Synthesis of a chloroplast membrane polypeptide on thylakoid-bound ribosomes during the cell cycle of Chlamydomonas reinhardii 137+

A novel rhodanese is required to maintain chloroplast translation in Chlamydomonas

Rhodanese-domain proteins (RDPs) are widespread in plants and other organisms, but their biological roles are mostly unknown. Here we report on a novel RDP from Chlamydomonas that has a single rhodanese domain, and a predicted chloroplast transit peptide. The protein was produced in Escherichia coli with a His-tag, but lacking most of the N-terminal transit peptide, and after purification was found to have rhodanese activity in vitro. It was also used to elicit antibodies for western blot analysis, which showed that the native Chlamydomonas protein migrated slower on SDS gels (apparent M(r) =34 kDa) than its predicted size (27 kDa), and co-fractionated with chloroplasts. To assess function in vivo, the tandem-RNAi approach was used to generate Chlamydomonas strains that had reductions of 30-70% for the mRNA and ~20-40% for the 34-kDa protein. These strains showed reduced growth under all trophic conditions, and were sensitive to even moderate light; properties reminiscent of chloroplast translation mutants. Pulse-labeling in the presence of cycloheximide indicated that chloroplast protein synthesis was broadly reduced in the RNAi strains, and transcript analysis (by RT-PCR and northern blotting) indicated the effect was mainly translational. These results identify a novel rhodanese-like protein that we have named CRLT, because it is required to maintain chloroplast translation.

Structure, function and assembly of Photosystem II and its light-harvesting proteins

Photosystem II (PSII) is a multisubunit chlorophyll-protein complex that drives electron transfer from water to plastoquinone using energy derived from light. In green plants, the native form of PSII is surrounded by the light-harvesting complex (LHCII complex) and thus it is called the PSII-LHCII supercomplex. Over the past several years, understanding of the structure, function, and assembly of PSII and LHCII complexes has increased considerably. The unicellular green alga Chlamydomonas reinhardtii has been an excellent model organism to study PSII and LHCII complexes, because this organism grows heterotrophically and photoautotrophically and it is amenable to biochemical, genetic, molecular biological and recombinant DNA methodology. Here, the genes encoding and regulating components of the C. reinhardtii PSII-LHCII supercomplex have been thoroughly catalogued: they include 15 chloroplast and 20 nuclear structural genes as well as 13 nuclear genes coding for regulatory factors. This review discusses these molecular genetic data and presents an overview of the structure, function and assembly of PSII and LHCII complexes.

Low density membranes are associated with RNA-binding proteins and thylakoids in the chloroplast of Chlamydomonas reinhardtii

Chloroplast subfractions were tested with a UV cross-linking assay for proteins that bind to the 5' untranslated region of the chloroplast psbC mRNA of the green alga Chlamydomonas reinhardtii. These analyses revealed that RNA-binding proteins of 30-32, 46, 47, 60, and 80 kD are associated with chloroplast membranes. The buoyant density and the acyl lipid composition of these membranes are compatible with their origin being the inner chloroplast envelope membrane. However, unlike previously characterized inner envelope membranes, these membranes are associated with thylakoids. One of the membrane-associated RNA-binding proteins appears to be RB47, which has been reported to be a specific activator of psbA mRNA translation. These results suggest that translation of chloroplast mRNAs encoding thylakoid proteins occurs at either a subfraction of the chloroplast inner envelope membrane or a previously uncharacterized intra-chloroplast compartment, which is physically associated with thylakoids.

A nuclear suppressor overcomes defects in the synthesis of the chloroplast psbD gene product caused by mutations in two distinct nuclear genes of Chlamydomonas

Mutations in two distinct nuclear genes, called NAC 1 and AC-115, of the unicellular green alga Chlamydomonas reinhardtii cause a specific and dramatic reduction in the synthesis of the chloroplast-encoded D2 polypeptide of Photosystem II. The psbD transcript which encodes the D2 protein is present in the mutant strains, but protein pulse-labeling and immunoprecipitation experiments demonstrate that the synthesis of the D2 protein does not occur normally in these cells. These phenotypes are suppressed by an extragenic nuclear suppressor isolated from a pseudorevertant of a nac 1 mutant. This suppressor is neither allele- nor gene-specific in its suppression and is able to overcome the effects of two different mutations in the NAC 1 gene, as well as a mutation in AC-115. The suppressor seems to be specific in its ability to remedy blocks in psbD mRNA translation in the chloroplast. It is not able to restore the translation of another chloroplast-encoded mRNA which is blocked by another nuclear mutation. The suppressor may identify a new nuclear gene specifically involved in the synthesis of the D2 protein in the chloroplast.

