The redox state of the plastoquinone pool controls the level of the light-harvesting chlorophyll a/b binding protein complex II (LHC II) during photoacclimation

A cytochrome b ₆ f deficient mutant of Lemna perpusilla maintains a constant and lower level of the light-harvesting chl a/b-binding protein complex II (LHC II) as compared to the wild type plants at low-light intensities. Inhibition of the plastoquinone pool reduction increases the LHC II content of the mutant at both low- and high-light intensities but only at high-light intensity in the wild type plants. Proteolytic activity against LHC II appears during high-light photoacclimation of wild type plants. However, the acclimative protease is present in the mutant at both light intensities. These and additional results suggest that the plastoquinone redox state serves as the major signal-transducing component in the photoacclimation process affecting both, synthesis and degradation of LHC II and appearance of acclimative LHC II proteolysis. The plastoquinol pool cannot be oxidized by linear electron flow in the mutant plants which are locked in a ‘high light’ acclimation state. The cytochrome b ₆ f complex may be involved indirectly in the regulation of photoacclimation via 1) regulation of the plastoquinone redox state; 2) regulation of the redox-controlled thylakoid protein kinase allowing exposure of the dephosphorylated LHC II to acclimative proteolysis. This revised version was published online in June 2006 with corrections to the Cover Date.     

Localization of light-harvesting complex apoproteins in the chloroplast and cytoplasm during greening ofChlamydomonas reinhardtii at 38°C

Assembly of the major light-harvesting complex (LHC II) and development of photosynthetic function were examined during the initial phase of thylakoid biogenesis inChlamydomonas reinhardtii cells at 38°C. Continuous monitoring of LHC II fluorescence showed that these processes were initiated immediately upon exposure of cells to light. However, mature-size apoproteins of LHC II (Lhcb) increased in amount in an alkali-soluble (non-membrane) fraction in parallel with the increase in the membrane fraction. Alkali-soluble Lhcb were not integrated into membranes when protein synthesis was inhibited, suggesting that they were not active intermediates in LHC II assembly, nor were they recovered in a purified chloroplast preparation. Immunocytochemical analysis of greening cells revealed Lhcb inside the chloroplast near the envelope and in clusters deeper in the organelle. Antibody binding also detected Lhcb in granules within vacuoles in the cytosol, and Lhcb were recovered in granules purified from greening cells. Our results suggest that the cytosolic granules serve as receptacles of Lhcb synthesized in excess of the amount that can be accommodated by thylakoid membrane formation within the plastid envelope.     

Freeze fracture immunocytochemistry of light-harvesting pigment complexes in a cryptophyte

Summary Immunocytochemical techniques using colloidal gold as the marker have been used to examine the location of the two light harvesting pigment-protein complexes in cryptophyte chloroplasts. A comparison of post-embedding thin section labelling and freeze fracture labelling has been carried out onRhodomonas salina using polyclonal antibodies to a chlorophylla/c ₂ light-harvesting complex, phycoerythrin and the -subunit of phycoerythrin. The effect of different fixation procedures on the intensity of labelling and ac curacy of antigen location have been examined and the effectiveness of uranyl acetate and tannic acid in improving both the preservation of thylakoid structure and labelling density of phycoerythrin has been demonstrated. Freeze fracture labelling gives better spatial res olution of the different antigens than post-embedding labelling, as well as better definition of thylakoid membranes. It confirms the location of phycoerythrin in the thylakoid lumen and the location of the chlorophylla/c ₂ LHC in both appressed and unappressed thylakoid membranes.Abbreviations PE phycoerythrin - chl chlorophyll - LHC light-har-vesting complex     

CHARACTERIZATION OF THE LIGHT-HARVESTING COMPLEX OF GIRAUDYOPSIS STELLIFER (CHRYSOPHYCEAE) AND EFFECTS OF LIGHT STRESS1

The light-harvesting complex (LHC) of Giraudyopsis stellifer Dangeard was isolated on a sucrose density gradient after digitonin treatment, and the pigment composition was analyzed by reverse-phase high-pressure liquid chromatography. The LHC is a chlorophyll (Chl) a/c/ fucoxanthin/violaxanthin complex, depleted of β-carotene, comparable to the LHC of Fucophyceae. The excitation transfer from Chl c and fucoxanthin to Chl a is efficient in whole cells. Immunological reactions indicate a close relationship between Chrysophyceae and Fucophyceae. The immunocytochemical labeling confirms the lack of segregation of the LHC in the appressed membranes of the three associated thylakoids and its localization in the intrapyrenoid thylakoid. The violaxanthin-antheraxanthin-zeaxanthin cycle is operative in the cells and efficiently protects photosystem II reaction centers against photoinhibition.     

