Genetic inactivation of the psaB gene in Synechocystis sp. PCC 6803 disrupts assembly of photosystem I

The reaction center of photosystem (PS) I is comprised of a heterodimer of homologous polypeptides, PsaA and PsaB. In order to investigate the biogenesis of PS I, the psaB gene was inactivated by targeted mutagenesis in the unicellular cyanobacterium Synechocystis 6803. This mutation resulted in disruption of stable PS I assembly, but PS II assembled normally. Expression of the psaA gene was not affected by the mutation, but PsaA protein was not detected, indicating that stable PsaA homodimers did not form. The ability to inactivate psaB makes it a viable target for site-directed mutagenesis.     

Analysis of the proposed Fe-SX binding region of Photosystem 1 by site directed mutation of PsaA in Chlamydomonas reinhardtii

The psaA and psaB genes of the chloroplast genome in oxygenic photosynthetic organisms code for the major peptides of the Photosystem 1 reaction center. A heterodimer of the two polypeptides PsaA and PsaB is thought to bind the reaction center chlorophyll, P700, and the early electron acceptors A₀, A₁ and Fe-S_X. Fe-S_X is a 4Fe4S center requiring 4 cysteine residues as ligands from the protein. As PsaA and PsaB have only three and two conserved cysteine residues respectively, it has been proposed by several groups that Fe-S_X is an unusual inter-peptide center liganded by two cysteines from each peptide. This hypothesis has been tested by site directed mutagenesis of PsaA residue C575 and the adjacent D576. The C575D mutant does not assemble Photosystem 1. The C575H mutant contains a photoxidisable chlorophyll with EPR properties of P700, but no other Photosystem 1 function has been detected. The D576L mutant assembles a modified Photosystem 1 in which the EPR properties of the Fe-S_A/B centers are altered. The results confirm the importance of the conserved cysteine motif region in Photosystem 1 structure.Dedicated to the memory of Daniel I. Arnon.     

Fluorescence changes accompanying short-term light adaptations in photosystem I and photosystem II of the cyanobacterium <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 and phycobiliprotein-impaired mutants: State 1/State 2 transitions and carotenoid-induced quenching of phycobilisomes

The features of the two types of short-term light-adaptations of photosynthetic apparatus, State 1/State 2 transitions, and non-photochemical fluorescence quenching of phycobilisomes (PBS) by orange carotene-protein (OCP) were compared in the cyanobacterium Synechocystis sp. PCC 6803 wild type, CK pigment mutant lacking phycocyanin, and PAL mutant totally devoid of phycobiliproteins. The permanent presence of PBS-specific peaks in the in situ action spectra of photosystem I (PSI) and photosystem II (PSII), as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 690 nm (PSII) and 725 nm (PSI) showed that PBS are constitutive antenna complexes of both photosystems. The mutant strains compensated the lack of phycobiliproteins by higher PSII content and by intensification of photosynthetic linear electron transfer. The detectable changes of energy migration from PBS to the PSI and PSII in the Synechocystis wild type and the CK mutant in State 1 and State 2 according to the fluorescence excitation spectra measurements were not registered. The constant level of fluorescence emission of PSI during State 1/State 2 transitions and simultaneous increase of chlorophyll fluorescence emission of PSII in State 1 in Synechocystis PAL mutant allowed to propose that spillover is an unlikely mechanism of state transitions. Blue–green light absorbed by OCP diminished the rout of energy from PBS to PSI while energy migration from PBS to PSII was less influenced. Therefore, the main role of OCP-induced quenching of PBS is the limitation of PSI activity and cyclic electron transport under relatively high light conditions.     

