A major polypeptide of chloroplast membranes of Chlamydomonas reinhardi. Evidence for synthesis in the cytoplasm as a soluble component

Chloroplast vesicle transport

Photosynthesis is a well-known process that has been intensively investigated, but less is known about the biogenesis of the thylakoid membrane that harbors the photosynthetic machinery. Thylakoid membranes are constituted by several components, the major ones being proteins and lipids. However, neither of these two are produced in the thylakoid membranes themselves but are targeted there by different mechanisms. The interior of the chloroplast, the stroma, is an aqueous compartment that prevents spontaneous transport of single lipids and/or membrane proteins due to their hydrophobicities. Thylakoid targeted proteins are encoded either in the nucleus or plastid, and thus some cross the envelope membrane before entering one of the identified thylakoid targeting pathways. However, the pathway for all thylakoid proteins is not known. Lipids are produced at the envelope membrane and have been proposed to reach the thylakoid membrane by different means: invaginations of the envelope membrane, direct contact sites between these membranes, or through vesicles. Vesicles have been observed in chloroplasts but not much is yet known about the mechanism or regulation of their formation. The question of whether proteins can also make use of vesicles as one mechanism of transport remains to be answered. Here we discuss the presence of vesicles in chloroplasts and their potential role in transporting lipids and proteins. We additionally discuss what is known about the proteins involved in the vesicle transport and the gaps in knowledge that remain to be filled.

Proteins affecting thylakoid morphology - the key to understanding vesicle transport in chloroplasts?

We recently showed that a Rab protein, CPRabA5e (CP = chloroplast localized), is located in chloroplasts of Arabidopsis thaliana where it is involved in various processes, such as thylakoid biogenesis and vesicle transport. Using a yeast two-hybrid method, CPRabA5e was shown to interact with a number of chloroplast proteins, including the CURVATURE THYLAKOID 1A (CURT1A) protein and the light-harvesting chlorophyll a/b binding protein (LHCB1.5). CURT1A has recently been shown to modify thylakoid architecture by inducing membrane curvature in grana, whereas LHCB1.5 is a protein of PSII (Photosystem II) facilitating light capture. LHCB1.5 is imported to chloroplasts and transported to thylakoid membranes using the post-translational Signal Recognition Particle (SRP) pathway. With this information as starting point, we here discuss their subsequent protein-protein interactions, given by the literature and Interactome 3D. CURT1A itself and several of the proteins interacting with CURT1A and LHCB1.5 have relations to vesicle transport and thylakoid morphology, which are also characteristics of cprabA5e mutants. This highlights the previous hypothesis of an alternative thylakoid targeting pathway for LHC proteins using vesicles, in addition to the SRP pathway.

The role of chlorophyll b in photosynthesis: hypothesis

BACKGROUND

The physico-chemical properties of chlorophylls b and c have been known for decades. Yet the mechanisms by which these secondary chlorophylls support assembly and accumulation of light-harvesting complexes in vivo have not been resolved.

PRESENTATION

Biosynthetic modifications that introduce electronegative groups on the periphery of the chlorophyll molecule withdraw electrons from the pyrrole nitrogens and thus reduce their basicity. Consequently, the tendency of the central Mg to form coordination bonds with electron pairs in exogenous ligands, a reflection of its Lewis acid properties, is increased. Our hypothesis states that the stronger coordination bonds between the Mg atom in chlorophyll b and chlorophyll c and amino acid sidechain ligands in chlorophyll a/b- and a/c-binding apoproteins, respectively, enhance their import into the chloroplast and assembly of light-harvesting complexes.

TESTING

Several apoproteins of light-harvesting complexes, in particular, the major protein Lhcb1, are not detectable in leaves of chlorophyll b-less plants. A direct test of the hypothesis--with positive selection--is expression, in mutant plants that synthesize only chlorophyll a, of forms of Lhcb1 in which weak ligands are replaced with stronger Lewis bases.