Correlation of pigment deprivation and ultrastructural organization of thylakoid membranes incryptomonas maculata following nutrient deficiency

Summary The photosynthetic pigments of the marine algaCryptomonas maculata are decreased under energy fluence rates of 4.4 Wm^–2 (high light=HL). That this is a result of nitrogen deprivation following an increased cell growth rate triggered by high light in comparison to the control under 1.28 Wm^–2 (low light=LL) is evident from combined pigment, growth rate and nutrient analyses. Fine structural studies by electron microscopy revealed that the electron opaque material in the thylakoid lumen of the plastid is lost under high light treatment parallel to the severe loss of almost 90% phycoerythrin-545. The reduction of chlorophylla andc is accompanied by a reversible disorganization of the thylakoid packing and the thylakoid membranes.In a combined freeze fracture study of HL and LL cells ofCryptomonas maculata it is demonstrated that the exoplasmic fracture face of the thylakoid membranes in HL cells possessed only 10 to 15% of the particles of the LL control; the 12.5 nm particle class was almost lacking, whereas particle populations with main sizes of 10 and 7.5 nm are preserved.The protoplasmic face, on the other hand, was less severely affected with only slight reduction in the particle frequency and a shift of the particle size from two populations with peaks at 10 and 7.5 nm to one class centred around 7.0 nm.     

Adhesion between liposomes mediated by the chlorophyll a/b light- harvesting complex isolated from chloroplast membranes

A highly purified chlorophyll a/b light-harvesting complex (chl a/b LHC; chl a/b ratio 1.2) was obtained from Triton-solubilized chloroplast membranes of pea and barley according to the method of Burke et al. (1978, Arch. Biochem. Biophys. 187: 252--263). Gel electrophoresis of the cation-precipitated chl a/b LHC from peas reveals the presence of four polypeptides in the 23- to 28-kdalton size range. Three of these peptides appear to be identical to those derived from re-electrophoresed CPII and CPII* bands. In freeze-fracture replicas, the cation-precipitated chl a/b LHC appears as a semicrystalline aggregate of membranous sheets containing closely spaced granules. Upon removal of the cations by dialysis, the aggregates break up into their constituent membranous sheets without changing their granular substructure. These membranous sheets can be resolubilized in 1.5% Triton X-100, and the chl a/b LHC particles then reconstituted into soybean lecithin liposomes. Freeze-fracture micrographs of the reconstituted chl a/b LHC vesicles suspended in a low salt medium reveal randomly dispersed approximately 80-A particles on both concave and convex fracture faces as well as some crystalline particle arrays, presumably resulting from incompletely solubilized fragments of the membranous sheets. Based on the approximately 80-A diameter of the particles, and on the assumption that one freeze- fracture particle represents the structural unit of one chl a/b LHC aggregate, a theoretical mol wt of approximately 200 kdalton has been calculated for the chl a/b LHC. Deep-etching and negative-staining techniques reveal that the chl a/b LHC particles are also exposed on the surface of the bilayer membranes. Addition of greater than or equal to 2 mM MgCl2 or greater than or equal to 60 mM NaCl to the reconstituted vesicles leads to their aggregation and, with divalent cations, to the formation of extensive membrane stacks. At the same time, the chl a/b LHC particles become clustered into the adhering membrane regions. Under these conditions the particles in adjacent membranes usually become precisely aligned. Evidence is presented to aupport the hypothesis that adhesion between the chl a/b LHC particles is mediated by hydrophobic interactions, and that the cations are needed to neutralize surface charges on the particles.     

