Characterisation of senescence-induced changes in light harvesting complex II and photosystem I complex of thylakoids of Cucumis sativus cotyledons: age induced association of LHCII with photosystem I

Structure and function of chloroplasts are known to after during senescence. The senescence-induced specific changes in light harvesting antenna of photosystem II (PSII) and photosystem I (PSI) were investigated in Cucumis cotyledons. Purified light harvesting complex II (LHCII) and photosystem I complex were isolated from 6-day non-senescing and 27-day senescing Cucumis cotyledons. The chlorophyll a/b ratio of LHCII obtained from 6-day-old control cotyledons and their absorption, chlorophyll a fluorescence emission and the circular dichroism (CD) spectral properties were comparable to the LHCII preparations from other plants such as pea and spinach. The purified LHCII obtained from 27-day senescing cotyledons had a Chl a/b ratio of 1.25 instead of 1.2 as with 6-day LHCII and also exhibited significant changes in the visible CD spectrum compared to that of 6-day LHCII, indicating some specific alterations in the organisation of chlorophylls of LHCII. The light harvesting antenna of photosystems are likely to be altered due to aging. The room temperature absorption spectrum of LHCII obtained from 27-day senescing cotyledons showed changes in the peak positions. Similarly, comparison of 77K chlorophyll a fluorescence emission characteristics of LHCII preparation from senescing cotyledons with that of control showed a small shift in the peak position and the alteration in the emission profile, which is suggestive of possible changes in energy transfer within LHCII chlorophylls. Further, the salt induced aggregation of LHCII samples was lower, resulting in lower yields of LHCII from 27-day cotyledons than from normal cotyledons. Moreover, the PSI preparations of 6-day cotyledons showed Chl a/b ratios of 5 to 5.5, where as the PSI sample of 27-day cotyledons had a Chl a/b ratio of 2.9 suggesting LHCII association with PSI. The absorption, fluorescence emission and visible CD spectral measurements as well as the polypeptide profiles of 27-day cotyledon-PSI complexes indicated age-induced association of LHCII of PSII with PSI obtained from 27-day cotyledons. We modified our isolation protocols by increasing the duration of detergent Triton X-100 treatment for preparing the PSI and LHCII complexes from 27-day cotyledons. However, the PSI complexes isolated from senescing samples invariably proved to have significantly low Chl a/b ratio suggesting an age induced lateral movement and possible association of LHCII with PSI complexes. The analyses of polypeptide compositions of LHCII and PSI holocomplexes isolated from 6-day control and 27-day senescing cotyledons showed distinctive differences in their profiles. The presence of 26-28 kDa polypeptide in PSI complexes from 27-day cotyledons, but not in 6-day control PSI complexes is in agreement with the notion that senescence induced migration of LHCII to stroma lamellae and its possible association with PSI. We suggest that the migration of LHCII to the stroma lamellae region and its possible association with PSI might cause the destacking and flattening of grana structure during senescence of the chloroplasts. Such structural changes in light harvesting antenna are likely to alter energy transfer between two photosystems. The nature of aging induced migration and association of LHCII with PSI and its existence in other senescing systems need to be estimated in the future.     

Lateral mobility of the light-harvesting complex in chloroplast membranes controls excitation energy distribution in higher plants

Chloroplast thylakoid protein phosphorylation produces changes in light-harvesting properties and in membrane structure as revealed by freeze-fracture electron microscopy. Protein phosphorylation resulted in an increase in the 77 °K fluorescence signal at 735 nm relative to that at 685 nm. In addition, a decrease in connectivity between Photosystem II centers (PS II) and a dynamic quenching of the room temperature variable fluorescence was observed upon phosphorylation. Accompanying these fluorescence changes was a 23% decrease in the amount of stacked membranes. Microscopic analyses indicated that 8.0-nm particles fracturing on the P-face moved from the stacked into the unstacked regions upon phosphorylation. The movement of the 8.0-nm particles was accompanied by the appearance of chlorophyll b and 25 to 29 kD polypeptides in isolated stroma lamellae fractions. We conclude that phosphorylation of a population of the light-harvesting chlorophyll $\text{a}\text{b}$ protein complexes (LHC) in grana partitions causes the migration of these pigment proteins from the PS II-rich appressed membranes into the Photosystem I (PS I) enriched unstacked regions. This increases the absorptive cross section of PS I. In addition, we suggest that the mobile population of LHC functions to interconnect PS II centers in grana partitions; removal of this population of LHC upon phosphorylation limits PS II → PS II energy transfer and thereby favors spillover of energy from PS II to PS I.     

