Sites of synthesis of chloroplast lamellar proteins in Vicia faba

Sequence of two genes in pea chloroplast DNA coding for 84 and 82 kD polypeptides of the photosystem I complex

The genes encoding the two P700 chlorophyll a-apoproteins of the photosystem I complex were localized on the pea (Pisum sativum) chloroplast genome. The nucleotide sequence of the genes and the flanking regions has been determined. The genes are separated by 25 bp and are probably cotranscribed. The 5' terminal gene (psaA1) codes for a 761-residue protein (MW 84.1 kD) and the 3' terminal gene (psaA2) for a 734-residue protein (MW 82.4 kD). Both proteins are highly hydrophobic and contain eleven putative membrane-spanning domains. The homology to the corresponding polypeptides from maize are 89% and 95% for psaA1 and psaA2, respectively. A putative promoter has been identified for the psaA1 gene, and potential ribosome binding sites are present before both genes.

Isolation of an intrinsic antenna chlorophyll a-protein from the photosystem I reaction center complex of the thermophilic cyanobacterium Synechococcus sp

A chlorophyll-protein was isolated from a Synechococcus P700-chlorophyll a-protein complex free from small subunits (CP1-e) by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis after treatment with 2% 2-mercaptoethanol and 2% SDS. In contrast to CP1-e which, when electrophoresed under denaturating conditions, showed two polypeptide bands of 62 and 60 kDa, the chlorophyll-protein contained only the 60-kDa polypeptide and hence is called CP60. The yield of CP60 was maximal with 1-2% SDS and 2-4% sulfhydryl reagents because the chlorophyll-protein was denatured at higher concentrations of the reagents. The absorption spectrum of CP60, which retained more than half of the chlorophyll alpha molecules originally associated with the 60-kDa subunit of the photosystem I reaction center complex, showed a red band maximum at 672 nm and a small absorption band around 700 nm at liquid nitrogen temperature. CP60 emitted a fluorescence band at 717 to 725 nm at 77 degrees K. The temperature dependence of the far red band of CP60 was essentially the same as that of CP1-e between 77 and 273 degrees K. No photoresponse of P700 was detected in CP60. The results suggest that the two polypeptides resolved by SDS-gel electrophoresis from CP1-e are apoproteins of two distinct chlorophyll-proteins and that CP60 represents a chlorophyll-bearing 60-kDa subunit functioning as an intrinsic antenna protein of the photosystem I reaction center complex. It will also be shown that the temperature dependence of the far red fluorescence band is not related to the photosystem I photochemistry.

Sexual agglutinin of mating-type minus gametes in Chlamydomonas reinhardii. I. Loss and recovery of agglutinability of gametes treated with EDTA

The effect of EDTA on the mating-type-specific agglutinins located on the flagellar surfaces of Chlamydomonas reinhardii gametes was investigated. The mating-type minus (mt-) gametes lost their agglutinability without apparent loss of motility soon after addition of EDTA at low concentrations (1-2 mM). At the same time, the cells released into the medium agglutinins which can elicit agglutinative responses of mating-type plus (mt+) gametes specifically. When EDTA was neutralized with Mg2+ or removed by centrifugation, the mt- cells quickly replaced agglutinins by protein synthesis: the recovery process was sensitive to cycloheximide, but not to tunicamycin. The EDTA-treated mt+ gametes lost their agglutinins much more slowly than the mt- gametes. The replacement of mt+ agglutinins was inhibited by both cycloheximide and tunicamycin.

Regulation of photosynthesis by reversible phosphorylation of the light-harvesting chlorophyll a/b protein

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The presence and synthesis of the NADPH-protochlorophyllide oxidoreductase in barley leaves with a high temperature-induced deficiency of plastid ribosomes

High-temperature-induced deficiency of plastid ribosomes in barley plants (Hordeum vulgare L.) was used as a system for studying the role of the cytoplasm in the synthesis of the NADPH-protochlorophyllide oxidoreductase. The enzyme is present in 33° C-grown plants. The failure of high-temperature-grown plants to accumulate chlorophyll during illumination is not caused by the absence of the protochlorophyllide-reducing enzyme. The synthesis of the NADPH-protochlorophyllide oxidoreductase was studied by feeding [(35)S]methionine to the seedling and by following the incorporation of the radioactively labeled amino acid into plastid proteins. The NADPH-protochlorophyllide oxidoreductase was labeled in high-temperature-grown barley plants to the same extent as in control plants grown at 25° C. It is concluded that the 36,000-Mr polypeptide of the NADPH-protochlorophyllide oxidoreductase is synthesized outside the plastid on cytoplasmic 80S ribosomes.