IMPLICATIONS

The mechanistic explanation for the effects of deficiencies in chlorophyll b or c points to the need for further research on manipulation of coordination bonds between these chlorophylls and chlorophyll-binding proteins. Understanding these interactions will possibly lead to engineering plants to expand their light-harvesting antenna and ultimately their productivity.

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.

Ribulose bisphosphate carboxylase from a mutant strain of Chlamydomonas reinhardii deficient in chloroplast ribosomes : The absence of both subunits and their pattern of synthesis during enzyme recovery

The ac-20 mutant strain of the unicellular green alga, Chlamydomonas reinhardii, lacks both chloroplast ribosomes and ribulose bisphosphate carboxylase activity when grown on organic medium. Under these conditions, the cells do not posses pools of either the large or small subunit of this enzyme. When transferred to inorganic medium, the carboxylase activity recovers. During this recovery, de novo synthesis of both subunits occurs. Synthesis of both subunits is inhibited by chloramphenicol even when possible free subunit pools rather than just the subunits incorporated into whole enzyme are examined.

Changes in polypeptide initiation and elongation rates during the cell cycle of Chlamydomonas reinhardi

Changes in the rate of protein synthesis during the cell cycle of Chlamydomonas reinhardi have been measured by determining changes in the separate rates of polypeptide chain initiation and elongation and in the rate of incorporation of a radioactive amino acid. The rate of polypeptide chain elongation, determined from the relative rates of labeling of two size classes of polyribosomes, varies only about twofold during the cell cycle. The rate of polypeptide chain initiation, determined from an analysis of the distribution of ribosomes in monoribosomes (and ribosomal subunits) and polyribosomes, varies more than 25-fold. Also, the overall rate of protein synthesis during the cell cycle varies to the same extent as the rate of chain initiation. Measurement of protein synthetic rates using incorporation of a radioactive amino acid ([3H]arginine) underestimates the actual change in the rate of protein synthesis during the cell cycle. The vast changes in the initiation rate during the cell cycle suggest a mechanism for selecting specific messenger RNAs for translation at different cell-cycle stages.

The cell cycle program of polypeptide labeling in Chlamydomonas reinhardtii

The cell cycle program of polypeptide labeling in syndhronous cultures of wild-type Chlamydomonas reinhardtii was analyzed by pulse-labeling cells with 35SO4 = or [3H]arginine at different cell cycle stages. Nearly 100 labeled membrane and soluble polypeptides were resolved and studied using one-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The labeling experiments produced the following results. (a) Total 35SO4 = and [3H]arginine incorporation rates varied independently throughout the cell cycle. 35SO4 = incorporation was highest in the mid-light phase, while [3H]arginine incorporation peaked in the dark phase just before cell division. (b) The relative labeling rate for 20 of 100 polypeptides showed significant fluctuations (3-12 fold) during the cell cycle. The remaining polypeptides were labeled at a rate commensurate with total 35SO4 = or [3H]arginine incorporation. The polypeptides that showed significant fluctuations in relative labeling rates served as markers to identify cell cycle stages. (c) The effects of illumination conditions on the apparent cell cycle stage-specific labeling of polypeptides were tested. Shifting light-grown asynchronous cells to the dark had an immediate and pronounced effect on the pattern of polypeptide labeling, but shifting dark-phase syndhronous cells to the light had little effect. The apparent cell cycle variations in the labeling of ribulose 1,5-biphosphate (RUBP)-carboxylase were strongly influenced by illumination effects. (d) Pulse-chase experiments with light-grown asynchronous cells revealed little turnover or inter-conversion of labeled polypeptides within one cell generation, meaning that major polypeptides, whether labeled in a stage-specific manner or not, do not appear transiently in the cell cycle of actively dividing, light-grown cells. The cell cycle program of labeling was used to analyze effects of a temperature-sensitive cycle blocked (cb) mutant. A synchronous culture of ts10001 was shifted to restrictive temperature before its block point to prevent it from dividing. The mutant continued its cell cycle program of polypeptide labeling for over a cell generation, despite its inability to divide.