The effect of the dark interval in intermittent light on thylakoid development: photosynthetic unit formation and light harvesting protein accumulation

Etiolated bean plants were grown in intermittent light with dark intervals of shorter or longer duration, to modulate the rate of chlorophyll accumulation, relative to that of the other thylakoid components formed. We thus produced conditions under which chlorophyll becomes more or less a limiting factor. We then tested whether LHC complexes can be incorporated in the thylakoid. It was found that an equal amount of chlorophyll, formed under the same total irradiation received, may be used for the stabilization of few and large-in-size PS units containing LHC components (short dark-interval intermittent light), or for the stabilization of many and small-in-size PS units with no LHC components (long dark-interval intermittent light). The size of the PS units diminishes as the dark-interval duration is increased, with no further change after 98 minutes. The PSII/cytf ratio remains constant throughout development in intermittent light and equal to that of mature chloroplasts (PSII/cytf = 1) except in the case of very long dark-interval regimes, where about half PSII units per cytf are present. The PSII/PSI ratio was found to be correlated with the PSII unit size (the larger the size, the lower the ratio). The number of PSI units operating on the same electron transfer chain varied depending on the size of the PSII unit (the larger the PSII unit size, the more the PSI units per chain). The results suggest that it is not the chlorophyll content per se which regulates the stabilization of LHC in developing thylakoids and consequently the size of the PS units, but rather the rate by which it is accumulated, relative to that of the other thylakoid components.Abbreviations Chl Chlorophyll - CL Continuous light - CPa the reaction center complex of PSII - CPI the reaction center complex of PSI - CPIa Chlorophyll protein complex containing the CPI and the light harvesting complex of PSI - fr w fresh weight - LDC Light dark cycles - LHC-I Light-harvesting complex of PSI - LHC-II Light harvesting complex of PSII - PS photosystem - PSI photosystem I - PSII photosystem II     

Prolonged high light treatment of plant cells results in changes of the amount,the localization and the electrophoretic behavior of several thylakoid membrane proteins

The effect of a 30 h high light treatment on the amount and the localization of thylakoid proteins was analysed in low light grown photoautotrophic cells of Marchantia polymorpha and Chenopodium rubrum. High light treatment resulted in a net loss of D1 protein which was accompanied by comparable losses of other proteins of the PS II core (reaction center with inner antenna). LHC II proteins were not reduced correspondingly, indicating that these complexes are less affected by prolonged high light. High light influenced the distribution of PS II components between the grana and the stroma region of the thylakoid membrane, probably by translocation of the respective PS II proteins. Additionally, modifications of several thylakoid proteins were detected in high light treated cells of C. rubrum. These effects are discussed in relation to photoinhibitory damage and repair processes.Abbreviations BCA bioinchonic acid - chl chlorophyll - CF₁ coupling factor - CYC cycloheximide - GT grana thylakoids - HL high light - LL low light - PAGE polyacrylamide gel electrophoresis - PFD photon flux density - PS I Photosystem I - PS II Photosystem II - RC reaction center - SDS sodium dodecylsulfate - ST stroma thylakoids - Thyl unfractionated thylakoids     

Reconstitution of light-harvesting complexes and photosystem II cores into galactolipid and phospholipid liposomes

Chlorophyll a/b light-harvesting complexes (chl a/b LHC) and photosystem II (PSII) cores were isolated from an octyl glucoside-containing sucrose gradient after solubilization of barley thylakoid membranes with Triton X-100 and octyl glucoside. No cation precipitation step was necessary to collect the chl a/b LHC. PAGE under mildly denaturing and fully denaturing conditions showed that the chl a/b LHC fraction contained chlorophyll-protein complexes CP27, CP29, and CP64. The PSII core material contained CP43 and CP47, and little contamination by other nonpigmented polypeptides. Freeze-fracture electron microscopy of the chl a/b LHC after reconstitution into digalactosyldiglyceride (DG) or phosphatidylcholine (PC) vesicles showed that the protein particles (approximately 7.5 +/- 1.6 nm) were approximately 99 and 90% randomly dispersed, respectively, in the liposomes. Addition of Mg++ produced particle aggregation and membrane adhesion in chl a/b LHC-DG liposomes in a manner analogous to that described for LHC-PC liposomes. Reconstitution of PSII cores into DG vesicles also produced proteoliposomes with randomly dispersed particles (approximately 7.5 +/- 1.6 nm). In contrast, PSII-PC mixtures formed convoluted networks of tubular membranes that exhibited very few fracture faces. Most of the protein particles (approximately 7.0 +/- 1.5 nm) were seen trapped between, rather than embedded in, the membranes. The interaction between the zwitterionic head group of the phosphatidyl choline and the negatively charged PSII core may be responsible for the unusual membrane structures observed.     

