Changes in the polypeptide composition of internal membranes of barley plastids during greening

The regulation of enzymes involved in chlorophyll biosynthesis

All living organisms contain tetrapyrroles. In plants, chlorophyll (chlorophyll a plus chlorophyll b) is the most abundant and probably most important tetrapyrrole. It is involved in light absorption and energy transduction during photosynthesis. Chlorophyll is synthesized from the intact carbon skeleton of glutamate via the C5 pathway. This pathway takes place in the chloroplast. It is the aim of this review to summarize the current knowledge on the biochemistry and molecular biology of the C5-pathway enzymes, their regulated expression in response to light, and the impact of chlorophyll biosynthesis on chloroplast development. Particular emphasis will be placed on the key regulatory steps of chlorophyll biosynthesis in higher plants, such as 5-aminolevulinic acid formation, the production of Mg(2+)-protoporphyrin IX, and light-dependent protochlorophyllide reduction.

Synthesis of chlorophyll a regulates translation of chlorophyll a apoproteins P700, CP47, CP43 and D2 in barley etioplasts

Accumulation of plastid-encoded chlorophyll apoproteins and chlorophyll synthesis are controlled by light in angiosperms. An in vitro system utilizing isolated and lysed barley (Hordeum vulgare L.) etioplasts revealed the specific accumulation of P700, CP47, CP43 and D2 triggered by de novo synthesis of chlorophyll. Accumulation rates of radiolabelled chlorophyll apoproteins were linear for about 30 min. Pulse/chase translation assays showed that synthesis of chlorophyll does not result in increased chlorophyll apoprotein stability. Instead turnover rates of chlorophyll apoproteins were higher in the presence than in the absence of chlorophyll. Chlorophyll-dependent accumulation of chlorophyll apoproteins must therefore be regulated on the level of translation. Translation of chlorophyll apoproteins was blocked to about 50% by addition of 30-50 microM aurintricarboxylic acid or 20 microM kasugamycin. The kinetics of chlorophyll-dependent translation indicated that the in vitro translation system is capable of translation initiation. The capability of translation initiation was lost in lysed etioplasts after preincubation for at least 5 min without chlorophyll synthesis. The results suggest that initiation is involved in chlorophyll-dependent regulation of translation.

The major light-harvesting complex of Photosystem II: aspects of its molecular and cell biology

The light-harvesting complex of photosystem II (LHC II) contains one major (LHC IIb) and at least three minor chlorophyll-protein components. The apoproteins of LHC IIb (LHCP) are encoded by nuclear genes and synthesized in the cytoplasm as a higher molecular weight precursor(s) (pLHCP). Several genes coding for pLHCP have been cloned from various higher plant species. The expression of these genes is dependent upon a variety of factors such as light, the developmental stage of the plastids and the plant. After its synthesis in the cytoplasm, pLHCP is imported into plastids, inserted into thylakoids, processed to its mature form, and assembled into LHC IIb. The pathway of assembly of LHC IIb in the thylakoid membranes is currently being investigated in several laboratories. We present a model that gives some details of the steps in the assembly process. Many of the steps involved in the synthesis and assembly are dependent on light and the stage of plastid development.

Probing in vitro translation products with monoclonal antibodies to chlorophyll a/b-binding proteins of barley thylakoids

The polypeptides of the barley light-harvesting protein of photosystem I (LHCI) share certain epitopes. At least two of these common epitopes are present in chlorophyll a/b-protein 1 (Chla/b-P1 = CP29), as shown by cross-reacting monoclonal antibodies (14). These antibodies were employed for immunological identification of polypeptides translated in vitro in an mRNA-dependent cell-free rabbit reticulocyte lysate. The monoclonal antibody CMpLHCI:2 precipitated only one polypeptide of molecular weight 28 kD from in vitro translates primed with polyA+ RNA. No 28 kD precipitation band was found, if this antibody was mixed with a PSI-200 preparation before it was added to the translate. The translational capacity of the LHCI transcripts isolated from 12 hours greened barley was much higher than those isolated from 6 hours greened barley. Transcripts for LHCI polypeptides were also found among the polyA+ RNA of the mutant viridis-k23, which is devoid of LHCI polypeptides in its thylakoid membranes. The monoclonal antibody CMpCh1a/b-P1:1 precipitated a polypeptide of molecular weight 31 kD from in vitro translates primed with polyA+ RNA. Thus, the cross-reactivity the two antibodies show with the mature proteins is not found when the antibodies are reacted with the precursor proteins.