The distribution of nascent polypeptide chains among intact polyribosomes from Chlamydomonas reinhardi

A model of polyribosome function based on tape theory has been applied to the analysis of intact polyribosomes from Chlamydomonas reinhardi. The distribution of nascent polypeptide chains found on polyribosomes does not conform to the expected pattern in which small polypeptides are synthesized on small polyribosomes and large polypeptides on correspondingly large polyribosomes. This discrepancy was revealed in the analysis of specific activity of polyribosomes (radioactivity in nascent chains per ribosome) versus polyribosome size at labeling saturation. It was found that the specific activity of small polyribosomes was higher than predicted and that of large polyribosomes was lower. This finding was validated by measuring the sizes of nascent chains from various polyribosome size classes by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The presence of large polypeptides on small polyribosomes could be partially accounted for by the synthesis of polypeptides on chloroplast (chloramphenicol-sensitive) polyribosomes. A maximum peptide interval time of 10 s was estimated from the labeling kinetics of the nascent chains of mid-sized (cytoplasmic) polyribosomes. This rate of translation is comparable to that reported in other eucaryotic cells.

Synthesis of Proteins by Isolated Euglena gracilis Chloroplasts

Intact Euglena gracilis chloroplasts, which had been purified on gradients of silica sol, incorporated [(35)S]methionine or [(3)H]leucine into soluble and membrane-bound products, using light as the only source of energy. The chloroplasts were osmotically shocked, fractionated on discontinuous gradients of sucrose, and the products of protein synthesis of the different fractions characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The soluble fraction resolved into three zones of radioactivity, the major one corresponding to the large subunit or ribulose diphosphate carboxylase. The thylakoid membrane fraction contained nine labeled polypeptides, the two most prominent in the region of 31 and 42 kilodaltons. The envelope fraction contained a major radioactive peak of about 48 kilodaltons and four other minor peaks. The patterns of protein synthesis by isolated Euglena chloroplasts are broadly similar to those observed with chloroplasts of spinach and pea.

Kinetics and regulation of synthesis of the major polypeptides of thylakoid membranes in Chlamydomonas reinhardtii y-1 at elevated temperatures

Etiolated cells of Chlamydomonas reinhardtii y-1 exhibit rapid and linear initial kinetics of greening when exposed to light at 38 degrees C. The initial rate of chlorophyll accumulation under these conditions is greater than the maximal rate during greening at 25 degrees C. Synthesis of the major polypeptides of thylakoid membranes within intact cells was assayed during greening by the incorporation of [3H]leucine and the subsequent electrophoresis of total cellular protein on polyacrylamide gels in the presence of sodium dodecyl sulfate. At 38 degrees C the major membrane polypeptides (about 28,000 and 24,000 daltons in mass) were synthesized at a linear rate after exposure of the cells to light, with no evidence of a lag period. A 1-2 h preincubation in the dark at the higher temperature was necessary to achieve linear initial kinetics. Actinomycin D inhibited synthesis of the membrane polypeptides if added at the beginning of a 2 h dark preincubation, but not when added near the end. These results suggested that transcription of the messenger RNA for the membrane polypeptides occurred during the dark period at 38 degrees C. But the major membrane polypeptides were not made by y-1 cells in the dark. The wavelengths of light most effective in eliciting production of the membrane polypeptides were the same as those allowing chlorophyll synthesis. In contrast, wild type cells, which are capable of chlorophyll synthesis in the dark, also make the membrane polypeptides in the dark. The data indicate that at elevated temperatures synthesis of the major thylakoid membrane polypeptides is controlled at a posttranscriptional step, and that this reaction normally proceeds only under conditions which permit reduction of protochlorophyllide.