Immunological characterization of chlorophyll a/b-binding proteins of barley thylakoids

Monoclonal antibodies have been raised against the light-harvesting chlorophyll a/b-binding proteins of photosystem I (LHCI) using a photosystem (PS) I preparation (PSI-200) wild-type from barley (Hordeum vulgare L. cv. Svaløf's Bonus) as the antigen. These antibodies cross-reacted with a minor light-harvesting chlorophyll a/b-protein of PSII (Chla/b-P1=CP29), but not with the major one, LHCII (=Chla/b-P2^**). Similarly, a monoclonal antibody to Chla/b-P1, elicited by a PSII preparation as the antigen, cross-reacted with LHCI, but not LHCII. This explains why an antigen consisting of LHCII, free of LHCI, but contaminated with Chla/b-P1, can elicit antibodies which cross-react with LHCI. Immunoblot assays showed that LHCI and Chla/b-P1 have at least two epitopes in common. Immunogold labelling of thin-sectioned wild-type thylakoids confirmed a preferential localisation of Chla/b-P1 in grana partition membranes and LHCI in stroma lamellae. The presence of LHCI was demonstrated in barley mutants lacking the PSI reaction centre (viridis-zb ⁶³) and chlorophyll b (chlorina-f2), and was correlated with the presence of long-wavelength (730 nm) fluorescence emission at 77 K. The mutant viridis-k ²³, which has a 77 K long-wavelength fluorescence peak at 720 nm, was shown by immune-blot assay to lack LHCI, although Chla/b-P1 was present.Abbreviations Chl-P chlorophyll-protein - CM Carlsberg Monoclonal - Da dalton - LHC light-harvesting complex - PAGE polyacrylamide gel electrophoresis - PSI, II photosystem I, II - PSI-200 PSI containing LHCI polypeptides - SDS sodium dodecyl sulphate     

Characterization of a Synechococcus sp. strain PCC 7002 mutant lacking Photosystem I. Protein assembly and energy distribution in the absence of the Photosystem I reaction center core complex

A Synechococcus sp. strain PCC 7002 psaAB::cat mutant has been constructed by deletional interposon mutagenesis of the psaA and psaB genes through selection and segregation under low-light conditions. This strain can grow photoheterotrophically with glycerol as carbon source with a doubling time of 25 h at low light intensity (10 E m^–2 s^–1). No Photosystem I (PS I)-associated chlorophyll fluorescence emission peak was detected in the psaAB::cat mutant. The chlorophyll content of the psaAB::cat mutant was approximately 20% that of the wild-type strain on a per cell basis. In the absence of the PsaA and PsaB proteins, several other PS I proteins do not accumulate to normal levels. Assembly of the peripheral PS I proteins PsaC,PsaD, PsaE, and PsaL is dependent on the presence of the PsaA and PsaB heterodimer core. The precursor form of PsaF may be inserted into the thylakoid membrane but is not processed to its mature form in the absence of PsaA and PsaB. The absence of PS I reaction centers has no apparent effect on Photosystem II (PS II) assembly and activity. Although the mutant exhibited somewhat greater fluorescence emission from phycocyanin, most of the light energy absorbed by phycobilisomes was efficiently transferred to the PS II reaction centers in the absence of the PS I. No light state transition could be detected in the psaAB::cat strain; in the absence of PS I, cells remain in state 1. Development of this relatively light-tolerant strain lacking PS I provides an important new tool for the genetic manipulation of PS I and further demonstrates the utility of Synechococcus sp. PCC 7002 for structural and functional analyses of the PS I reaction center.Abbreviations ATCC American type culture collection - Chl chlorophyll - DCMU 3-(3,4-dichlorophyl)-1,1-dimethylurea - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - HEPES N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid] - PCC Pasteur culture collection - PS I Photosystem I - PS II Photosystem II - SDS sodium dodecyl sulfate     

Contrasting patterns of photosynthetic acclimation to the light environment are dependent on the differential expression of the responses to altered irradiance and spectral quality

Chl, chlorophyll
Chl a/b, ratio of chlorophyll a to chlorophyll b
Cyt f, cytochrome f
FR, far-red light
LFR, low irradiance, far-red enriched growth light
LHCII, light harvesting complex associated with PSII
LW, low irradiance, white growth light
MW, moderate irradiance, white growth light
PAR, photosynthetically active radiation
P_max, light and CO₂ saturated photosynthetic rate
PSI, photosystem I
PSII, photosystem II