Fine structure of the chloroplast membranes ofEuglena gracilis as revealed by freeze-cleaving and deep-etching techniques

Summary The chloroplasts ofEuglena gracilis have been examined by freeze-cleaving and deep-etching techniques.The two chloroplast envelope membranes exhibit distinct fracture faces which do not resemble any of the thylakoid fracture faces.Freeze-cleaved thylakoid membranes reveal four split inner faces. Two of these faces correspond to stacked membrane regions, and two to unstacked regions. Analysis of particle sizes on the exposed faces has revealed certain differences from other chloroplast systems, which are discussed. Thylakoid membranes inEuglena are shown to reveal a constant number of particles per unit area (based on the total particle number for both complementary faces) whether they are stacked or unstacked.Deep-etchedEuglena thylakoid membranes show two additional faces, which correspond to true inner and outer thylakoid surfaces. Both of these surfaces carry very uniform populations of particles. Those on the external surface (the A surface) are round and possess a diameter of approximately 9.5 nm. Those on the inner surface (the D surface) appear rectangular (as paired subunits) and measure approximately 10 nm in width and 18 nm in length. Distribution counts of particles show that the number of particles per unit area revealed by freeze-cleaving within the thylakoid membrane approximates closely the number of particles exposed on the external thylakoid surface (the A surface) by deep-etching. The possible significance of this correlation is discussed. The distribution of rectangular particles on the inner surface of the thylakoid sac (D surface) seems to be the same in both stacked and unstacked membrane regions. We have found no correlation between the D surface particles and any clearly defined population of particles on internal, freeze-cleaved membrane faces. These and other observations suggest that stacked and unstacked membranes are similar, if not identical in internal structure.     

STUDIES ON THE AUTECOLOGY OF THE MARINE DIATOM THALASSIOSIRA NORDENSKIOELDII. II. THE INFLUENCE OF CELL SIZE ON GROWTH RATE,AND CARBON,NITROGEN, CHLOROPHYLL a AND SILICA CONTENT1

The effect of cell size on growth rates and some cellular contents of Thalassiosira nordenskioeldii Cleve has been measured at 0 and 10 C. At 0 C the growth rate did not vary with cell size. The 2 smallest clones at this temperature had reduced growth rates because of the induction of sexuality in that size range. The clones grown at 10 C showed a significant negative relationship between growth rate and valve diameter with the cell surface area/volume ratio positively related to growth rate. At both temperatures the smaller cells had proportionately more carbon and nitrogen/unit cell volume. The amount of chlorophyll a and silica/unit cell surface area increased with increasing cell surface area at both 0 and 10 C. Both the C/N and C/chl a ratios showed no significant change with cell size at either temperature but there was a significant increase in the C/chl a ratio at 0 C. The C/Si ratio decreased with increasing cell size at both 0 and 10 C.     

Life time deficiency of photosynthetic pigment-protein complexes CP1, A1, AB1, and AB2 in two cecidomyiid galls derived from <Emphasis Type="Italic">Machilus thunbergii</Emphasis> leaves

Two kinds of cecidomyiid galls induced by Daphnephila on Machilus thunbergii Sieb. & Zucc. leaves at various developmental stages, i.e., young, growing, and mature, were analyzed for their biochemical composition of photosynthetic pigment-protein complexes located in thylakoid membranes using the Thornber and MARS electrophoretic fractionation systems. Both kinds of galls were totally deficient in the pigment-protein complexes CP1, and A1, AB1, and AB2 through the whole period of gall formation. Immunoblotting of antibody against light-harvesting complex 2b (LHC2b) apoprotein confirmed this deficiency in gall’s lifetime, which never recovered under any condition. Electron microscopy demonstrated that already at the early developmental stage the gall chloroplasts had thylakoid morphology like that in a normal leaf.     