Translational regulation of chloroplast gene expression during the light-dark cell cycle of Chlamydomonas: evidence for control by ATP/energy supply

In Chlamydomonas reinhardtii growing synchronously under a light-dark cycle, the major chloroplast mRNAs are constitutively present but are translated only during the light period. We show that translation of these mRNAs can be induced during the normal dark period by light or by acetate and the induction is blocked by an inhibitor of ATP synthesis, CCCP. Moreover, ATP levels in synchronous cells were found to be 2-5-fold lower during the dark than in the light period; the administration of acetate or light at the mid-dark period increased the ATP level 2-3-fold. These results exclude cell-cycle mediated control and suggest that the regulation of chloroplast translation in light-dark grown Chlamydomonas is mediated, at least in part, by ATP levels.

Synthesis of EF-Tu and distribution of its mRNA between stroma and thylakoids during the cell cycle of Chlamydomonas reinhardii

In Chlamydomonas reinhardii the elongation factor EF-Tu is encoded in the chloroplast DNA. We identified EF-Tu in the electrophoretic product pattern of chloroplast-made proteins and showed that this protein is only synthesized in the first half of the light period in synchronized cells. The newly synthesized EF-Tu contributed little to the almost invariable content of EF-Tu in chloroplasts during the light period of the cell cycle. However, increasing cell volume and the lack of EF-Tu synthesis in the second half of the light period led to a decrease in the concentration of EF-Tu in chloroplasts. At different times in the vegetative cell cycle, the RNA was extracted from whole chloroplasts and from free and thylakoid-bound chloroplast polysomes. The content of mRNA of EF-Tu in chloroplasts and the distribution between stroma and thylakoids were determined. During the light period, the content of the mRNA for EF-Tu varied in parallel to the rate of EF-Tu synthesis. However, in the dark, some mRNA was present even in the absence of EF-Tu synthesis. Most of the mRNA was bound to thylakoids during the whole cell cycle. This suggests that synthesis of EF-Tu is associated with thylakoid membranes.

Light-dependent, but phytochrome-independent, translational control of the accumulation of the P700 chlorophyll-a protein of photosystem I in barley (Hordeum vulgare L.)

This work reports on the regulation of synthesis of the P700 chlorophyll-a apoprotein of photosystem I in barley. The mRNA for the P700 apoprotein is almost exclusively confined to the plastid membrane-bound polysomes. However, the mRNA for the 32-kDa herbicide-binding protein of photosystem II is found in both the soluble and membrane-bound polysomes.The mRNA for the P700 apoprotein is found in similar amounts in dark-grown and light-grown wild-type as well as mutant xantha-l(81) barley. The latter mutant is deficient in chlorophyll biosynthesis. However, while wild-type leaves accumulate the P700 chlorophyll-a protein only in the light, mutant leaves never accumulate the P700 apoprotein.A more sensitive approach was taken using isolated plastids to study P700 apoprotein synthesis. Etioplasts did not synthesize detectable P700 apoprotein even when the etioplasts were exposed to light. However, only a 1-min exposure of leaves to light was necessary to induce P700 apoprotein synthesis by isolated plastids.Phytochrome involvement in controlling P700 apoprotein synthesis was tested by using red/farred light treatment of leaves. These treatments showed no far-red reversibility of red-induced P700-apoprotein synthesis in isolated plastids even after 3 h of darkness after the light treatments. From these data we conclude that the accumulation of P700 apopootein is not under the control of phytochrome and that the light induction of P700 apoprotein is most likely mediated through the protochlorophyllide/chlorophyllide system. This control, however, may also involve cytoplasmic signals as the synthesis of the P700 apoprotein is not turned on in illuminated etioplasts.