Four plant species (Chamerion angustifolium, Digitalis purpurea, Brachypodium sylvaticum and Plantago lanceolata) which have previously been shown to demonstrate contrasting photosynthetic acclimatory responses to the light environment ( 33 , Plant, Cell and Environment 20, pp. 438–448) were analysed at a biochemical level. Plants were grown under low irradiance with a shade-type spectrum (LFR: 50μmol quanta m^–2 s^–1), moderately high white light (MW: 300μmol quanta m^–2 s^–1) and low irradiance white light (LW: 50μmol quanta m^–2 s^–1). The effects of light quality upon chlorophyll content and photosynthetic capacity were found to be species-dependent. A far-red dependent reduction in chlorophyll was found in three species, and an irradiance-dependent reduction was found in B. sylvaticum, which showed the greatest alteration in the xanthophyll cycle pool size of all species tested under these conditions. Chlorophyll a/b ratios were sensitive to both light quality and quantity in C. angustifolium and D. purpurea, being highest in MW, lowest in LFR, and intermediate in LW, whilst the other species showed no response. Ratios of photosystem II to photosystem I (PSII and PSI) demonstrated a strong irradiance-associated increase in all species except B. sylvaticum, whereas an increase in PSII/PSI in LFR compared to LW conditions was present in all species. A change in chlorophyll a/b was not always associated with a change in PSII/PSI, suggesting that the level of LHCII associated with each PSII varied in some species. Cytochrome f content showed an irradiance-dependent effect only, indicating a relationship with the capacity of electron transport. It is concluded that differing strategies of acclimation to the light environment demonstrated by these species results from differing strengths of expression of a series of independently regulated changes in the levels of photosynthetic components.     

Influence of tris(2,3-dibromopropyl) isocyanurate on the expression of photosynthesis genes of Nannochloropsis sp.

The proteins encoded by psaA and psaB form a heterodimer, an essential compound of photosystem; while the protein encoded by psbC binds with chlorophyll a in photosystem II, serving as antennae in photosystem. Here we report that a heterocyclic brominated flame retardant, tris(2,3-dibromopropyl) isocyanurate (TBC), inhibited the expression of psaA and psbC, then leads to the decrease of Nannochloropsis sp.'s growth biomass. TBC exposure inhibited the expression of psaA and psbC at 10, 100 ng/mL slightly and 1000 ng/mL significantly. In addition, TBC was found to slow down the growth of Nannochloropsis sp. at concentrations ranging from 10 to 1000 ng/mL. These results indicated that TBC influenced both photosynthesis and growth performance of Nannochloropsis sp.     

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A₂ (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP³ as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.Abbreviations CP1 chlorophyll a-protein complex of PSI - CPa chlorophyll a-protein complex of PSII - D₁₀ digitonin subchloroplast particles enriched in PSII - D₁₄₄ digitonin subchloroplast particles enriched in PSI - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - LHCP^1–3 light harvesting chlorophyll a/b protein complexes - PAGE polyacrylamide gel electrophoresis - PSI photosystem I - PSII photosystem II - SDS sodium dodecyl sulphate - TCA trichloroacetic acid - Tricine N-Tris-(hydroxymethyl)-methylglycine - Tris Tris-(hydroxymethyl)-aminomethan     

NaCl胁迫加重强光胁迫下超大甜椒叶片的光系统II和光系统I的光抑制

快速叶绿素荧光动力学可以在无损情况下探知叶片光合机构的损伤程度, 快速叶绿素荧光测定和分析技术(JIP-test)将测量值转化为多种具有生物学意义的参数, 因而被广泛应用于植物光合机构对环境的响应机制研究。该文研究了超大甜椒(Capsicum annuum)幼苗在强光及不同NaCl浓度胁迫下的荧光响应情况。与单纯强光胁迫相比, NaCl胁迫引起了叶绿素荧光诱导曲线的明显改变, 光系统II (PSII)光抑制加重, 同时PSII反应中心和受体侧受到明显影响, 而且高NaCl浓度胁迫下PSII供体侧受伤害明显, 同时PSI反应中心活性(P700⁺)在盐胁迫下明显降低。这些结果表明, NaCl胁迫会增强强光对超大甜椒光系统的光抑制, 并且浓度越高抑制越明显, 但对PSI的抑制作用低于PSII。高NaCl浓度胁迫易对PSII供体侧造成破坏, 且PSI光抑制严重。     