Effects of nuclear gene mutations on the structure and function of plastids in pea : The light-harvesting chlorophyll a/b protein

Two chlorophyll-deficient mutants of Pisum sativum L. were investigated to elucidate the effect of mutations of nuclear genes on the synthesis and function of the light-harvesting chlorophyll a/b protein (LHCP). Both mutants, a chlorotic as well as a chlorophyll b-less mutant, exhibited chloroplasts differentiated into grana and stroma regions, but, for the most part, grana formation was reduced and the number of lamellae per granum was decreased. The major LHCP-apoprotein was reduced in the thylakoid membranes of the chlorotic mutant and was missing (or very greatly reduced) in the membranes of the chlorophyll b-less mutant. The polyadenylated RNA of both mutants was isolated from young green leaves and translated in a wheat germ cell-free system. Using immunoprecipitation, it was demonstrated that both mutants contained the mRNA for the LHCP-apoprotein and that this message is taken up into polysomes, as shown by read-through translation. From these experimental data it is concluded that transcription and translation of the major protein of the light-harvesting complex are not affected by these genetic defects. A close correlation between the amount of chlorophyll b and the amount of LHCP incorporated into the membrane was observed.

Biosynthetic pathways of two polypeptide subunits of the light-harvesting chlorophyll a/b protein complex

We have used an in vitro reconstitution system, consisting of cell-free translation products and intact chloroplasts, to investigate the pathway from synthesis to assembly of two polypeptide subunits of the light-harvesting chlorophyll-protein complex. These polypeptides, designated 15 and 16, are integral components of the thylakoid membranes, but they are products of cytoplasmic protein synthesis. Double immunodiffusion experiments reveal that the two polypeptides share common antigenic determinants and therefore are structurally related. Nevertheless, they are synthesized in vitro from distinct mRNAs to yield separate precursors, p15 and p16, each of which is 4,000 to 5,000 daltons larger than its mature form. In contrast to the hydrophobic mature polypeptides, the precursors are soluble in aqueous solutions. Along with other cytoplasmically synthesized precursors, p15 and p16 are imported into purified intact chloroplasts by a post-translational mechanism. The imported precursors are processed to the mature membrane polypeptides which are recovered exclusively in the thylakoids. The newly imported polypeptides are assembled correctly in the thylakoid lipid bilayer and they bind chlorophylls. Thus, these soluble membrane polypeptide precursors must move from the cytoplasm through the two chloroplast envelope membranes, the stroma, and finally insert into the thylakoid membranes, where they assemble with chlorophyll to form the light-harvesting chlorophyll protein complex.

The protochlorophyllide holochrome of barley (Hordeum vulgare L.). Phytochrome-induced decrease of translatable mRNA coding for the NADPH: protochlorophyllide oxidoreductase

During the illumination of dark-grown barley plants light induces a rapid decrease of a translatable mRNA which codes for a polypeptide of Mr 44000. This component was identified as a precursor of the NADPH:protochlorophyllide oxidoreductase. The precursor has an Mr larger than the authentic protein by approximately 8000. The light-induced change in the level of translatable mRNA can be induced by a 15-s red-light pulse followed by 5 h of darkness. The red-light effect is reversed by a subsequent far-red-light treatment. It is concluded that the light-induced decline of translatable mRNA for the NADPH:protochlorophyllide oxidoreductase is controlled by phytochrome. The significance of this finding for present concepts of light-dependent control of chloroplast development and chlorophyll synthesis is discussed.

Uniparental inheritance of a chloroplast photosystem II polypeptide controlling herbicide binding

The ability of atrazine to inhibit Photosystem II electron transport and the rate of electron transfer from the primary to the secondary quinone electron acceptors in the photosystem II complex were examined in triazine-resistant and -susceptible parental biotypes of Brassica campestris L. and their F1 progeny derived from reciprocal crosses. The lack of herbicide inhibitory activity and the presence of functional properties which decreased the Q- to B electron transport rate constant were inherited in parallel through the maternal parent. We conclude that the herbicide receptor protein is uniparentally inherited through the female parent. These data are discussed in relation to other studies which indicate that the binding site is a 32 000-dalton polypeptide which determines the functional properties of B (the secondary Photosystem II electron acceptor).