Immunological characterization of chlorophyll a/b-binding proteins of barley thylakoids

Monoclonal antibodies have been raised against the light-harvesting chlorophyll a/b-binding proteins of photosystem I (LHCI) using a photosystem (PS) I preparation (PSI-200) wild-type from barley (Hordeum vulgare L. cv. Svaløf's Bonus) as the antigen. These antibodies cross-reacted with a minor light-harvesting chlorophyll a/b-protein of PSII (Chla/b-P1=CP29), but not with the major one, LHCII (=Chla/b-P2(**)). Similarly, a monoclonal antibody to Chla/b-P1, elicited by a PSII preparation as the antigen, cross-reacted with LHCI, but not LHCII. This explains why an antigen consisting of LHCII, free of LHCI, but contaminated with Chla/b-P1, can elicit antibodies which cross-react with LHCI. Immunoblot assays showed that LHCI and Chla/b-P1 have at least two epitopes in common. Immunogold labelling of thin-sectioned wild-type thylakoids confirmed a preferential localisation of Chla/b-P1 in grana partition membranes and LHCI in stroma lamellae. The presence of LHCI was demonstrated in barley mutants lacking the PSI reaction centre (viridis-zb (63)) and chlorophyll b (chlorina-f2), and was correlated with the presence of long-wavelength (730 nm) fluorescence emission at 77 K. The mutant viridis-k (23), which has a 77 K long-wavelength fluorescence peak at 720 nm, was shown by immune-blot assay to lack LHCI, although Chla/b-P1 was present.

Phosphorylation of thylakoid proteins during chloroplast biogenesis in greening etiolated and light-grown wheat leaves

Phosphorylation of polypeptides in isolated thylakoids was examined during chloroplast biogenesis in greening etiolated wheat leaves and 4 day-old wheat leaves grown under a diurnal light regime. At early stages of plastid development standard thylakoid preparations were heavily contaminated with nuclear proteins, which distorted the polypeptide phosphorylation profiles. Removal of contamination from membranes by sucrose density centrifugation demonstrated that the major membrane phosphoprotein in etioplasts was at 35 kDa. During etioplast greening a number of phosphoproteins appeared, of which the 25-27 kDa apoproteins of the light-harvesting chlorophylla/b protein complex associated with photosystem II (LHCII) became the most dominant. At the early stages of thylakoid development found at the base of the 4-day-old light grown leaf the LHCII apoproteins were evident as phosphoproteins; however the major phosphoprotein was polypeptide atca. 9kDA. Phosphorylation of both the LHCII apoproteins and the 9 kDa polypeptide in these thylakoids was not light-dependent. In the older thylakoids isolated from the leaf tip the LHCII apoproteins were the major phosphoproteins and their phosphorylation had become light-regulated; however phosphorylation of the 9 kDa polypeptide remained insensitive to light.

Insertion of the precursor of the light-harvesting chlorophylla/b-protein into the thylakoids requires the presence of a developmentally regulated stromal factor

The precursor of the major light-harvesting chlorophylla/b-proteins of photosystem II was synthesizedin vitro from a gene fromLemna gibba. When the labelled precursor was incubated with developing barley plastids, the precursor and the processed polypeptide were incorporated in the thylakoids in proportions that varied depending on the developmental stage of plastids. At early stages of development most of the precursor associated with the thylakoids could be removed by washing with 0.1 M NaOH, while in more mature plastids most of its was resistant to a NaOH wash. Insertion of the precursor into thylakoids required the presence of a stromal factor and Mg-ATP. The stromal factor is probably a protein. The insertion reaction has an optimal temperature of 25°C and a pH of 8. The appearance of the stromal factor and the thylakoid membrane's receptivity for the insertion of the precursor depended on the stage of plastid development. These observations are consistent with the hypothesis that the insertion of the precursor into the thylakoid prior to its proteolytic processing, is one of the steps involved in the assembly of the light-harvesting complex of photosystem II.

Light-induced changes in the distribution of the 36000-Mr polypeptide of NADPH-protochlorophyllide oxidoreductase within different cellular compartments of barley (Hordeum vulgare L.) : I. Localization by immunoblotting in isolated plastids and total leaf extracts

Changes in the relative content of NADPH-protochlorophyllide oxidoreductase during the light-induced greening of barley plants were measured both in the total leaf extract as well as in intact and broken plastids. The enzyme protein was identified by its apparent molecular weight and its immunological crossreactivity with an antiserum directed against the NADPH-protochlorophyllide oxidoreductase. The monospecificity of the antiserum was tested by two different criteria: i. The antiserum was purified by affinity chromatography. ii. It was demonstrated that the antiserum crossreacts with only those polypeptides which appear to be enzymatically active. In the fraction of broken plastids isolated from leaves of briefly illuminated barley plants the concentration of the enzyme protein was reduced drastically. Our results indicate that this decrease in enzyme protein content is the consequence of an artificial proteolytic breakdown of the membrane-bound enzyme protein. In intact plastids and in the total leaf extract the concentration of the enzyme protein did not change dramatically during the first 4 to 6 h of illumination. However, when the exposure to continuous white light was extended further the concentration of the enzyme protein in intact plastids began to decline rapidly while in total leaf extracts the concentration remained almost constant for the next 10 h of light. These results indicate that part of the enzyme protein may be localized outside of the plastid compartment.