Detection of point mutations in chloroplast genes of Antirrhinum majus L. I. Identification of a point mutation in the psaB gene of a photosystem I plastome mutant

A point mutation in the plastome-encoded psaB gene of the mutant en:alba-1 of Antirrhinum majus L. was identified by an analysis of chloroplast DNA with a modified PCR-SSCP technique. Application of this technique is indicated when a gene or a group of genes is known in which the point mutation is located. Analysis of primary photosynthetic reactions in the yellowish white plastome mutant indicated a dysfunction of photosystem (PS) 1. The peak wavelength of PS I-dependent chlorophyll (Chl) fluorescence emission at 77 K was shifted by 4 nm to 730 nm, as compared to fluorescence from wild-type. There were no redox transients of the reaction center Chl P₇₀₀ upon illumination of leaves with continuous far-red light or with rate-saturating flashes of white light. The PS I reaction center proteins PsaA and PsaB are not detectable by SDS-PAGE in mutant plastids. Hence, plastome encoded PS I genes were regarded as putative sites of mutation. In order to identify plastome mutations we developed a modified SSCP (single-strand conformation polymorphism) procedure using a large PCR fragment which can be cleaved with various restriction enzymes. When DNA from wild-type and en:alba-1 was submitted to SSCP analysis, a single stranded Hinf I fragment of a PCR product of the psaB gene showed differences in electrophoretic mobility. Sequence analysis revealed that the observed SSCP was caused by a single base substitution at codon 136 (TAT TAG) of the psaB gene. The point mutation produces a new stop codon that leads to a truncated PsaB protein. The results presented indicate that the mutation prevents the assembly of a functional PS I complex. The applicability to other plastome mutants of the new method for detection of point mutations is discussed.     

Changes in the photosynthetic characteristics and photosystem stoichiometries in wild-type and Chl <Emphasis Type="Italic">b</Emphasis>-deficient mutant rice seedlings under various irradiances

By using a wild-type rice (Oryza sativa L. cv. Norin No. 8) and the chlorophyll (Chl) b-deficient mutant derived from Norin No. 8 (chlorina 11), the present study monitored the oxygen evolution, contents of Chl a and b, β-carotene, and lutein in leaf and the contents of cytochrome f, and the reaction centres of photosystem I (PSI) and photosystem II (PSII) in thylakoids. The oxygen evolution, maximal quantum yield of PSII (F_v/F_m) and Chl concentration remained constant in both Norin No. 8 and chlorina 11 under 5 and 2% of full sunlight for six days. On the other hand, on the thylakoid level, the PSII reaction centre of chlorina 11 was more stable even under high irradiance, while approximately 40% decrease in levels of the PSII reaction centre occurred under 2% of full sunlight for six days. However, under such conditions, by regulating the stoichiometry of active PSII and PSI centres, the light absorption balance in both rice types was adjusted between the two photosystems. The present study attempted to examine whether the light absorption balance between PSII and PSI is altered to effectively conduct photosynthesis in the wild-type and Chl b-deficient mutant rice seedlings.     