Incorporation of polypeptides into thylakoid membranes of Chlamydomonas reinhardtii. Cyclic variations

A purified fraction of unstacked thylakoid membranes (TMF1u) has been obtained from homogenates of Chlamydomonas reinhardtii (wild type 137+) by using repeated centrifugates in sucrose density gradients and low salt treatment. The contaminants of the fraction are reduced to a few mitochondria (approximately 3% of the total mitochondrial population), a few osmiophilic granules, and fragments of chloroplast envelopes. By SDS-polyacrylamide gel electrophoresis the polypeptide components of TMF1u were resolved into at least 30 bands. To determine the relative rates of assembly of newly synthesized polypeptides into thylakoid membranes, synchronized algal cells were doubly labeled in vivo with L-[14C] and L-[3H]arginine--used for long- and short-term labeling, respectively. TMF1u's were isolated from the labeled cells at selected time points during the cycle and the distribution of radioactivity was assayed in the gel electrophoretograms of their solubilized polypeptides. Incorporation of newly synthesized polypeptides into the bands of the gels was found to occur continuously but differentially throughout the cycle. Maximal rates of incorporation for the majority of the polypeptides were detected shortly after cell division (6D-7D; equivalent to early G1 phase). The rates of radioactive labeling decreased gradually to a low level at the end of the dark period and then rose slightly at the beginning of the next light period. The findings suggest that, in addition to the light/dark control postulated in the past, assembly of newly synthesized proteins into thylakoid membranes is activated by signals at work in the early G1 phase.

A mutant strain of Chlamydomonas reinhardi exhibiting altered ribulosebisphosphate carboxylase

A mutant, ac i72, of Chlamydomonas reinhardi possessing an altered ribulosebisphosphate carboxylase and unable to grow on minimal medium has been isolated and characterized. Comparison of ribulosebisphosphate carboxylase purified from both wild type and ac i72 strains is given. The enzyme from ac i72 shows alterations in several characteristics: (a) the specific activity is reduced to 35% that of wild type, (b) the V for both substrates is reduced 3-6 fold, (c) the Mg2+ requirement for maximal activity is 3 times greater, (d) the inhibitory effect of Cl- is greater, and (e) the isoelectric point is changed (6.0 for wild type and 5.8 for ac i72). However, the ribulosebisphosphate carboxylase from ac i72 is identical to that from wild type with respect to pH requirement, temperature sensitivity, subunit structure, and sedimentation characteristic. Other photosynthetic properties of wild type and ac i72 cells were also compared. CO2 fixation in ac i72 in vivo is reduced proportionally to the reduction in activity of the enzyme, but the level of O2 evolution is the same as in wild-type cells. Photosynthetic electron transport, 70-S ribosome content, and chlorophyll content are unaltered in ac i72. The chloroplast ultrastructure of ac i72 cells is distinctly different from that of wild-type cells. The inheritance of the mutation is Mendelian.

The Light-harvesting Chlorophyll a/b-Protein Complex of Chlamydomonas reinhardii

The molecular organization of chlorophyll in Chlamydomonas reinhardii has been shown to be essentially similar to that in higher plants. Some 50% of the chlorophyll in Chlamydomonas reinhardii chloroplast membranes has been shown to be located in a chlorophyll a/b-protein complex. The complex was isolated in a homogeneous form by hydroxylapatite chromatography of sodium dodecyl sulfate extracts of the chloroplast membranes. Its absorption spectrum exhibits two maxima in the red region at 670 and 652 nm due to the presence of equimolar quantities of chlorophylls a and b in the complex. Preparations of the chlorophyll-protein also contain some of each of the carotenoids observed in the intact chloroplast membrane, but not in the same proportions. The native complex (S value = 2.3S) exhibits a molecular weight of 28,000 +/- 2,000 on calibrated sodium dodecyl sulfate-polyacrylamide gel electrophoresis. However, on the basis of its amino acid composition and other data a more probable molecular weight of about 35,000 was calculated. Each 35,000 dalton unit contains three chlorophyll a and three chlorophyll b molecules, and on the average one carotenoid molecule conjugated with probably a single polypeptide of 29,000 daltons. Comparison of spectral and biochemical characteristics demonstrates that this algal chlorophyll-protein is homologous to the previously described major light-harvesting chlorophyll a/b-protein of higher plants. It is anticipated that the Chlamydomonas complex functions solely in a light-harvesting capacity in analogy to the function determined for the higher plant component.

