferricyanideand 2,6-dichlorobenzoquinone as electron acceptors in the presenceof Ca²⁺ as an essential cofactor, and approximately 90% of theactivity was blocked by 10 µM DCMU. The core complex exhibitedthe thermoluminescence Q-band but not the B-band regardlessof the presence or absence of DCMU, although both bands wereobserved in the His-tagged thylakoids. The core complex wasfree from PSI and contained one Y_D, Tyr 160 of the D2 protein,four Mn atoms, two cytochrome b-559, about 46 Chl a molecules,and probably one Q_A, the primary acceptor quinone of PSII. Itwas inferred from these results that His-tagging at the C-terminusof the D2 protein does not affect the functional and structuralintegrity of the PSII core complex, and that the ‘His-tagstrategy’ is highly useful for biochemical, physicochemical,and structural studies of Chlamydomonas PSII. (Received October 22, 1998; Accepted December 25, 1998)     

Thylakoid Membrane Development and Differentiation: Assembly of the Chlorophyll a-b Light-Harvesting Complex and Evidence for the Origin of Mr=19, 17.5 and 13.4 kDa Proteins

Pea plants were grown under intermittent illumination (ImL)conditions. The low dosage of light given to ImL plastids limitedthe rate of chlorophyll (Chl) a and Chl b biosynthesis and,therefore, it retarded the rate of photosynthetic unit formationand thylakoid membrane development. Depending on the developmentalstage of the photosynthetic unit, ImL plastids had variableChl a/Chl b ratios (2.7 <Chl a/Chlb<20) and showed distinctintermediates in the assembly of the chlorophyll a–b light-harvestingcomplex (LHC) of photosystem-II (PSII). The results are consistentwith a step-wise increment in the PSII antenna size involvingthree distinct forms of the PSII unit: (i) a PSII-core formwith about 37 Chl a molecules; (ii) a PSIL_ß form containingthe PSII-core and the LHC-II-inner antenna with a total of about130 Chl (a + b) molecules, and (iii) the mature PSII_a form containingPSII_ß and the LHC-II-peripheral antenna with a totalof 210–300 Chl (a + b) molecules. The thylakoid membranecontained polypeptide subunits b, c and d (the Lhcb1, 2 and3 gene products, respectively) when only the LHC-II-inner waspresent. Polypeptide subunit a, (the apoprotein of the chlorophyll-proteinknown as CP29), along with increased amounts of b and c appearedlater in the development of thylakoids, concomitant with theassembly of the LHC-II-peripheral. The results suggest thatpolypeptide subunit d has priority of assembly over subunita. It is implied that, of all LHC-II constituent proteins, subunitd is most proximal to the PSII-core complex and that it servesas a linker in the transfer of excitation energy from the bulkLHC-II (subunits b and c) to the PSII-core. The work also addressesthe origin of low-molecular-weight proteins (M_r = 19, 17.5 and13.4 kDa) which co-isolate with intact developing plastids andwhose abundance decreases during plastid development. Aminoacid compositional and immunoblot analyses show a nuclear histoneorigin for these low-molecular-weight proteins and suggest co-isolationof histone-containing nuclear vesicles along with intact developingplastids. ¹Present address: Plant Physiology Research Group, The Universityof Calgary, Department of Biological Sciences, 2500 UniversityDrive N.W., Calgary, Alberta CANADA T2N 1N4.     

Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Light-harvesting capacities of photosystem I (PSI) and photosystemII (PSII) in a wild-type and three chlorophyll b-deficient mutantstrains of rice were determined by measuring the initial slopeof light-response curve of PSI and PSII electron transport andkinetics of light-induced redox changes of P-700 and Q_A, respectively.The light-harvesting capacity of PSI determined by the two methodswas only moderately reduced by chlorophyll b-deficiency. Analysisof the fluorescence induction that monitors time course of Q_Aphotoreduction showed that both relative abundance and antennasize of PSII_a decrease with increasing deficiency of chlorophyllb and there is only PSII_rß in chlorina 2 which totallylacks chlorophyll b. The numbers of antenna chlorophyll moleculesassociated with the mutant PSII centers were, therefore, threeto five times smaller than that of PSII_a in the wild type rice.Rates of PSII electron transport determined on the basis ofPSII centers in the three mutants were 60–70% of thatin the normal plant at all photon flux densities examined, indicatingthat substantial portions of the mutant PSII centers are inactivein electron transport. The initial slopes of light-responsecurves of PSII electron transport revealed that the functionalantenna sizes of the active populations of PSII centers in themutants correspond to about half that of PSII in the wild typerice. Thus, the numbers of chlorophyll molecules that serveas antenna of the oxygen-evolving PSII centers in the mutantsare significantly larger than those that are actually associatedwith each PSII center. It is proposed that the inactive PSIIserves as an antenna of the active PSII in the three chlorophyllb-deficient mutants of rice. In spite of the reduced antennasize of PSII, therefore, the total light-harvesting capacityof PSII approximately matches that of PSI in the mutants. (Received July 29, 1994; Accepted February 7, 1996)     