The light-dependent accumulation of the P700 chlorophyll a protein of the photosystem I reaction center in barley. Evidence for translational control

The light-dependent accumulation of the P700 chlorophyll a protein of the photosystem I reaction center has been studied in greening barley (Hordeum vulgare L.) seedlings. Immunoblot analysis of total cellular protein fractions and immunogold labelling of the P700 chlorophyll a protein in ultrathin sections of Lowicryl-embedded leaf tissue revealed that the concentration of this chlorophyll-binding protein in plastids of dark-grown barley seedlings is below the limit of detection. Upon illumination with white light, a rapid accumulation of this protein is induced. This light effect seems not to be regulated at the level of transcription. The gene for the P700 chlorophyll a protein has been mapped within the large single-copy region of the plastid DNA of barley. High levels of transcripts of this gene are present already in dark-grown seedlings and remain fairly constant throughout an extended illumination period. Polysomes were isolated from etioplasts and chloroplasts. The same high relative concentration of mRNA encoding the P700 chlorophyll a protein was present in both polysome fractions. This result suggests that the light-dependent accumulation of the P700 chlorophyll a protein during chloroplast formation in barley seedlings is regulated at the translational, or posttranslational, level.

Chlorophyll-protein complex composition and photochemical activity in developing chloroplasts from greening barley seedlings

The time course for the observation of intact chlorophyll-protein (CP) complexes during barley chloroplast development was measured by mild sodium dodecyl sulfate polyacrylamide gel electrophoresis. The procedure required extraction of thylakoid membranes with sodium bromide to remove extrinsic proteins. During the early stages of greening, the proteins extracted with sodium bromide included polypeptides from the cell nucleus that associate with developing thylakoid membranes during isolation and interfere with the separation of CP complexes by electrophoresis. Photosystem I CP complexes were observed before the photosystem II and light-harvesting CP complexes during the initial stages of barley chloroplast development. Photosystem I activity was observed before the photosystem I CP complex was detected whereas photosystem II activity coincided with the appearance of the CP complex associated with photosystem II. Throughout chloroplast development, the percentage of the total chlorophyll associated with photosystem I remained constant whereas the amount of chlorophyll associated with photosystem II and the light-harvesting complex increased. The CP composition of thylakoid membranes from the early stages of greening was difficult to quantitate because a large amount of chlorophyll was released from the CP complexes during detergent extraction. As chloroplast development proceeded, a decrease was observed in the amount of chlorophyll released from the CP complexes by detergent action. The decrease suggested that the CP complexes were stabilized during the later stages of development.

The NADPH-protochlorophyllide oxidoreductase is the major protein constituent of prolamellar bodies in wheat (Triticum aestivum L.)

A fraction of highly purified prolamellar bodies was isolated from etioplasts of wheat (Triticum aestivum L. cv. Starke II, Weibull), as previously described by Ryberg and Sundqvist (1982, Physiol. Plant., 56, 125-132). Studies on the protein composition revealed that only one major polypeptide of an apparent molecular weight of 36000 is present in the fraction of prolamellar bodies. This polypeptide was identified as the NADPH-protochlorophyllide oxidoreductase. The highest specific activity of the enzyme in etiolated leaf tissue was confirmed to be in the fraction of prolamellar bodies.

Differences in chlorophyll-protein complexes and composition of polypeptides between thylakoids from bundle sheaths and mesophyll cells in maize

Thylakoids from enzymatically separated bundle sheath and mesophyll tissue chloroplasts were examined for their chlorophyll-proteins by tube sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE). Differences were found in distribution of chlorophyll among peaks. The chlorophyll-protein a peak (CPa), considered to be the photosystem II (PSII) reaction centre by many authors, was seen to be absent in bundle sheath thylakoid samples. The slab SDS-PAGE revealed the absence of the polypeptides present in PSII preparations of chloroplast subfractions having only PSII activity. This finding confirms Anderson's hypothesis of the structure of grana and stroma thylakoids.