Chloroplast genes in Chlamydomonas affecting organelle ribosomes. Genetic and biochemical analysis of analysis of antibiotic-resistant mutants at several gene loci

Six chloroplast gene mutants of Chlamydomonas reinhardtii resistant to spectinomycin, erythromycin, or streptomycin have been assessed for antibiotic resistance of their chloroplast ribosomes. Four of these mutations clearly confer high levels of antibiotic resistance on the chloroplast ribosomes both in vivo. Although one mutant resistant to streptomycin and one resistant to spectinomycin have chloroplast ribosomes as sensitive to antibiotics as those of wild type in vivo, these mutations can be shown to alter the wildtype sensitivity of chloroplast ribosomes in polynucleotide-directed amino acid incorporation in vitro. Genetic analysis of these six chloroplast mutants and three similar mutants (Sager, 1972), two of which have been shown to affect chloroplast ribosomes (Mets and Bogorad, 1972; Schlanger and Sager, 1974), indicates that in Chlamydomonas at least three chloroplast gene loci can affect streptomycin resistance of chloroplast ribosomes and that two can affect erythromycin resistance. The three spectinomycin-resistant mutants examined appear to be alleles at a single chloroplast gene locus, but may represent mutations at two different sites within the same gene. Unlike wild type, the streptomycin and spectinomycin resistant mutants which have chloroplast ribosomes sensitive to antibiotics in vivo, grow well in the presence of antibiotic by respiring exogenously supplied acetate as a carbon source, and have normal levels of cytochrome oxidase activity and cyanide-sensitive respiration. We conclude that mitochondrial protein synthesis in these mutants is resistant to these antibiotics, whereas in wild type it is sensitive. To explain the behavior of these two chloroplast gene mutants as well as other one-step mutants which are resistant both photosynthetically and when respiring acetate in the dark, we have postulated that a mutation in a single chloroplast gene may result in alteration of both chloroplast and mitochondrial ribosomes. Mitochondrial resistance would appear to be the minimal necessary condition for survival of all such mutants, and antibiotic-resistant chloroplast ribosomes would be necessary for survival only under photosynthetic conditions.

Amino acid incorporation into protein by ribosomes bound to chloroplast thylakoid membranes: formation of discrete products

A system which incorporates amino acids into proteins of chloroplast membranes of Chlamydomonas reinhardti is described. It consists of chloroplast ribosomes bound to thylakoid membranes and cell extract. mRNA is present in this thylakoid-ribosome complex, since neither initiation nor RNA synthesis seems to be required for amino acid incorporation. Incorporation requires ATP, GTP and a soluble portion of cell extract. It is inhibited by chloramphenicol, but not cycloheximide. Most incorporated radioactivity remains bound to the membranes. Although a large portion of this labeled membrane-bound protein occurs as nascent polypeptides, a portion appears at least four products of discrete molecular weights. The major in vitro product migrates as a polypeptide of 23 000 daltons. We conclude that a portion of chloroplast membrane proteins is not only made within the chloroplast, but directly on the membranes. We had previously observed that release of membrane-bound ribosomes is partially dependent on puromycin, and concluded that some membrane-bound ribosomes were attached to the membranes through nascent protein chains. Thus, our results suggest that some chloroplast membrane proteins are inserted into the membranes as they are synthesized. This chloroplast membrane amino acid incorporation system offers a promising tool for studying biosynthesis of membrane proteins, and how they become inserted into chloroplast thylakoids to form functional membranes.