Synthesis of two proteins in chloroplasts and mRNA distribution between thylakoids and stroma during the cell cycle of Chlamydomonas reinhardii

Chloroplasts contain thylakoid-bound and free ribosomes and polysomes. Whether binding of polysomes plays an immediate role in the regulation of chloroplast protein synthesis is not yet clear. In the present work, variations of protein synthesis and of mRNA content were measured not in greening, but in fully differentiated chloroplasts during the cell cycle of synchronized cultures of Chlamydomonas reinhardii. At different times of the vegetative cell cycle, the RNA was extracted from free and thylakoid-bound chloroplast polysomes and the partition of mRNAs between stroma and thylakoids was measured for two proteins, i.e. the 32-kDa herbicide-binding membrane protein and the soluble large subunit of the ribulose-1,5-bisphosphate carboxylase. At the same time the rates of synthesis of these two proteins were also determined. At 2 h after the onset of light, the content of both mRNAs in chloroplasts had doubled and 75-90% of each of these mRNAs were found to be bound to the thylakoids. The rate of protein synthesis, however, increased 10-fold, but reached its maximum only after about 6 h in the light. The differences in the time courses, in the stimulation of the rate of protein synthesis, and in the mRNA-binding to thylakoids point to a translational regulation of protein synthesis. Furthermore, since a very high proportion of polysomes were bound to thylakoids, containing mRNA for both a membrane and a soluble protein, this light-induced binding of polysomes to thylakoids seems to be an essential, but not the only, prerequisite for protein synthesis in chloroplasts.

Chloroplast messenger RNAs of free and thylakoid-bound polysomes from Vicia faba L

Purified chloroplasts from developing leaves of Vicia faba L. were broken and separated into stroma and thylakoid fractions. Both fractions contained polysomes as demonstrated by analytical density gradient centrifugation and in-vitro read-out translation. Messenger RNAs of free and thylakoid-bound polysomes were isolated and analysed by hybridization with heterologous gene probes from spinach and tobacco. Transcripts of the chloroplast genes psaA, psbB, psbC, psbD and petA were found predominantly on thylakoidbound polysomes engaged in the synthesis and the contrasslational integration of membrane proteins. In contrast, transcripts of the genes rbcL, psbE, petD, atpA, atpB, atpE and atpH were found more frequently on free polysomes corresponding to a stroma-located translation of these mRNAs and a posttranslational integration of the encoded intrinsic membrane proteins. We conclude from these findings that chloroplast-encoded membrane proteins are integrated by co-and posttranslational mechanisms.

Protein synthesis by isolated chloroplasts

Isolated chloroplasts show substantial rates of protein synthesis when illuminated. This 'in organello' protein synthesis system has been advantageously utilised to elucidate the coding capacity of chloroplast and the regulation of chloroplast genes. The system is also being used recently to transcribe and translate homologous and heterologous templates. In this mini-review, we attempt to critically ecaluate the available literature and present the current and the prospective lines of research.

Chlorophyll antenna proteins of photosystem I: topology, synthesis, and regulation of the 20-kDa subunit of Chlamydomonas light-harvesting complex of photosystem I

The light-harvesting complex of photosystem I (LHCI) was isolated from wild-type cells of Chlamydomonas reinhardtii; the Chl a/b-protein complex contains four major polypeptides of approximately 27, 26, 24, and 20 kDa (polypeptides 14, 15, 17.2, and 22, respectively, in the nomenclature for Chlamydomonas thylakoid proteins). Antiserum against the 20-kDa subunit of LHCI was prepared and used to determine the membrane topology, subcellular site of synthesis, and cell-cycle regulation of this polypeptide. The results indicate that the 20-kDa subunit as well as the other major LHCI polypeptides are integral membrane proteins. Moreover, protease digestion experiments reveal that the 20-kDa polypeptide is completely protected by the membrane bilayer but the 27- and 26-kDa LHCI polypeptides are exposed at the membrane surface. In vivo synthesis of the 20-kDa polypeptide is sensitive to cycloheximide but not to chloramphenicol; the form of the polypeptide recovered from in vitro translations of polyadenylated RNA is approximately 24 kDa, 4 kDa larger than the mature polypeptide. It is concluded that this LHCI polypeptide is nuclear encoded and synthesized in the cytoplasm as a higher molecular weight precursor. Synthesis of the 20-kDa polypeptide is restricted to the light period in light-dark synchronized cells. Translatable mRNA for this polypeptide accumulates during the light but levels are dramatically reduced during the dark period. Thus, synthesis of the 20-kDa subunit of LHCI appears to be transcriptionally regulated during the cell cycle.