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein : Evidence for the requirement of chlorophyll a for the stabilization of the apoprotein

The effect of light on the biosynthesis of the light-harvesting chlorophyll a/b protein (LHCP) is investigated in wild-type barley (Hordeum vulgare L.) and in the chlorophyll b-less mutant chlorina f2. In dark-grown plants a short red light pulse triggers the appearance of mRNA activity for the LHCP. While the accumulation of this mRNA is controlled by phytochrome (Apel (1979) Eur. J. Biochem. 97, 183-188), the red light treatment is not sufficient to induce the appearance of the LHCP within the membrane. Thus, at least one of the subsequent steps in the biosynthetic pathway leading to the assembly of the LHCP is controlled by light. The red light-induced mRNA is taken up into the polysomes during the subsequent dark period and is translated in vitro in a cell-free protein synthesizing system. However, an accumulation of the freshly synthesized polypeptide within the plant is not observed. The apparent instability of the polypeptide might be explained by the deficiency of chlorophyll in the red light-treated plants. In the chlorophyll b-less barley mutant chlorina f2 an accumulation of the freshly synthesized apoprotein of the LHCP can be observed in the light. Thus, chlorophyll a formation seems to be a light-dependent step which is required for the stabilization of the LHCP.

Protein synthesis by isolated Acetabularia chloroplasts. In vitro synthesis of the apoprotein of the P-700-chlorophyll alpha-protein complex (CP i)

Acetabularia chloroplasts can incorporate radioactive amino acids for up to several hours in vitro. The incorporation is sensitive to chloramphenicol and lincomycin, insensitive to cycloheximide, and completely light-dependent. At least 35 discrete labelled bands can be separated by SDS-polyacrylamide gel electrophoresis: 20--24 in the soluble fraction and 13--15 in the membrane fraction. Most of the label (80--85%) is in the membrane fraction, and 90% of that is in a polypeptide of 32 000 daltons. Chlorophyll-protein complexes were purified from in vitro labelled chloroplasts by SDS-polyacrylamide gel electrophoresis. CP I (P-700-chlorophyll alpha-protein complex) and its apoprotein were both labelled. This shows that the apoprotein is synthesized on chloroplast ribosomes, and can be integrated correctly into the thylakoid membrane in the absence of any cytoplasmic contribution. In contrast, no label was incorporated into the two polypeptides of CP II, the light-harvesting chlorophyll a/b complex.

Events surrounding the early development of Euglena chloroplasts. 16. Plastid thylakoid polypeptides during greening

Using sulfolipid to locate plastid thylakoid membranes in gradients from dark-grown resting cells it has been possible to study the plastid thylakoid membrane polypeptides of Euglena gracilis var. bacillaris undergoing light-induced chloroplast development. All plastid thylakoid bands seen in dark-growing wild-type cells and in mutant W3BUL in which plastid DNA is undetectable, are observed to increase in amount during plastid development. Others, which are undetectable in dark-grown wild-type and W3BUL increase greatly during plastid development and appear to be those associated with pigment-protein complexes. The data obtained from experiments where the polypeptides were labeled with 35S during development, either continuously or in pulses, were consistent with these findings. Cycloheximide strongly inhibited the increases in amount in all bands and chloramphenicol or streptomycin produced a lower level of inhibition in all bands indicating tight control of theformation of each plastid membrane constituent by the others. The formation of a polypeptide band of 25 000 molecular weight, thought to be a part of a pigment-protein complex of the thylakoid, and chlorophyll synthesis were inhibited identically by these antibiotics.

Photosynthetic apparatus in chilling-sensitive plants : IV. Changes in ATP and protein levels in cold and dark stored and illuminated tomato leaves in relation to Hill reaction activity

Changes in the levels of both ATP and protein in relation to Hill reaction activity following cold and dark storage and illumination of leaves of Lycopersicon esculentum Mill. were studied. Loss of Hill reaction activity observed during cold and dark storage of leaves for 3-4 days was accompanied by about 50% decrease of both ATP and protein levels while the content of chlorophyll was not affected Illumination of cold and dark stored leaves (8000 lx for 2 h) resulted in almost a complete restoration of both ATP and protein levels as well as Hill reaction activity. The latter process proceeded, however with different kinetics than the former ones. The rate of Hill reaction activity increase very rapidly from the beginning of illumination while the ATP level diminished during the first hour of illumination. In addition there was a lag in the increases in protein content. By about two hours of illumination all these processes reached the maximum values. Following illumination of leaf dises stored in the cold and dark in the presence of either cycloheximide or DCMU, both ATP and proteins levels as well as Hill reaction activity were greatly diminished. These data seem to suggest that the lack of reactivation of Hill reaction activity in the presence of these two inhibitors is due to inhibition of ATP synthesis required primarily for manganese reincorporation into the thylakoid membrane and theraby restoration of Hill reaction activity (Kaniuga, Zabek and Sochanowicz, Planta 1978b). Contribution of cytoplasmic protein synthesis in this process appears to be of secondary importance, although the inactivation and reactivation of electron transport are accompanied by a large loss (as high as 50%) and the restoration of the initial protein content in leaves following illumination.