A large photoreactive particle from Chromatium vinosum chromatophores

Large photoreactive particles from Chromatium vinosum are obtained pure and in high yield by using a mixture of detergents at high ionic strength to dissociate the chromatophore membrane. The particles contain all of the secondary electron acceptor of the chromatophores and about half of the cytochrome. Their content of ubiquinone is greatly enridhed as compared with chromatophores. Th individual particles have an estimated molecular weight of between 650,000 and 810,000. Gel electrophoresis of the preparation in sodium dodecylsulfate shows polypeptides with molecular weights of 50-45,000, 30,000, 27,000, 22,000 and 12,000. The 50-45,000 components are cytochromes. The 30,000, 27,000 and 22,000 components may be analogous to the triad of polypeptides present in Rhodopseudomonas spheroides reaction centers. The non-cytochrome components are partly soluble in chloroform/methanol. Aggregates of particles appear in these preparations. Electron microscopy of the aggregates demonstrates rectilinear lattices of isodiametric particles, 120 A in diameter. These sheet-like structures are one unit thick and typically contain 9-16 members. They appear to arise by aggregation during isolation but are probably similar to native aggregates apparent within chromatophores after treatment with detergents at low salt concentration.

Synthesis and turnover of ribulose biphosphate carboxylase and of its subunits during the cell cycle of Chlamydomonas reinhardtii

The chloroplast enzyme ribulose-1,5-bisphosphate (Ru-1,5-P2) carboxylase (EC 4.1 1.39) is made up ot two nonidentical subunits, one synthesized in the chloroplast and the other outside. Both of these subunits of the assembled enzyme are synthesized in a stepwise manner during the synchronous cell cycle of the green alga Chlamydomonas reinhardtii. The activity of this enzyme increases in the light and this increase is due to de novo protein synthesis as shown by the measurement of the amount of protein and by the pulse incorporation of radioactive arginine in the 18S enzyme peak in linear sucrose density gradients. During the dark phase of the cell cycle, there is little change in the enzymatic activity as well as in the amount of this enzyme. Pulse-labeling studies using radioactive arginine indicated that there is a slow but detectable rate of synthesis of the carboxylase and of its subunits in the dark. Ru-1,5-P2 carboxylase, prelabeled with radioactive arginine throughout the entire light period, shows a similarly slow rate of degradation in the following dark period. This slow turnover of the enzyme in the dark accounts for the steady levels of carboxylase protein and of enzymatic activity during this period. A wide variety of inhibitors of protein synthesis by 70S and 80S ribosomes abolished the incorporation of [3H]arginine into total Ru-1,5-P2 carboxylase during short-term incubation. These results suggest a tight-coordinated control of the biosynthesis of the small and large subunits of the enzyme. This stringent control is further substantiated by the finding that both subunits are synthesized in sychrony with each other, that the ratio of radioactivity of the small to the large subunit remains constant throughout the entire light-dark cycle, and that the rates of synthesis and of degradation of both subunits are similar to that of the assembled enzyme.

Comparative studies on the polypeptide composition of chloroplast lamellae and lamellar fractions

The polypeptide composition of spinach chloroplast membranes and membrane fractions has been examined by the technique of sodium dodecylsulfate-polyacrylamide gel electrophoresis. Chloroplasts were fragmented into grana (Photosystem II enriched) and stroma lamellae (Photosystem I in character) by the French press technique. The grana lamellae were further fractionated by the use of digitonin into two fractions, one enriched in Photosystem II and the other enriched in Photosystem I. These membranes are composed of at least 15 polypeptides two of which, with approximate weights of 39 and 50 kdaltons, are observed only in granal fractions. Quantitatively the primarily Photosystem II fractions are enriched in polypeptides in the 30-23 kdalton range whereas the Photosystem I (or Photosystem I-enriched) fractions are enriched in polypeptides in the 60-54 kdalton region. The experiments reported show that contamination by soluble proteins or other membranes is negligible. The results indicate that subtle differences in composition account for the large differences in structure and function within the chloroplast membrane system.

