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When are ribosomes used in the process of protein synthesis

(1974)Protein synthesis in chloroplasts. PhD thesis, University of Warwick.

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Ribosomes - Protein Synthesis - Cronodon

A distinctive feature of chloroplasts of plants and algae is their extensive, internal, green, chlorophyll-containing membrane system, called thylakoid membranes, where the primary reactions of photosynthesis occur. This system of photosynthetic reaction centers converts light energy into chemical energy, which is used to drive cellular metabolism. Besides their important role in photosynthesis, chloroplasts are also involved in several biochemical pathways, such as the biosynthesis of amino acids, fatty acids, tetrapyrroles including chlorophyll and heme, carotenoids, isoprenoids and pyrimidines. Chloroplasts are also involved in carbon metabolism and in nitrogen and sulfur assimilation (1). Like mitochondria, chloroplasts possess their own genetic system, which cooperates closely with the nucleus in biosynthesizing numerous organellar components. Chloroplasts represent one type of plastid derived from colorless proplastids in the meristematic cells of plant leaves and shoots, which have only a rudimentary internal membrane system (1). Light profoundly affects the development of proplastids. They differentiate into chloroplasts in the presence of light, whereas in its absence they differentiate into etioplasts, which lack chlorophyll and contain a prolamellar body. Upon subsequent illumination, the prolamellar body gives rise to lamellae of the thylakoid membrane. Depending on the plant tissues, the developmental stage, and the environmental conditions, proplastids also differentiate into chromoplasts in petals or fruits, into leucoplasts in roots, or into amyloplasts in tubers in which starch is accumulated. Proplastids also develop into elaioplasts in glands, certain fruits and seeds, where they are involved in synthesizing lipids, terpenoids, carotenoids, and carbohydrates. Although these various plastid forms have rather distinct morphologies, plastid differentiation is reversible to a large extent, because chloroplasts develop from leucoplasts or amyloplasts, and viceversa. During transitions from chloroplasts to the other plastid forms, the expression of most organellar genes is reduced, whereas specific nuclear genes encoding plastid proteins are activated (1, 2). An important point is that all plastid types contain an internal membrane system that is crucial for their interconversion.

Exogenous ATP was not able to induce this in organello protein synthesis to an appreciable degree.

In contrast, the strictly ATP-dependent protein synthesis was stimulated very efficiently by glyceraldehyde-3-phosphate, dihydroxyacetone phosphate and glycerol-3-phosphate, but strongly inhibited by 3-phosphoglycerate.

Protein synthesis in chloroplasts - WRAP: Warwick …

Spermidine, which activates initiation of translation in chloroplasts, enhanced triose phosphate-stimulated protein synthesis even further.

The chloroplast genomes of nongreen algae contain twice as many genes as those of higher plants. Additional genes include those required for photosynthesis that are nucleus-encoded in plants and green algae, genes involved in the synthesis of fatty acids, amino acids, and pigments, genes required for protein folding and transport, and additional genes of unknown function (9). The smallest plastid genome identified, that of the white parasitic plant Epifagus virginiana, is only 70 kbp in size. It has lost all the genes involved in photosynthesis, and the remaining genes encode mostly components of the plastid protein synthesizing system.

It is generally admitted that plastids originated as the result of an endosymbiotic event in which a prokaryotic photosynthetic organism, probably similar to a cyanobacterium, invaded a primitive eukaryotic cell. Strong support for this endosymbiotic hypothesis arises from the considerable similarity between the transcriptional and translational systems of prokaryotes and plastids. It is thought that during evolution genetic information from the intruder was gradually lost and transferred to the nucleus of the host. The question thus arises why chloroplast DNA has been maintained. One possibility is that this evolutionary plastid genome size reduction is still in progress and has not yet reached its final stage. Another possibility is that the plastid protein synthesizing apparatus is essential for synthesizing the large hydrophobic polypeptides of the photosynthetic reaction centers, which cannot be translocated across the plastid envelope membrane. A third recently advanced hypothesis is that the presence of the plastid protein synthesizing system is essential to allow a rapid response of plastid gene expression to environmental changes (10).

Chloroplast ribosomes and protein synthesis. - …

carries the coding that determines protein synthesis.