Cloning and nucleotide sequence of a cDNA encoding a major fucoxanthin-, chlorophylla/c-containing protein from the chrysophyteIsochrysis galbana: implications for evolution of thecab gene family

We investigated the primary structure of a cDNA encoding a light-harvesting protein from the marine chrysophyteIsochrysis galbana. Antibodies raised against the major fucoxanthin, chlorophylla/c-binding light-harvesting protein (FCP) ofI. galbana were used to select a cDNA clone encoding one of the FCP apoproteins. The nucleic acid and deduced amino acid sequences reveal conserved regions within the first and third transmembrane spans with Chla/b-binding proteins and with FCPs of another chromophyte. However, the amino acid identity betweenI. galbana FCP and othercab genes of FCPs is only ca. 30%. Phylogenetic analyses demonstrated that the FCP genes of both diatoms and chrysophytes sequenced to date are more closely related tocab genes encoding LHC I, CP 29, and CP 24 of higher plants than tocab genes encoding LHC II of chlorophytes. We propose that LHC I, CP 24 and CP 29 and FCP might have originated from a common ancestral chl binding protein and that the major LHC II of Chla/b-containing organisms arose after the divergence between the chromophytes and the chlorophytes.     

Circadian expression of the light-harvesting protein of Photosystem II in etiolated bean leaves following a single red light pulse: Coordination with the capacity of the plant to form chlorophyll and the thylakoid-bound protease

The appearance of the light harvesting II (LHC II) protein in etiolated bean leaves, as monitored by immunodetection in LDS-solubilized leaf protein extracts, is under phytochrome control. A single red light pulse induces accumulation of the protein, in leaves kept in the dark thereafter, which follows circadian oscillations similar to those earlier found for Lhcb mRNA (Tavladoraki et al. (1989) Plant Physiol 90: 665–672). These oscillations are closely followed by oscillations in the capacity of the leaf to form Chlorophyll (Chl) in the light, suggesting that the synthesis of the LHC II protein and its chromophore are in close coordination. Experiments with levulinic acid showed that PChl(ide) resynthesis does not affect the LHC II level nor its oscillations, but new Chl a synthesis affects LHC II stabilization in thylakoids, implicating a proteolytic mechanism. A proteolytic activity against exogenously added LHC II was detected in thylakoids of etiolated bean leaves, which was enhanced by the light pulse. The activity, also under phytochrome control, was found to follow circadian oscillations in verse to those in the stabilization of LHC II protein in thylakoids. Such a proteolytic mechanism therefore, may account for the circadian changes observed in LHC II protein level, being implicated in pigment-protein complex assembly/stabilization during thylakoid biogenesis.Abbreviations Chl chlorophyll - CL continuous light - D dark - FR far-red light - LA levulinic acid - LHC II light-harvesting complex serving Photosystem II - PChl(ide) protochlorophyllide - PCR protochlorophyllide oxidoreductase - R red light     

Evidence for the role of surface-exposed segments of the light-harvesting complex in cation-mediated control of chloroplast structure and function.

Chloroplast membranes contain a light-harvesting pigment-protein complex (LHC) which binds chlorophylls a and b. A mild trypsin digestion of intact thylakoid membranes has been utilized to specifically alter the apparent molecular weights of polypeptides of this complex. The modified membrane preparations were analyzed for altered functional and structural properties. Cation-induced changes in room temperature fluorescence intensity and low temperature chlorophyll fluorescence emission spectra, and cation regulation of the quantum yield of photosystem I and II partial reactions at limiting light were lost following the trypsin-induced alteration of the LHC. Electron microscopy revealed that cations can neither maintain nor promote grana stacking in membranes which have been subjected to mild trypsin treatment. Freeze-fracture analysis of these membranes showed no significant differences in particle density or average particle size of membrane subunits on the EF fracture face; structural features of the modified lamellae were comparable to membranes which had been unstacked in a “low salt” buffer. Digitonin digestion of trypsin-treated membranes in the presence of cations followed by differential centrifugation resulted in a subchloroplast fractionation pattern similar to that observed when control chloroplasts were detergent treated in cation-free medium. We conclude that: (a) the initial action of trypsin at the thylakoid membrane surface of pea chloroplasts was the specific alteration of the LHC polypeptides, (b) the segment of the LHC polypeptides which was altered by trypsin is necessary for cation-mediated grana stacking and cation regulation of membrane subunit distribution, and (c) cation regulation of excitation energy distribution between photosystem I and II involves the participation of polypeptide segments of the LHC which are exposed at the membrane surface.     