Protein synthesis during chloroplast development in Spirodela oligorhiza. Coordinated synthesis of chloroplast-encoded and nuclear-encoded subunits of ATPase and ribulose-1,5-bisphosphate carboxylase

We have studied qualitative and quantitative changes of several parameters during chloroplast development in Spirodela oligorhiza (duckweed). On a dry weight basis, the amount of protein increases from 2.5% (w/w) in dark-grown to 7.8% (w/w) in light-grown fronds. At the same time the amount of starch drops from 50% to 27% (w/w). Using an immunochemical quantification method we have found that during greening of etiolated plants the amount of all subunits of the ATPase complex per frond increases 10-fold, whereas the level of the subunits of ribulose-1,5-biphosphate carboxylase increases 50-fold. Cytochrome f was found to be present in dark-grown Spirodela and the amount of this polypeptide per frond increases about 30-fold. The concentration of a polypeptide that possibly represents a cytochrome b6 subunit increases about 10-fold upon greening. The molar ratio of the CF1-beta and CF1-gamma subunits of the ATPase complex varies over 2-3, while in all stages of chloroplast development studied the molar ratio of the carboxylase subunits is about 1. As these values are in agreement with the stoichiometrical amounts in the native protein complexes, we conclude that the synthesis of CF1-beta and CF1-gamma, as well as the synthesis of the large and small carboxylase subunits, are strictly coordinated during chloroplast biogenesis in Spirodela oligorhiza.

Plastid gene expression in a yellow-green leaf mutant of Petunia hybrida

We have analyzed the morphology and gene expression in plastids of a yellow-green leaf mutant of Petunia hybrida (E 5059). Under normal light intensities (20,000 lx), yellow-green leaves develop with a typical proplastid morphology (few membranes, incomplete stacking). When such plants are grown under low light intensities (3,000 lx), the newly formed leaves are green. The plastids in these green leaves have a wild-type like chloroplast morphology (thylakoids and grana structure). Pre-existing green leaves remain green in 20,000 lx, indicating that chlorophyll is not degraded. An analysis of polypeptides synthesized in isolated plastids from the yellow-green and green leaves of this mutant plant shows several differences. In the yellow-green leaf plastids only a very small amount of the large subunit of ribulose-1,5-bisphosphate carboxylase (RuBPCase) is present, while in green plastids this polypeptide is present in much higher amounts. Hybridization experiments indicated that in plastids from the yellow-green leaves the mRNA coding for the large subunit polypeptide is present in much lower amounts than in plastids from the 3,000 lx green leaves of this mutant or in chloroplasts from wild type plants. These results indicate regulation at the mRNA level. Furthermore, in yellow-green leaf plastids eleven polypeptides are present with high molecular wieght (higher than 67,000 d). Five of them are synthesized by the yellow-green leaf plastid itself. Such high molecular weight polypeptides are also synthesized by proplastids isolated from white petunia cell suspension cultures, but are not synthesized by 3,000 lx green leaf plastids, or by isolated normal leaf chloroplasts. These results indicate that the synthesis of these polypeptides is specific for the proplastid stage of chloroplast biogenesis.

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.

Events surrounding the early development of Euglena chloroplasts. 15. Origin of plastid thylakoid polypeptides in wild-type and mutant cells

Techniques are described for the isolation of plastid thylakoid membranes from light-grown and dark-grown cells of Euglena gracilis var. bacillaris, and from mutants affecting plastid development. These membranes, which have minimal contamination with other cell fractions, are localized in sucrose gradients by using the thylakoid membrane sulfolipid as a specific marker. The plastid thylakoid membrane polypeptides isolated from these membranes were separated on SDS polyacrylamide gels and yielded patterns containing 30-40 polypeptides. Light-grown strain Z gave patterns identical with bacillaris. Since the plastid thylakoid polypeptide patterns obtained from dark-grown wild-type cells and from a bleached mutant W3BUL in which plastid DNA is undetectable are identical, it appears that the proplastid thylakoid polypeptides of wild-type cannot be coded in plastid DNA and are probably coded in nuclear DNA. The plastid thylakoid polypeptide patterns obtained from various dark-grown mutants, making large but abnormal chloroplasts, show a correlation between the amount of chlorophyll formed and the amount of a plastid thylakoid polypeptide thought to be associated wtth one of the pigment-protein light-harvesting complexes. Treatment with SAN 9789 (4-chloro-5-(methylamino)-2(alpha, alpha, alpha,-trifluoro-m-tolyl)-3-(2H(pyridazinone) known to block carotenoid synthesis at the level of phytoene, causes a progressive loss of all plastid thylakoid polypeptides during growth in darkness and results in the establishment of a new, lowere steady-state level of sulfolipid. At least ten of the plastid thylakoid polypeptides become labeled when isolated chloroplasts are supplied with radioactive amono acids; of these six are undectable in W3BUL and are, therefore, candidates for coding by plastid DNA.