Regulation of Synthesis of PSI in the Cyanophytes Synechocystis PCC6714 and Plectonema boryanum during the Acclimation of the Photosystem Stoichiometry to the Light Quality

The effects of light quality on the formation of the PSI complexwere examined in Synechocystis PCC6714 and in Plectonema boryanum.The rate of increase in levels of core polypeptides of PSI,PsaA/B, doubled upon shift from Chl a-absorbed light (PSI light)to phycobilisome-ab-sorbed light (PSII light). The elevatedrate was decreased upon the reverse shift. Half time of theacceleration was approximately 10 min, and that of the decreasewas approximately 4 min. The rate of degradation of the polypeptideswas far lower than the rate of the increase under either lightregime. Neither synthesis nor degradation of the PsbA and PsbCpolypeptides of PSII was significantly altered by the lightquality. We conclude that synthesis of the PSI complex is chromaticallyregulated to allow adjustments in photosystem stoichiometry.The level of mRNA for PsaA/B was not altered by the light regime.Anomalous inhibition by chloramphenicol suggested that the regulationoccurs at a step(s) other than the peptide elongation step,perhaps at the initiation of the ribosome cycle or at the insertionof Chl a for the stabilization of the polypeptides. The pho-toreductionof protochlorophyllide (Pchlide) was compared with the synthesisof the polypeptides in a mutant of Plectonema boryanum thatlacked Pchlide dark reductase (YFC1004). The results indicatedthat the synthesis of stable PsaA/B polypeptides was not limitedby the reduction of Pchlide, although the synthesis did dependon a supply of Chl a. ¹Present address: Department of Plant Biology, University ofMaryland at College Park, MD 20742, U.S.A. ²Present address: Department of Marine Bioscience, Fukui Pre-fecturalUniversity, Obama, Fukui, 917 Japan     

The action of lipase on chloroplast membranes: II. Polypeptide patterns of bean galactolipase- and phospholipase A2-treated thylakoid membranes

Thylakoid membranes obtained from bean chloroplasts treated with bean galactolipase or phospholipase A₂ (from Crotalus terr. terr.) showed marked changes in their polypeptide patterns when separated on SDS-PAGE. The obtained results have been discussed with regard to the relationship between chloroplast lipids and polypeptides originating from chlorophyll-protein complexes of bean thylakoids. A coexistence between galactolipids and the peripheral antennae in PS I complex and LHCP³ as well as a conspicuous role of phospholipids in PSI and PSII centre chlorophyll-protein complexes has to be underlined.Abbreviations CP1 chlorophyll a-protein complex of PSI - CPa chlorophyll a-protein complex of PSII - D₁₀ digitonin subchloroplast particles enriched in PSII - D₁₄₄ digitonin subchloroplast particles enriched in PSI - DCMU 3-(3,4-dichlorophenyl)-1, 1-dimethylurea - LHCP^1-3 light harvesting chlorophyll a/b protein complexes - PAGE polyacrylamide gel electrophoresis - PSI photosystem I - PSII photosystem II - SDS sodium dodecyl sulphate - TCA trichloroacetic acid - Tricine N-Tris-(hydroxymethyl)-methylglycine - Tris Tris-(hydroxymethyl)-aminomethan     

The BtpA protein stabilizes the reaction center proteins of photosystem I in the cyanobacterium Synechocystis sp. PCC 6803 at low temperature

Specific inhibition of photosystem I (PSI) was observed under low-temperature conditions in the cyanobacterium Synechocystis sp. strain PCC 6803. Growth at 20 degrees C caused inhibition of PSI activity and increased degradation of the PSI reaction center proteins PsaA and PsaB, while no significant changes were found in the level and activity of photosystem II (PSII). BtpA, a recently identified extrinsic thylakoid membrane protein, was found to be a necessary regulatory factor for stabilization of the PsaA and PsaB proteins under such low-temperature conditions. At normal growth temperature (30 degrees C), the BtpA protein was present in the cell, and its genetic deletion caused an increase in the degradation of the PSI reaction center proteins. However, growth of Synechocystis cells at 20 degrees C or shifting of cultures grown at 30 degrees C to 20 degrees C led to a rapid accumulation of the BtpA protein, presumably to stabilize the PSI complex, by lowering the rates of degradation of the PsaA and PsaB proteins. A btpA deletion mutant strain could not grow photoautotrophically at low temperature, and exhibited rapid degradation of the PSI complex after transfer of the cells from normal to low temperature.     