Temperature Dependent Alterations in the Pattern of Photochemical and Non-Photochemical Quenching and Associated Changes in the Photosystem II Conditions of the Leaves

The alterations in the PSII activity of leaves, subsequent toa mild or severe heat stress were characterized by monitoringthe Chl a fluorescence and thermoluminescence emission fromintact leaves. The Chl a fluorescence measurements were carriedout in leaves adapted to either ‘state I’ or ‘stateII’ since under these two conditions the photosyntheticapparatus is known to have distinctly different structure-functionrelationships. The pattern of Chl a fluorescence induction instate II-adapted leaves was different from that of state I-adaptedleaves due to the alterations in the extent of photochemical(q_Q) and non-photochemical (q_E) quenching during the time courseof induction. The pattern of changes in q_Q and q_E values wasalso altered by heat treatment depending on the severity ofheat stress; severe heat stress (47°C) suppressing theseparameters drastically. Mild heat treatment (42°C) did notaffect the ability of leaves to undergo state I to state IItransition whereas the severe heat stress totally abolishedsuch transition. The fluorescence and thermoluminescence characteristicsof the leaves that have been exposed to the severe heat stresssuggest that a large number of affected PSII units retain afunctional water-oxidizing complex at the donor side. (Received June 14, 1994; Accepted July 19, 1995)     

Stoichiometries of Photosystem I and Photosystem II in Rice Mutants Differently Deficient in Chlorophyll b

Stoichiometries of photosystem I (PSI) and photosystem II (PSII)reaction centers in a cultivar of rice, Norin No. 8, and threechlorophyll b-deficient mutants derived from the cultivar wereinvestigated. Quantitation of PSI by photooxidation of P-700and chromatographic assay of vitamin K₁ showed that, on thebasis of chlorophyll, the mutants have higher concentrationsof PSI than the wildtype rice. Greater increases were observedin the PSII contents measured by photoreduction of Q_A, bindingof a radioactive herbicide and atomic absorption spectroscopyof Mn. Consequently, the PSII to PSI ratio increased from 1.1–1.3in the wild-type rice to 1.8 in chlorina 2, which contains noChl b, and to 2.0–3.3 in chlorina 11 and chlorina 14,which have chlorophyll a/b ratios of 9 and 13, respectively.Measurement of oxygen evolution with saturating single-turnoverflashes revealed that, whereas at most 20% of PSII centers areinactive in oxygen evolution in the wildtype rice, the non-functionalPSII centers amount to about 50% in the three mutant strains.The fluorescence induction kinetics was also analyzed to estimateproportions of the inactive PSII in the mutants. The data obtainedsuggest that plants have an ability to adjust the stoichiometryof the two photosystems and the functional organization of PSIIin response to the genetically induced deficiency of chlorophyllb. (Received July 29, 1994; Accepted February 7, 1996)     

Light-Harvesting for Photosynthesis in Four Strains of the Red Alga Porphyra yezoensis Having Different Phycobilin Contents

The light-harvesting system of photosynthesis was studied infour strains of Porphyra yezoensis differing in their phycoerythrin(PE) content; the red strain, richer in PE than the wild strain,and the green and the yellow strains, poorer in PE. Specialattention was given to possible alteration of pigment systemin response to PE content, especially in the green and the yellowstrains. The relative quantum yields of Chi a fluorescence at–196°C and O² evolution were compared. Four strains commonly showed a low yield in pigment system II(PS II) fluorescence on Chl a excitation. The yield was as lowas in those algae in which PS II has only a small portion ofChl a as the light harvester. Measurement of O₂ evolution gavethe same results. Results indicate that the functional compositionof Chl a system remains unaltered in four strains with differentPE content. PS II in the green and the yellow strains reflectsa reduction in the size of light-harvesting components, suggestingthat pigmentation in these strains is fixed genetically as asun-type. ⁴ On leave from University of Washington, Seatle, Washington,U.S.A. (Received September 18, 1982; Accepted December 25, 1982)     

Reversible changes in structure and function of photosynthetic apparatus of pea (Pisum sativum) leaves under drought stress

The effects of drought on photosynthesis have been extensively studied, whereas those on thylakoid organization are limited. We observed a significant decline in gas exchange parameters of pea (Pisum sativum) leaves under progressive drought stress. Chl a fluorescence kinetics revealed the reduction of photochemical efficiency of photosystem (PS)II and PSI. The non-photochemical quenching (NPQ) and the levels of PSII subunit PSBS increased. Furthermore, the light-harvesting complexes (LHCs) and some of the PSI and PSII core proteins were disassembled in drought conditions, whereas these complexes were reassociated during recovery. By contrast, the abundance of supercomplexes of PSII-LHCII and PSII dimer were reduced, whereas LHCII monomers increased following the change in the macro-organization of thylakoids. The stacks of thylakoids were loosely arranged in drought-affected plants, which could be attributed to changes in the supercomplexes of thylakoids. Severe drought stress caused a reduction of both LHCI and LHCII and a few reaction center proteins of PSI and PSII, indicating significant disorganization of the photosynthetic machinery. After 7 days of rewatering, plants recovered well, with restored chloroplast thylakoid structure and photosynthetic efficiency. The correlation of structural changes with leaf reactive oxygen species levels indicated that these changes were associated with the production of reactive oxygen species.     