The plastid membranes of barley (Hordeum vulgare). Light-induced appearance of mRNA coding for the apoprotein of the light-harvesting chlorophyll a/b protein

Illumination of dark-grown barley plants induces a massive insertion of the light-harvesting chlorophyll a/b protein into the developing thylakoid membrane. In addition to the onset of chlorophyll synthesis, light induces specifically the appearance of a prominent mRNA species which codes for a polypeptide of Mr 29500. This component was identified as a precursor of the apoprotein of the light-harvesting chlorophyll a/b protein. The precursor has an Mr larger than the authentic protein by approximately 4000. Studies of the chlorophyll-b-less mutant chlorina f2 of barley offer the first clue to the mechanism which controls the light-dependent mRNA formation. The induction of the mRNA coding for the aproprotein of the light-harvesting chlorophyll a/b protein does not seem to be linked directly to the assembly process of the light-harvesting structure and does not require chlorophyll b. It is proposed that light exerts its influence on the mRNA formation by a reaction which is different from the phototransformation of protochlorophyll(ide) to chlorophyll(ide).

Appearance and composition of chlorophyll-protein complexes I and II during chloroplast membrane biogenesis in Chlamydomonas reinhardi y-1

The chlorophyll-protein complexes I and II have been isolated and anlyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis during greening and degreening of Chlamydomonas reinhardi y-1. At all stages of membrane formation, the complexes, when present, have a constant composition. Chlorophyll-protein complex I consists of a major polypeptide(s) of molecular weight 64,000 synthesized in the chloroplast, to which about 29 chlorophyll a molecules are bound. The complex is not detected when other polypeptides of chloroplastic origin, related to both Photosystem I and Photosystem II activities, are not synthesized. However, Photosystem I activity can develop in membranes in which chlorophyll-protein complex I is not detectable. Chlorophyll-protein complex II consists of two polypeptides of cytoplasmic origin, molecular weights 24,000 and 22,000, which bind 12 chlorophylls (a and b). The chlorophyll-protein complex II can be detected in membranes in which the development of photosystem II activity is prevented. Clipping of a Mr = 2000 fragment(s) from the Mr = 22,000 polypeptide following trypsin digestion of membranes, does not affect the complex. The detection of the complexes is possible only in membranes in which the simultaneous synthesis of both the chlorophyll and the corresponding polypeptides occurs. The 28,000 dalton polypeptide, reported to be present in the chlorophyll-protein complex II, comigrates with the complex but apparently is not part of the complex itself. The apparent molecular weight of the chlorophyll-protein complexes I and II are 88,000 and 28,000, respectively. The minimal true value for complex I is 89,000 or 154,000 and for complex II is 56,000.

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.

Thylakoid membrane polypeptides of Chlamydomonas reinhardtii: wild-type and mutant strains deficient in photosystem II reaction center

Unstacked thylakoid membrane vesicles were obtained from a homogenate of Chlamydomonas reinhardtii by flotation in a 1.8 M sucrose layer containing 5 mM HEPES (N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid)-10 mM EDTA (pH 7.5). Sodium dodecyl sulfate-gradient gel electrophoresis showed that the wildtype membranes have a total of at least 33 polypeptides ranging in molecular weights from 68,000 to less than 10,000. The wild-type and three non-photosynthetic mutant strains were studied with respect to their photosynthetic electron transport properties, their fluorescence rise kinetics, and their membrane polypeptide compositions. The results showed a strong correlation between the presence of a membrane polypeptide (molecular weight = 47,000) and the activity of the photosystem II reaction center. This polypeptide is missing from F34 (a mendelian mutant lacking Q, the primary electron acceptor of photosystem II), but is partially restored in a suppressed strain of F34 in which there is an incomplete recovery of photosystem II activity. In a thermosensitive mutant, T4, the same polypeptide is present in reduced amount only in cells grown at 35 degrees but not in those grown at 25 degrees. Evidence from fluorescence rise kinetics and partial photochemical reactions show that the cells grown at 25 degree are similar to wild-type cells but the cells grown at 35 degrees are greatly deficient in Q.