Photosystem II solubilizes as a monomer by mild detergent treatment of unstacked thylakoid membranes*

We studied the aggregation state of Photosystem II in stacked and unstacked thylakoid membranes from spinach after a quick and mild solubilization with the non-ionic detergent n-dodecyl-α,D-maltoside, followed by analysis by diode-array-assisted gel filtration chromatography and electron microscopy. The results suggest that Photosystem II (PS II) isolates either as a paired, appressed membrane fragment or as a dimeric PS II-LHC II supercomplex upon mild solubilization of stacked thylakoid membranes or PS II grana membranes, but predominantly as a core monomer upon mild solubilization of unstacked thylakoid membranes. Analysis of paired grana membrane fragments reveals that the number of PS II dimers is strongly reduced in single membranes at the margins of the grana membrane fragments. We suggest that unstacking of thylakoid membranes results in a spontaneous disintegration of the PS II-LHC II supercomplexes into separated PS II core monomers and peripheral light-harvesting complexes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Lateral heterogeneity in the distribution of chlorophyll-protein complexes of the thylakoid membranes of spinach chloroplasts

The lateral distribution of the main chlorophyll-protein complexes between appressed and non-appressed thylakoid membranes has been studied. The reaction centre complexes of Photosystems I and II and the light-harvesting complex have been resolved by an SDS-polyacrylamide gel electrophoretic method which permits most of the chlorophyll to remain protein-bound.

The analyses were applied to subchloroplast fractions shown to be derived from different thylakoid regions. Stroma thylakoids were separated from grana stacks by centrifugation following chloroplast disruption by press treatment or digitonin. Vesicles derived from the grana partitions were isolated by aqueous polymer two-phase partition. A substantial depletion in the amount of Photosystem I chlorophyll-protein complex and an enrichment in the Photosystem II reaction centre complex and the light-harvesting complex occurred in the appressed grana partition region. The high enrichment in this fraction compared to grana stack fractions derived from press or digitonin treatments, suggests that the grana Photosystem I is restricted mainly to the non-appressed grana end membranes and margins, and that the grana partitions possess mainly Photosystem II reaction centre complex and the light-harvesting complex.

In contrast, stroma thylakoids are highly enriched in the Photosystem I reaction centre complex. They possess also some 10–20% of the total Photosystem II reaction centre complex and the light-harvesting complex.

The ratio of light-harvesting complex to Photosystem II reaction centre complex is rather constant in all subchloroplast fractions suggesting a close association between these complexes. This was not so for the ratio of light-harvesting complex and the Photosystem I reaction centre complex.

The lateral heterogeneity in the distribution of the photosystems between appressed and non-appressed membranes must have a profound impact on current understanding of both the distribution of excitation energy and photosynthetic electron transport between the photosystems.     