Solubilization of green plant thylakoid membranes with n-dodecyl-α,D-maltoside. Implications for the structural organization of the Photosystem II,Photosystem I,ATP synthase and cytochrome b6 f complexes

A biochemical and structural analysis is presented of fractions that were obtained by a quick and mild solubilization of thylakoid membranes from spinach with the non-ionic detergent n-dodecyl-α,D-maltoside, followed by a partial purification using gel filtration chromatography. The largest fractions consisted of paired, appressed membrane fragments with an average diameter of about 360 nm and contain Photosystem II (PS II) and its associated light-harvesting antenna (LHC II), but virtually no Photosystem I, ATP synthase and cytochrome b ₆ f complex. Some of the membranes show a semi-regular ordering of PS II in rows at an average distance of about 26.3 nm, and from a partially disrupted grana membrane fragment we show that the supercomplexes of PS II and LHC II represent the basic structural unit of PS II in the grana membranes. The numbers of free LHC II and PS II core complexes were very high and very low, respectively. The other macromolecular complexes of the thylakoid membrane occurred almost exclusively in dispersed forms. Photosystem I was observed in monomeric or multimeric PS I-200 complexes and there are no indications for free LHC I complexes. An extensive analysis by electron microscopy and image analysis of the CF₀F₁ ATP synthase complex suggests locations of the δ (on top of the F₁ headpiece) and ∈ subunits (in the central stalk) and reveals that in a substantial part of the complexes the F₁ headpiece is bended considerably from the central stalk. This kinking is very likely not an artefact of the isolation procedure and may represent the complex in its inactive, oxidized form. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influences of Blue and Red Light on the Photosynthetic Apparatus of Chlorella kessleri*

In white light of 33.2 μmol . m^?2 . s^?1 oxygen evolution of Chlorella kessleri is about 30 % higher after growth in blue light than after growth in red light of the same quantum fluence rate. When determined by the light-induced absorbance change at γ 820 nm, blue light-adapted cells possess about 60% more reaction centres per total chlorophyll in photosystem II. Correspondingly, the cells exhibit about 30% more Hill activity of PS II. Conversely, red light-adapted cells contain relatively more reaction centres and higher electron flow capacities of photosystem I. The distribution of total chlorophyll among the pigment-protein complexes, CPI, CPIa, CPa, and LHC II, corresponds to these data. There is more chlorophyll associated with the light-harvesting complex of PS II, LHC II, in cells under blue light conditions, but more chlorophyll bound to both complexes of PS I, CPI and CPIa, in cells under red light conditions. The respective ratios of chlorophyll a/chlorophyll b of all complexes are identical for blue and red light-adapted cells. This results in a higher relative amount of chlorophyll b in blue light-adapted cells. Total carotenoids per total chlorophyll are increased by 20% in red light-adapted cells. Their distribution among the pigment-protein complexes is unknown, however the ratios of lutein, neoxanthin and violaxanthin extractable from LHC II are different in blue (32.1:35.9:32.0) and in red (51.4:26.7:21.9) light-adaptod cells.     