Increase in the rate of photosynthetic linear electron transport in cyanobacteruim <Emphasis Type="Italic">Synechocystis</Emphasis> sp. PCC 6803 lacking phycobilisomes

Compensating changes in the pigment apparatus of photosynthesis that resulted from a complete loss of phycobilisomes (PBS) were investigated in the cells of a PAL mutant of cyanobacterium Synechocystis sp. PCC 6803. The ratio PBS/chlorophyll calculated on the basis of the intensity of bands in the action spectra of photosynthetic activity of two photosystems in the wild strain was 1: 70 for PSII and 1: 300 for PSI. Taking into consideration the number of chlorophyll molecules per reaction center in each photosystem, these ratios could be interpreted as association of PBS with dimers of PSII and trimers of PSI as well as greater dependence of PSII as compared with PSI on light absorption by PBS. The ratio PSI/PSII determined by photochemical cross-section of the reactions of two photosystems was 3.5: 1.0 for wild strain of Synechocystis sp. PCC 6803 and 0.7: 1.0 for the PAL mutant. A fivefold increase in the relative content of PSII in pigment apparatus corresponds to a 5-fold increase in the intensity of bands at 685 and 695 nm as related to the band of PSI at 726 nm recorded in low-temperature fluorescence spectrum of the PAL mutant. Inhibition of PSII with diuron resulted in a pronounced stimulation of chlorophyll fluorescence in the PAL mutant as compared to the wild strain of Synechocystis sp. PCC 6803; these data suggested an activation of electron transfer between PSII and PSI in the mutant cells. Thus, the lack of PBS in the mutant strain of Synechocystis sp. PCC 6803 was compensated for by the higher relative content of PSII in the pigment apparatus of photosynthesis and by a rise in the rate of linear electron transport.     

Changes of Photosystem Ⅱ Electron Transport in the ChlorophyⅡ-deficient Oilseed Rape Mutant Studied by ChlorophyⅡ Fluorescence and Thermoluminescence

The photosystem Ⅱ （PSII） complex of photosynthetic membranes comprises a number of chlorophyll-binding proteins that are important to the electron flow. Here we report that the chlorophyll b-deficient mutant has decreased the amount of light-harvesting complexes with an increased amount of some core polypeptldes of PSII, including CP43 and CP47. By means of chlorophyll fluorescence and thermolumlnescence, we found that the ratio of Fv/Fm, qP and electron transport rate in the chlorophyll b-deficient mutant was higher compared to the wild type. In the chlorophyll lPdeflclent mutant, the decay of the primary electron acceptor quinones （QA-） reoxidation was decreased, measured by the fluorescence. Furthermore, the thermoluminescence studies in the chlorophyll bdeficient mutant showed that the B band （S2/S3QB-） decreased slightly and shifted up towards higher temperatures. In the presence of dlchlorophenyl-dlmethylurea, which is inhibited in the electron flow to the second electron acceptor quinines （QB） at the PSll acceptor side, the maximum of the Q band （S2QA-） was decreased slightly and shifted down to lower temperatures, compared to the wild type. Thus, the electron flow within PSll of the chlorophyⅡ b-deficient mutant was down-regulated and characterized by faster oxidation of the primary electron acceptor quinine QA-via forward electron flow and slower reduction of the oxidation S states.     

Effect of light quality on chloroplast-membrane organization and function in pea

The effect of light quality during plant growth of chloroplast membrane organization and function in peas (Pisum sativum L. cv. Alaska) was investigated. In plants grown under photosystem (PS) I-enriched (far-red enriched) illumination both the PSII/PSI stoichiometry and the electrontransport capacity ratios were high, about 1.9. In plants grown under PSII-enriched (far-red depleted) illumination both the PSII/PSI stoichiometry and the electron-transport capacity ratios were significantly lower, about 1.3. In agreement, steady-state electron-transport measurements under synchronous illumination of PSII and PSI demonstrated an excess of PSII in plants grown under far-red-enriched light. Sodium dodecylsulfate polyacrylamide gel electrophoretic analysis of chlorophyll-containing complexes showed greater relative amounts of the PSII reaction center chlorophyll-protein complex in plants grown under farred-enriched light. Additional changes were observed in the ratio of light-harvesting chlorophyll a/b protein to PSII reaction center chlorophyll-protein under the two different light-quality regimes. The results demonstrate the dynamic nature of chloroplast structure and support the notion that light quality is an important factor in the regulation of chloroplast membrane organization and-function.Abbreviations and symbols Chl chlorophyll - CPa PSII reaction center chlorophyll protein complex - CPI PSI chlorophyll protein complex - FR-D light depleted in far-red sensitizing primarily PSII - FR-E light enriched in far-red sensitizing primarily PSI - LHCP PSII light-harvesting chlorophyll a/b protein complex - P ₇₀₀ primary electron donor of PSI - PSI, PSII photosystems I and II, respectively - Q primary electron acceptor of PSII     