Electrophoretic characterization of membrane proteins during chloroplast development in barley

Membranes of plastids isolated from greening 15-cm (6 days) barley seedling were analysed electrophoretically using acid-soaked polyacrylamide gels. During greening five new major classes of membrane-bound proteins appeared having apparent molecular weights of 100 000, 63 000, 41 000, 39 000, and 34 000, respectively. As greening progressed these proteins became the prominent feature of the electrophoretic pattern. Chloramphenicol and cycloheximide each had different inhibitory effects on the appearance of the new protein bands. Mutants of barley (xantha-f, g, h) blocked at an early stage in chloroplast development lacked the light-induced bands. Conversely, mutants xantha-b-12 and b-18 with lamellar systems organized into giant grana lacked some, but not all, of the light-induced bands. At the early stages of greening the newly formed membrane proteins and chlorophyll were inserted into existing membranes. At later stages, all membrane components appeared to be synthesized. Evidence is discussed that certain membrane proteins are specific for grana, while others are associated with stroma lamellae.

Synthesis of chloroplast membrane polypeptides during synchronous growth of Chlamydomonas reinhardtii

The synthesis of the major chloroplast membrane polypeptides has been studied during synchronous growth of Chlamydomonas reinhardtii. Under these conditions, chlorophyll is synthesized during the latter part of the light period and cell division takes place during the dark period. The profile of the chloroplast membrane polypeptides of C. reinhardtii has been well characterized and shown to contain two major classes by size (Hoober, J. 1970. J. Biol. Chem.245:4327). Polypeptides of group I have a mol wt range of 50,000-55,000 daltons. The second region consists of at least three polypeptide groups, IIa, IIb, and IIc, having mol wt of 40,000, 31,000, and 27,000 daltons, respectively. The synthesis of these polypeptides has been measured using a double-labeling technique and a computer-aided statistical analysis. The rate of labeling of group I polypeptides is highest during the early light period and decreases after 6 h of growth. Group IIa is labeled from the beginning of the light period, but little synthesis of IIb occurs before 3 h, and significant amounts of label are not found in IIc before 5 h of growth. After approximately 8 h of light, groups IIb and IIc are synthesized at rates significantly greater than those of the other membrane polypeptides. The synthesis of the major polypeptide groups ceases in the dark. We conclude that the biosynthesis of the chloroplast membranes is a sequential or stepwise process.

Rubulose diphosphate carboxylase synthesis in Chlamydomonas reinhardii: Inhibition by chloramphenicol and stimulation by cycloheximide

Chloramphenicol at 50 and 100 μg/ml inhibited synthesis of ribulose diphosphate carboxylase in the ac-20 mutant strain of Chlamydomonas reinhardii. Cycloheximide at 0.5 and 1.0 μg/ml significantly stimulated synthesis of this enzyme. By comparison, chloramphenicol had no effect on induction of isocitrate lyase by acetate, and cycloheximide completely inhibited this induction. The data suggest either that the carboxylase is made completely on 70S ribosomes or, less likely, that pools of subunit made on 80S ribosomes exist with long life times in the C. reinhardii cell.