Complex formation in plant thylakoid membranes. Competition studies on membrane protein interactions using synthetic peptide fragments

Thylakoid membranes of pea were used to study competition between extra-membrane fragments and their parental membrane-bound proteins. Phosphorylated and unphosphorylated fragments of light harvesting complex II (LHC II) from higher plants were used to compete with LHC II for interactions with itself and with other thylakoid protein complexes. Effects of these peptide fragments of LHC II and of control peptides were followed by 80 K chlorophyll fluorescence spectroscopy of isolated thylakoids. The phosphorylated LHC II fragment competes with membrane-bound phosphoproteins in the phosphatase reaction. The same fragment accelerates the process of dark-to-light adaptation and decreases the rate of the light-to-dark adaptation when these are followed by fluorescence spectroscopy. In contrast, the non-phosphorylated LHC II peptide does not affect the rate of adaptation but produces results consistent with inhibition of formation of a quenching complex. In this quenching complex we propose that LHC II remains inaccessible to the LHC II kinase, explaining an observed decrease in LHC II phosphorylation in the later stages of the time-course of phosphorylation. The most conspicuous protein which is steadily phosphorylated during the time-course of phosphorylation is the 9 kDa (psbH) protein. The participation of the phosphorylated form of psbH in the quenching complex, where it is inaccessible to the phosphatase, may explain its anomalously slow dephosphorylation. The significance of the proposed complex of LHC II with phospho-psbH is discussed.Abbreviations LHC II light harvesting complex II - PS II Photosystem II - PS I Photosystem I     

Supramolecular photosystem II organization in grana thylakoid membranes: evidence for a structured arrangement

The distribution of photosystem (PS) II complexes in stacked grana thylakoids derived from electron microscopic images of freeze-fractured chloroplasts are examined for the first time using mathematical methods. These characterize the particle distribution in terms of a nearest neighbor distribution function and a pair correlation function. The data were compared with purely random distributions calculated by a Monte Carlo simulation. The analysis reveals that the PSII distribution in grana thylakoids does not correspond to a random protein mixture but that ordering forces lead to a structured arrangement on a supramolecular level. Neighboring photosystems are significantly more separated than would be the case in a purely random distribution. These results are explained by structural models, in which boundary lipids and light-harvesting complex (LHC) II trimers are arranged between neighboring PSII. Furthermore, the diffusion of PSII was analyzed by a Monte Carlo simulation with a protein density of 80% area occupation (determined for grana membranes). The mobility of the photosystems is severely reduced by the high protein density. From an estimate of the mean migration time of PSII from grana thylakoids to stroma lamellae, it becomes evident that this diffusion contributes significantly to the velocity of the repair cycle of photoinhibited PSII.     

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     

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.     

Structural reorganisation of chloroplast thylakoid membranes in response to heat-stress

Chloroplasts isolated from broad bean (Vicia faba) show major structural reorganisations on heating to temperatures above 35°C. Exposure to increasing temperatures in the range 35–45°;C for 5 min, leads to a progressive destacking of the chloroplast membranes and the replacement of the normal granal arrangement by modified thylakoid attachment sites. An analysis of the size and packing densities of the freeze-fracture particles present in different membrane fracture-faces suggests that this rearrangement reflects the dissociation of the light-harvesting units of Photosystem II. The antennae complexes of Photosystem II appear to cluster together, maintaining regions of membrane adhesion, whilst excluding the core-complexes of Photosystem II and light-harvesting units of Photosystem I from these regions. If the chloroplasts are heated to higher temperatures, 45–55°C, phase-separated aggregates of non-bilayer-forming lipids are often observed. The release of these lipids from their normal constraints within the bilayer is consistent with the idea that they play a role in the packaging of the light-harvesting complexes within the thylakoid membrane.     

Genetic engineering of thylakoid protein complexes by chloroplast transformation in Chlamydomonas reinhardtii

Chloroplast transformation of Chlamydomonas reinhardtii has developed into a powerful tool for studying the structure, function and assembly of thylakoid protein complexes in a eukaryotic organism. In this article we review the progress that is being made in the development of procedures for efficient chloroplast transformation. This focuses on the development of selectable markers and the use of Chlamydomonas mutants, individually lacking thylakoid protein complexes, as recipients. Chloroplast transformation has now been used to engineer all four major thylakoid protein complexes, photosystem II, photosystem I, cytochrome b ₆/f and ATP synthase. These results are discussed with an emphasis on new insights into assembly and function of these complexes in chloroplasts as compared with their prokaryotic counterparts.Abbreviations ENDOR electron nuclear double resonance - ESEEM electron spin echo envelope modulation - LHC light harvesting complex - PSI Photosystem I - PS II Photosystem II - P₆₈₀ primary electron donor in PS II - P₇₀₀ primary electron donor in PS I     