Association of chlorophyll a/b-binding Pcb proteins with photosystems I and II in Prochlorothrix hollandica

Action spectra for photosystem II (PSII)-driven oxygen evolution and of photosystem I (PSI)-mediated H₂ photoproduction and photoinhibition of respiration were used to determine the participation of chlorophyll (Chl) a/b-binding Pcb proteins in the functions of pigment apparatus of Prochlorothrix hollandica. Comparison of the in situ action spectra with absorption spectra of PSII and PSI complexes isolated from the cyanobacterium Synechocystis 6803 revealed a shoulder at 650 nm that indicated presence of Chl b in the both photosystems of P. hollandica. Fitting of two action spectra to absorption spectrum of the cells showed a chlorophyll ratio of 4:1 in favor of PSI. Effective antenna sizes estimated from photochemical cross-sections of the relevant photoreactions were found to be 192 ± 28 and 139 ± 15 chlorophyll molecules for the competent PSI and PSII reaction centers, respectively. The value for PSI is in a quite good agreement with previous electron microscopy data for isolated Pcb-PSI supercomplexes from P. hollandica that show a trimeric PSI core surrounded by a ring of 18 Pcb subunits. The antenna size of PSII implies that the PSII core dimers are associated with ∼ 14 Pcb light-harvesting proteins, and form the largest known Pcb-PSII supercomplexes.     

Quantitative correlation of peripheral and intrinsic core polypeptides of photosystem II with photosynthetic electron-transport activity ofAcetabularia mediterranea in red and blue light

The high photosynthetic activity (O₂ production and CO₂ consumption) ofAcetabularia mediterranea Lamour. (=A. acetabulum (L.) Silva) characteristic of cells cultured in white light decreases slowly when cells are kept in continuous red light, and is less than 20% of the original activity after three weeks. Subsequent blue irradiation restores the original activity completely within 3–5 d. The polypeptide composition of the thylakoids from cells grown in either red or blue light and after transfer from red to blue light was analyzed mainly with regards to photosystem II (PSII). The P700-containing reaction-centre complex of photosystem I, CPI, showed only minor quantitative alterations as a consequence of the growth-light quality, which correlated well with the activity of photosystem I under these conditions. In PSII, no drastic changes occurred in the quantity of the reaction-centre components D1 (herbicide-binding polypeptide) and D2, as determined by immunoblots. Likewise, the proteins associated with the water-splitting apparatus did not change detectably in thylakoids from red- or blue-light-treated cells (the 16-kDa component could not be found inAcetabularia thylakoids). The level of the major light-harvesting complex was completely unaffected by the light quality. In contrast, the quantities of the chlorophyll a-protein complexes of the core antenna, CP43 and CP47 (and probably CP29), changed, with kinetics similar to those of total photosynthetic activity. We postulate that the function of the PSII antenna became increasingly impaired in the absence of blue light (i.e. in red light), while blue light had a restoring effect. The peripheral antenna, comprising the light-harvesting complexes, is probably functionally connected with the reaction-centre chlorophylls via the core antenna chlorophyll-protein complexes (CP43, CP47 and probably CP29). A deficiency of these complexes would lead to uncoupling of antenna and reaction centre in the majority of PSII complexes after long periods of red-light treatment.