Influence of gibberellic and abscisic acids and the growth retardant, CCC, upon plastid development

Gibberellic acid (GA3) enhances ultrastructural morphogenesis of plastids in greening cereals whilst abscisic acid (ABA) and CCC have the reverse effect over a shorter period. GA3 application counteracts the ABA and CCC inhibition of membrane development and, over longer periods of greening, the fastest rate of chloroplast development is shown in the presence of both GA3 and ABA. Experiments with isolated etioplasts show that the ABA inhibition of development also occurs and can be counteracted with GA3 treatment but no individual enhancement of plastid morphogenesis by GA3 was detected.Ribulose 1,5-diphosphate carboxylase (RuDPC) acitvity was also increased with applications of GA3 and reduced with ABA treatment. The initial levels of RuDPC activity in the presence of CCC were concentration dependant; high in 10(-3) M CCC and low in 10(-6) M CCC but activity returned to normal levels after CCC application was stopped. Experiments with isolated etio-chloroplasts gave similar results but levels of RuDPC activity declined rapidly in both illuminated and un-illuminated incubated suspensions of intact etioplasts.

Can a non-Mendelian mutation affect both chloroplast and mithchondrial ribosomes?

Chloroplast ribosomes isolated from a spectinomycin-resistant mutant (spr-1-27-3) of Chlamydomonas reinhardtii that displays non-Mendelian inheritance fail to bind labeled antibiotic, in contrast to ribosomes from wild-type cells. In vitro resistance of this mutant appears to result from the absence of a specific protein in the small subunit of the chloroplast ribosome. However, chloroplast protein synthesis in the mutant and wild type shows identical sensitivity to spectinomycin in short-term in vivo experiments where ribulosediphosphate carboxylase serves as the marker. Long-term experiments demonstrate that the mutant can grow in the presence of spectinomycin only when acetate is supplied as a carbon source. Mitochondrial structure and function of the mutant are not affected by the antibiotic, whereas chloroplast structure and function are. Apparently, the mitochondrion, rather than the chloroplast, of this mutant is resistant to spectinomycin in vivo. We hypothesize that the gene product of the spr locus is a protein common to both chloroplast and mitochondrial ribosomes. The mutant gene product, in vivo, confers resistance on mitochondrial, but not chloroplast, ribosomes. We suppose that the mutant spr protein loosely attaches to chloroplast ribosomes in vivo so that the antibiotic is bound and blocks protein synthesis, but it dissociates during isolation, resulting in loss of the binding site.

Developmental changes in the levels of SDS-extractable polypeptides during plastid morphogenesis

A quantitative estimation of the levels of plastidic SDS-extractable polypeptides as separated by polyacrylamide gel electrophoresis is described to demonstrate the practicality of such an approach. Using an internal standard of cytochrome c and expressing all polypeptide levels as cytochrome c relative stain equivalents, the levels of most polypeptides from developing Avena plastids change relative to the period of greening especially over the period 12-20 h. Some changes in certain polypeptides can be shown to be due to plastid senescence rather than plastid development. There is also a distinct difference in the pattern of polypeptides when plastids are isolated from different laminar regions. An incubation study using etioplasts showed of the original 8 polypeptides, six were retained, two were lost, another two were formed during incubation but eleven polypeptides found in the in situ study never appeared.

Transcription and translation for carotenoid synthesis in Chlamydomonas reinhardtii

The sites of transcription and translation of carotenoid pigments were studied in synchronously grown cells of Chlamydomonas reinhardtii Dang. Rifampicin, cycloheximide and spectinomycin were used to distinguish between the nuclear-cytoplasmic genetic system and the genetic system of the chloroplast. Since rifampicin is without effect, chloroplast DNA appears not to possess information required for the synthesis of carotenoids. Carotenoid synthesis parallels chlorophyll synthesis in these cells. Carotenoid synthesis is dependent on de novo protein synthesis both on cytoplasmic and chloroplast ribosomes, for both cycloheximide and spectinomycin are effective inhibitors. However, the cells are able to form about 40% of the expected increase in carotenoids when cytoplasmic and chloroplast ribosomes are simultaneously inhibited. Of the major carotenoids in C. reinhardtii, lutein appears the least dependent on de novo protein synthesis. The synthesis of β-carotene and trollein appears to be completely dependent on the function of cytoplasmic ribosomes.