Heterogeneity of thylakoid membranes studied by EPR spin probe

A lipophilic nitroxyl radical, 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl 1-adamantylacetate, has been applied to EPR spin probe study of chloroplasts and subchloroplast fragments of different types. The latter originate from grana and the grana core regions. The binding of the spin probe to the membranes was revealed by specific changes in a shape of the EPR spectra. A share of membrane-bound spin probe was different for chloroplasts and subchloroplast fragments, as well as its rotational correlation time and apparent enthalpy and entropy activation of nitroxide rotational motion. The binding of the spin probe induced a significant decrease in the amount of the oxidized P700 and changes in the kinetics of its light oxidation and dark recovery. This suggests that one of the sites of nitroxyl radical binding is the nearest surrounding of the pigment-protein complexes of Photosystem I (PSI). Distinctions in mobility of spin probe immobilized by chloroplasts and their fragments can be caused by the different environment of the PSI complexes located in various regions of thylakoid membranes. Published in Russian in Biokhimiya, 2007, Vol. 72, No. 5, pp. 690–698.     

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     

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     

Photosynthetic activities of cadmium-treated tomato plants

Tomato plants (Lycopersicum esculentum Mill. cv. Moneymaker) grown on nutrient medium containing cadmium exhibit reduced net photosynthesis and reduced contents of chlorophyll and accessory pigments. In chloroplasts isolated from cadmiumtreated plants photosystem II activity, as measured by 2,6-dichlorophenolindophenol photoreduction, and photosystem II + I activity (H₂O → methyl viologen) were both inhibited to about 60%. When 1,5-diphenylcarbazide was used as artificial electron donor, no significant cadmium effect was observed. Photosystem I activity was not affected by cadmium. The fine structure of chloroplasts in cadmium-treated plants was degenerated, similarly to senescence response. The principal symptom of cadmium action was the occurrence of large plastoglobules and a disorganization of the lamellar structure, mainly grana stacks. Transfer of cadmium-treated plants into a medium with increased manganese level caused grana stacking and restoration of photosystem II activity.     

Changes in lipid composition of Photosystem 1 particles from thylakoids phosphorylated under reductive or anaerobic conditions

Changes in lipid composition of Photosystem 1 (PS 1) particles isolated from thylakoids phosphorylated under reductive or anaerobic conditions have been studied. Under reductive conditions, there was an increase in monogalactosyldiacylglycerol containing highly saturated fatty acids and phosphatidylglycerol containing transhexadecenoic fatty acid. Under anaerobic conditions, the amount of all lipid classes was increased. As we have shown earlier (S. V. Manuilskaya, O. I. Volovik, A. I. Mikhno, A. I. Polischuk and S. M. Kochubey (1990) Photosynthetica 24: 419–423) these changes were due to a co-migration of some lipid species and light-harvesting chlorophyll a/b complex LHC II from PS 2 to PS 1. These data allow us to conclude that LHC II consists of the lipoproteins containing specific lipids. Different composition of lipids co-migrating with LHC II under various conditions of phosphorylation might be caused by the variety of LHC II subpopulations transferred under each reductive condition.Abbreviations PS 1 Photosystem 1 - PS 2 Photosystem 2 - LHC II light-harvesting chlorophyll a/b protein complex II - Chl chlorophyll - MGDG monogalactosyldiacylglycerol - DGDG digalactosyldiacylglycerol - PG phosphatidylglycerol - SQDG sulfoquinovosyldiacylglycerol     