Phosphatase activities in spinach thylakoid membranes-effectors, regulation and location

The dephosphorylation of seven phosphoproteins associated with Photosystem II or its chlorophyll a/b antenna in spinach thylakoids, was characterised. The rates were found to fall into two distinct groups. One, rapidly dephosphorylated, consisted of the two subunits (25 and 27 kD) of the major light harvesting complex of Photosystem II (LHC II) and a 12 kD polypeptide of unknown identity. A marked correlation between the dephosphorylation of these three phosphoproteins, strongly suggested that they were all dephosphorylated by the same enzyme. Within this group, the 25 kD subunit was consistently dephosphorylated most rapidly, probably reflecting its exclusive location in the peripheral pool of LHC II. The other group, only slowly dephosphorylated, included several PS II proteins such as the D1 and D2 reaction centre proteins, the chlorophyll-a binding protein CP43 and the 9 kD PS II-H phosphoprotein. No dephosphorylation was observed in either of the two groups in the absence of Mg²⁺-ions. Dephosphorylation of the two LHC II subunits took place in both grana and stroma-exposed regions of the thylakoid membrane. However, deposphorylation in the latter region was significantly more rapid, indicating a preferential dephosphorylation of the peripheral (or mobile) LHC II. Dephosphorylation of LHC II was found to be markedly affected by the redox state of thiol-groups, which may suggest a possible regulation of LHC II dephosphorylation involving the ferredoxin-thioredoxin system.Abbreviations CP 43 43 kD chlorophyll a- binding protein - D1 and D2 reaction centre proteins of PS II - LHC II light-harvesting complex of PS II - LHC II-25 25 kD subunit of LHC II - LHC II-27 27 kD subunit of LHC II - NEM N-ethylmaleimide - PP2C protein phosphatase 2C - PS II-H psb H gene product     

Leaf senescence in a non-yellowing mutant of Festuca pratensis Huds.

A study was made of linolenic acid-dependent oxidative chlorophyll bleaching (CHLOX) by thylakoid membranes from senescing leaf tissue of a normal cultivar (cv. Rossa) and a non-yellowing mutant genotype (Bf 993) of Festuca pratensis Huds. To overcome the problem of variation in levels of endogenous chlorophyll substrate in membranes from different sources, light-harvesting complex (LHC) was used to supplement thylakoid pigment. It was shown that CHLOX is associated with both Photosystem I and LHC-rich thylakoid subfractions but that purified LHC has negligible associated CHLOX activity and stimulates the rate of bleaching by isolated entire chloroplast membranes. Non-senescent tissue of Bf 993 and Rossa had essentially identical thylakoid CHLOX levels, which subsequently declined during senescence in darkness. The half-life of CHLOX from the mutant was three times greater than that of the normal genotype. In both cultivars, the amount of CHLOX assayed in thylakoids isolated at different times during senescence was more than adequate to support the corresponding in-vivo rate of pigment degradation as calculated from the half-life for chlorophyll. It was concluded that the non-yellowing mutation is not expressed through a lack of CHLOX activity. The role of linolenic acid metabolism in the regulation of thylakoid structure and function during senescence, and as a likely site of the non-yellowing lesion, are discussed.Abbreviations CHLOX linolenic acid-dependent oxidative chlorophyll bleaching activity - CHLPX chlorophyll peroxidase - CPI chlorophyll-protein complex I - LHC light-harvesting complex - LNA linolenic acid - PSI photosystem I - PSII photosystem II - S relative senescence rate - t _1/2 lialf time for degradation     

Biogenesis of thylakoid membranes with emphasis on the process in Chlamydomonas

Recent results obtained by electron microscopic and biochemical analyses of greening Chlamydomonas reinhardtii y1 suggest that localized expansion of the plastid envelope is involved in thylakoid biogenesis. Kinetic analyses of the assembly of light-harvesting complexes and development of photosynthetic function when degreened cells of the alga are exposed to light suggest that proteins integrate into membrane at the level of the envelope. Current information, therefore, supports the earlier conclussion that the chloroplast envelope is a major biogenic structure, from which thylakoid membranes emerge. Chloroplast development in Chlamydomonas provides unique opportunities to examine in detail the biogenesis of thylakoids.Abbreviations Rubisco ribulose bisphosphate carboxylase/oxygenase - CAB Chl a/b-binding (proteins) - Chlide chlorophyllide - LHC I light-harvesting complex of PS I - LHC II light-harvesting complex of PS II - Pchlide protochlorophyllide