The Photosynthetic Apparatus of Phytoplankton from a Perennially Ice-Covered Antarctic Lake: Acclimation to an Extreme Shade Environment

Phytoplankton in perennially ice-covered Lake Bonney (Antarctica)are exposed to a limited range of light variation both in termsof intensity (<1–3% of incident) and spectral distribution(blue-green) during the austral spring and summer. This relativeconstancy is due to continuous sunlight, optical filtering throughthe 4.2 m ice cap and an absence of vertical mixing. The effectsof this unique light environment on the structure and functionof the photosynthetic apparatus were studied using measurementsof P₇₀₀ reaction center content and spectral variation in photosystemII (PSII) fluorescence kinetics. Light-induced absorbance changeat both 700 nm and 810 nm was used to measure P₇₀₀ concentration.The average ratio of total Chl/P₇₀₀ was 743 (mol mol^–1),with a range of 480 to 1,039. These ratios were low in comparisonto previous studies of phytoplankton growing in low-light culturesor algae growing beneath Arctic sea ice. A sample from the deep(17 m) layer dominated by Chlamydomonas subcaudata was grownin enriched culture media. PSII fluorescence kinetics were measuredon thylakoid preparations in the presence of DCMU under blue-green(481 nm) and red (660 nm) light. C. subcaudata utilized blue-greenlight for photosynthesis more efficiently than the photobiologicallywell characterized C. reinhardtii (strain CC-124). These results,together with pigment analyses, suggest that carotenoids inLake Bonney phytoplankton are more important in light harvestingas opposed to photoprotection. (Received March 23, 1994; Accepted December 5, 1994)     

Structural and Immunological Characterization of the Cuscuta pentagona L. Chloroplast

Structural and immunochemical studies were used to determinethe photosynthetic potential of the dodder (Cuscuta pentagona)chloroplast. Ultrastructural studies revealed that thylakoidmembranes of pre-parasitic phase Cuscuta pentagona are almostall organized into long, overlapping grana stacks of mainlytwo to five thylakoids with little space between adjacent stacks.Immunoblots reveal chloroplast proteins associated with PSIand II, as well as cytochrome f and plastocyanin. Stromal extractscontained immmunologically-detectable RuBisCO and phosphoribulokinase.Cytochemical localizations of the oxidizing side of PSI showedproduct localization on the lumen side of the thylakoid. Immunocytochemicallocalizations of RuBisCO reveal exclusive labeling in the stroma,whereas antibodies to the PSII proteins, light-harvesting Chla/b complex and the oxygen-evolving complex of PSII, are concentratedover the thylakoids. A limited capacity for CO₂ fixation wasfound in seedlings by monitoring CO₂ exchange rates in the presenceand absence of atrazine. These data indicate that the chloroplastfrom this species of dodder contains a number of the proteinsrequired for a successful fixation of CO₂ and the proteins inthe thylakoids are organized much like other higher plants,with the exception of the large percentage of the thylakoidsorganized into grana. (Received August 10, 1998; Accepted April 3, 1999)     

Extensive remodeling of the photosynthetic apparatus alters energy transfer among photosynthetic complexes when cyanobacteria acclimate to far-red light

Some cyanobacteria remodel their photosynthetic apparatus by a process known as Far-Red Light Photoacclimation (FaRLiP). Specific subunits of the phycobilisome (PBS), photosystem I (PSI), and photosystem II (PSII) complexes produced in visible light are replaced by paralogous subunits encoded within a conserved FaRLiP gene cluster when cells are grown in far-red light (FRL; λ = 700–800 nm). FRL-PSII complexes from the FaRLiP cyanobacterium, Synechococcus sp. PCC 7335, were purified and shown to contain Chl a, Chl d, Chl f, and pheophytin a, while FRL-PSI complexes contained only Chl a and Chl f. The spectroscopic properties of purified photosynthetic complexes from Synechococcus sp. PCC 7335 were determined individually, and energy transfer kinetics among PBS, PSII, and PSI were analyzed by time-resolved fluorescence (TRF) spectroscopy. Direct energy transfer from PSII to PSI was observed in cells (and thylakoids) grown in red light (RL), and possible routes of energy transfer in both RL- and FRL-grown cells were inferred. Three structural arrangements for RL-PSI were observed by atomic force microscopy of thylakoid membranes, but only arrays of trimeric FRL-PSI were observed in thylakoids from FRL-grown cells. Cells grown in FRL synthesized the FRL-specific complexes but also continued to synthesize some PBS and PSII complexes identical to those produced in RL. Although the light-harvesting efficiency of photosynthetic complexes produced in FRL might be lower in white light than the complexes produced in cells acclimated to white light, the FRL-complexes provide cells with the flexibility to utilize both visible and FRL to support oxygenic photosynthesis.This article is part of a Special Issue entitled Light harvesting, edited by Dr. Roberta Croce.     

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.