Regreening of senescent Nicotiana leaves. II. Redifferentiation of plastids

Single senescent leaves attached to decapitated shoots of Nicotiana rustica L. regreened, especially when treated with cytokinin. Regreening caused an increase in leaf thickness, due to cell expansion. Senescent leaf plastids (gerontoplasts) were smaller than green chloroplasts, with degenerated membrane systems and stroma, and larger plastoglobuli. At advanced senescence, micrographs showed disintegrating gerontoplasts, reduced numbers of plastids were counted, and regreening became variable. The redevelopment of grana and stroma in regreening plastids was accelerated by cytokinin. All plastids in regreening leaves were identifiable as redifferentiating gerontoplasts because of their content of plastoglobuli and starch. Immunogold labelling showed significant association of POR with etioplasts in cotyledons, but with mature plastids in regreening leaves. No proplastids or dividing chloroplasts were observed in regreening leaves. Plastids numbers declined during senescence and did not increase again during regreening. It is concluded that the chloroplasts of regreening leaves arose by redifferentiation of gerontoplasts.Keywords: Chloroplasts, cytokinin, Nicotiana, senescence, regreening.      

Chloroplast ultrastructure and thylakoid polypeptide composition are affected by different salt concentrations in the halophytic plant Arthrocnemum macrostachyum

The effect of different external salt concentrations, from 0 mM to 1030 mM NaCl, on photosynthetic complexes and chloroplast ultrastructure in the halophyte Arthrocnemum macrostachyum was studied. Photosystem II, but not Photosystem I or cytochrome b6/f, was affected by salt treatment. We found that the PsbQ protein was never expressed, whereas the amounts of PsbP and PsbO were influenced by salt in a complex way. Analyses of Photosystem II intrinsic proteins showed an uneven degradation of subunits with a loss of about 50% of centres in the 0 mM NaCl treated sample. Also the shape of chloroplasts, as well as the organization of thylakoid membranes were affected by NaCl concentration, with many grana containing few thylakoids at 1030 mM NaCl and thicker grana and numerous swollen thylakoids at 0 mM NaCl. The PsbQ protein was found to be depleted also in thylakoids from other halophytes.     

Supramolecular organization of thylakoid membrane proteins in green plants

The light reactions of photosynthesis in green plants are mediated by four large protein complexes, embedded in the thylakoid membrane of the chloroplast. Photosystem I (PSI) and Photosystem II (PSII) are both organized into large supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with single copies of four different LHCI proteins and has binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain usually two to four copies of trimeric LHCII complexes. These supercomplexes have a further tendency to associate into megacomplexes or into crystalline domains, of which several types have been characterized. Together with the specific lipid composition, the structural features of the main protein complexes of the thylakoid membranes form the main trigger for the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. We suggest that the margins, the strongly folded regions of the membranes that connect the grana, are essentially protein-free, and that protein-protein interactions in the lumen also determine the shape of the grana. We also discuss which mechanisms determine the stacking of the thylakoid membranes and how the supramolecular organization of the pigment-protein complexes in the thylakoid membrane and their flexibility may play roles in various regulatory mechanisms of green plant photosynthesis.     

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     

A Comparison of Pigment-Protein Complexes among Normal, Chlorophyll-Deficient and Senescent Soybean Genotypes

Pigment-protein complexes were isolated from chloroplasts of normal green and several types of chlorophyll-deficient soybeans. The complexes were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and comparisons were made between normal and chlorophyll-deficient genotypes of the relative amounts of chlorophyll associated with Photosystem I (PSI), Photosystem II (PSII), light-harvesting, and free pigment complexes.

Chlorophyll-deficient genotypes, compared to normal green genotypes, have fewer light-harvesting complexes and a higher ratio of PSII to PSI complexes. Chlorophyll associated with PSII in yellow genotypes is in relatively higher amounts in spite of the fact that these genotypes have much less grana stacking than normal green genotypes. Although PSII activity has been associated with appressed regions of grana in normal plants, our work shows that the association does not always hold true.