On the molecular nature of chloroplast thylakoid membranes

Envelope- and stroma-free thylakoid membranes of Vicia faba chloroplasts were disintegrated and the electrophoretic behavior of the components studied with special regard to the pigment-protein complexes. The process of denaturation of the complexes was found to differ with respect to the other protein components. As the result of denaturation, the pigment-free protein moieties exhibit altered electrophoretic mobilities in relation to the intact complexes mainly conditioned by two processes contrary in their action, i.e. increase of change and change of the hydrodynamic properties. Exhaustive extraction of the thylakoid membranes with 6 M guanidine - HCl removes the proteins mainly associated by polar and weak hydrophobic interactions. The insoluble residue quantitatively exhibits the pigment-protein complexes including their denatured protein moieties, two extrinsic hydrophobic proteins as well as some protein traces. Electron-microscopic studies demonstrate the material still to have a high degree of order and preserved basic structure. After removing the lipids from the basic membrane, large amounts of the protein moeity of Complex II become soluble in guanidine -HCl. Since all other lamellar proteins are removable either by quanidine -HCl extraction or by trypsin digestion it is assumed the basic membrane of thylakoid to consist only of the pigment-protein complexes embedded into the lipid matrix.

Protein synthesis in chloroplasts. Characteristics and products of protein synthesis in vitro in etioplasts and developing chloroplasts from pea leaves

The function of plastid ribosomes in pea (Pisum sativum L.) was investigated by characterizing the products of protein synthesis in vitro in plastids isolated at different stages during the transition from etioplast to chloroplast. Etioplasts and plastids isolated after 24, 48 and 96h of greening in continuous white light, use added ATP to incorporate labelled amino acids into protein. Plastids isolated from greening leaves can also use light as the source of energy for protein synthesis. The labelled polypeptides synthesized in isolated plastids were analysed by electrophoresis in sodium dodecyl sulphate-ureapolyacrylamide gels. Six polypeptides are synthesized in etioplasts with ATP as energy source. Only one of these polypeptides is present in a 150 000g supernatant fraction. This polypeptide has been identified as the large subunit of Fraction I protein (3-phospho-D-glycerate carboxylyase EC 4.1.1.39) by comparing the tryptic 'map' of its L-(35S)methionine-labelled peptides with the tryptic 'map' of large subunit peptides from Fraction I labelled with L-(35S)methionine in vivo. The same gel pattern of six polypeptides is seen when plastids isolated from greening leaves are incubated with either added ATP or light as the energy source. However, the rates of synthesis of particular polypeptides are different in plastids isolated at different stages of the etioplast to chloroplast transition. The results support the idea that plastid ribosomes synthesize only a small number of proteins, and that the number and molecular weight of these proteins does not alter during the formation of chloroplasts from etioplasts.

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.

Ribosomes bound to chloroplast membranes in Chlamydomonas reinhardtii

The amount of chloroplast ribosomal RNAs of Chlamydomonas reinhardtii which sediment at 15,000 g is increased when cells are treated with chloramphenicol. Preparations of chloroplast membranes from chloramphenicol-treated cells contain more chloroplast ribosomal RNAs than preparations from untreated cells. The membranes from treated cells also contain more ribosome-like particles, some of which appear in polysome-like arrangements. About 50% of chloroplast ribosomes are released from membranes in vitro as subunits by 1 mM puromycin in 500 mM KCl. A portion of chloroplast ribosomal subunits is released by 500 mM KCl alone, a portion by 1 mM puromycin alone, and a portion by 1 mM puromycin in 500 mM KCl. Ribosomes are not released from isolated membranes by treatment with ribonuclease. Membranes in chloroplasts of chloramphenicol-treated cells show many ribosomes associated with membranes, some of which are present in polysome-like arrangements. This type of organization is less frequent in chloroplasts of untreated cells. Streptogramin, an inhibitor of initiation, prevents chloramphenicol from acting to permit isolation of membrane-bound ribosomes. Membrane-bound chloroplast ribosomes are probably a normal component of actively growing cells. The ability to isolate membrane-bound ribosomes from chloramphenicol-treated cells is probably due to chloramphenicol-prevented completion of nascent chains during harvesting of cells. Since chloroplasts synthesize some of their membrane proteins, and a portion of chloroplast ribosomes is bound to chloroplast membranes through nascent protein chains, it is suggested that the membrane-bound ribosomes are synthesizing membrane protein.

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.