The internal thylakoid membrane system consists of appressed and non-appressed flattened membrane vesicles, called grana and stroma lamellae, respectively (Fig. 1). The primary reactions of photosynthesis are catalyzed by four major protein-pigment complexes of the thylakoid membrane:

(i) photosystem II and (ii) photosystem I, and their associated chlorophyll antennae, (iii) the cytochrome b6/f complex, and (iv) the ATP synthase (Fig. 1; see also Photosynthesis). Briefly, light energy is captured by the antennae and channeled to the reaction centers of photosystem II and photosystem I. The energy is used to energize an electron in chlorophyll and to create a stable charge separation across the membrane. This triggers a series of oxido-reductions along the photosynthetic electron-transfer chain. At one end of this chain, water is oxidized by photosystem II with concomitant evolution of oxygen and release of protons into the lumen. Then electrons are transferred to plastoquinone, to the cytochrome b6/f complex, which acts as a proton pump, and to the soluble electron carrier plastocyanin in the thylakoid lumen. At the other end of the chain, photosystem I oxidizes plastocyanin upon light absorption and transfers electrons to ferredoxin and then to NADP to form NADPH. The resulting pH gradient is used by the fourth complex, ATP synthase, to produce ATP on the stromal side. This enzyme also functions in the opposite direction by hydrolyzing ATP to pump protons into the thylakoid lumen and thus generate a pH gradient. Because the abundance of the thylakoid membrane complexes facilitates their biochemical analysis and because the state of the redox cofactors is monitored readily by spectroscopic techniques, the thylakoid membrane has been studied intensively and represents one of the best-studied membrane systems.

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    chloroplasts and mitochondria have small ribosomes for protein synthesis

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Protein synthesis in chloroplasts

Identity of the in vitro product present in the soluble phase of the chloroplast with the large subunit of Fraction I protein was established by comparing a two-dimensional tryptic peptide map of its [S35] methionine-labelled peptides with a tryptic peptide map of the large subunit of Fraction I protein labelled in vivo with [S35] methionine. It may therefore be concluded that only one of the many proteins present in the soluble phase of the chloroplast, namely the large subunit of Fraction 1 protein, is synthesised on chloroplast ribosomes.

PubMedID: 7388030 | Protein synthesis in chloroplasts

Figure 1. Photosynthetic complexes in the thylakoid membrane of chloroplasts. PSII (photosystem II) is located within the appressed grana region, whereas PSI (photosystem I) is located within the nonappressed stroma lamellae. The photosynthetic electron transfer chain is shown starting with water as electron donor to PSII, to plastoquinone (PQ), to the cytochrome b6/f complex (cytb6/f), to the soluble electron transfer protein plastocyanin (PC), to pSi, to ferredoxin (Fd), to ferredoxin-NADP oxidoreductase (FNR), and to NADP as final electron acceptor. Electron flow is coupled to proton translocation into the lumen. The resulting pH gradient across the thylakoid membrane drives ATP synthesis. Both ATP and NADPH are used for CO2 fixation.

Protein Synthesis by Isolated Chloroplasts

Each of the four photosynthetic complexes contains numerous protein subunits, some of which are encoded by the chloroplast genome, whereas others are encoded by the nuclear genome (Table 1; Fig. 2). The two principal reaction center polypeptides of photosystems I and II are highly hydrophobic and contain 11 and 5 transmembrane a-helices, respectively, to which most of the redox cofactors and several chlorophylls are bound with an asymmetrical distribution across the thylakoid membrane. This asymmetry is crucial for the vectorial electron transport in the membrane.

Mechanism of Protein Biosynthesis in Chloroplasts - …

Figure 2. Biosynthesis of the photosynthetic apparatus and protein traffic in the chloroplast. Photosynthetic complexes consist of nucleus- and chloroplast-encoded subunits. The former are synthesized as precursors on cytosolic 80S ribosomes and targeted to the chloroplast. Upon import into the organelle, the N-terminal stromal transit peptide domain is cleaved, and the protein is directed to the stroma, to the envelope, or to the thylakoids. In the latter case, the protein contains an additional cleavable thylakoid targeting domain. Several posttranscriptional steps in the chloroplast, such as RNA stability, processing, splicing, editing, and translation, plus the assembly of protein complexes, require the action of numerous nucleus-encoded factors. Chlorophyll, the major pigment of the thylakoid membrane is synthesized entirely in the chloroplast. Synthesis starts from d-aminolevulinic acid (ALA) and involves several steps in common with heme biosynthesis until protoporphyrin IX (proto IX). One of the last steps of chlorophyll synthesis, conversion of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), requires light in land plants. Chlorophyll synthesis is tightly coordinated with the synthesis of its apoproteins. Expression of nuclear genes of photosynthetic proteins is strongly stimulated by light. Some of the chlorophyll precursors influence, directly or indirectly, expression of nuclear genes involved in photosynthesis.

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