Synthesis of large subunit of ribulosebisphosphate carboxylase by thylakoid-bound polyribosomes from spinach chloroplasts

Intact chloroplasts were isolated from developing first leaves of spinach. The chloroplasts were broken and separated into an extensively washed membrane (thylakoid) fraction and a soluble (stroma) fraction. The membrane fraction contained polyribosomes with properties similar to those of thylakoid-bound polyribosomes of other organisms. The distribution of mRNA for large-subunit ribulosebisphosphate carboxylase (LS) was determined by translating RNA from chloroplasts, thylakoids, and stroma in a wheat germ cell-free translation system. LS translation product was identified by immunoprecipitation with antibody to LS from spinach, electrophoresis of the immunoprecipitated product, and fluorography. At least 44% of translatable chloroplast LS-mRNA was in the washed thylakoid fraction. Thylakoid-bound LS-mRNA was in polyribosomes since LS was produced by thylakoids in an Escherichia coli cell-free translation system under conditions where initiation did not take place. Our results demonstrate that membrane-bound polyribosomes can synthesize the stroma-localized polypeptide LS, and suggest that the thylakoids may be an important site of its synthesis.

In vitro synthesis and assembly of photosystem II proteins of spinach chloroplasts

The synthesis and assembly of photosystem II (PS II) proteins of spinach chloroplasts were investigated in three different in vitro systems, i.e., protein synthesis in isolated chloroplasts (in organello translation), read-out translation of thylakoid-bound ribosomes, and transport of translation products from spinach leaf polyadenylated RNA into isolated chloroplasts. Polyacrylamide gel electrophoresis of labeled thylakoid polypeptides in the presence of sodium dodecyl sulfate revealed that the first two systems were capable of synthesizing the reaction center proteins of PS II (47 and 43 kDa), the herbicide-binding protein, and cytochrome b559. The reaction center proteins synthesized in organello were shown to bind chlorophyll and to assemble properly into the PS II core complex. One of the reaction center proteins translated by the thylakoid-bound ribosomes (47 kDa) was also found to be integrated in situ into the complex but was lacking bound chlorophyll. Incorporation of radioactivity into the three extrinsic proteins of the oxygen-evolution system (33, 24, and 18 kDa) was detected only when intact chloroplasts were incubated with the translation products from polyadenylated RNA, showing that these proteins are coded for by nuclear DNA. The occurrence of a precursor polypeptide 6 kDa larger than the 33-kDa protein was immunochemically detected in the translation products.

Photosynthetic ATPases: purification, properties, subunit isolation and function

Photosynthetic coupling factor ATPases (F1-ATPases) generally censist of five subunits named α, β, γ, δ and ε in order of decreasing apparent molecular weight. The isolated enzyme has a molecular weight of between 390,000 to 400,000, with the five subunits probably occurring in a 3:3:1:1:1 ratio. Some photosynthetic F1 ATPases are inactive as isolated and require treatment with protease, heat or detergent in order to elicit ATPase activity. This activity is sensitive to inhibition by free divalent cations and appears to be more specific for Ca(2+) vs. Mg(2+) as the metal ion substrate chelate. This preference for Ca(2+) can be explained by the higher inhibition constant for inhibition of ATPase activity by free Ca(2+). Methods for the assay of a Mg-dependent ATPase activity have recently been described. These depend on the presence of organic solvents or detergents in the reaction mixture for assay. The molecular mechanism behind the expression of either the Ca- or Mg-ATPase activities is unknown. F1-ATPases function to couple proton efflux from thylakoid membranes or chromatophores to ATP synthesis. The isolated enzyme may thus also be assayed for the reconstitution of 'coupling activity' to membranes depleted of coupling factor 1.The functions of the five subunits in the complex have been deduced from the results of chemical modification and reconstitution studies. The δ subunit is required for the functional binding of the F1 to the F0. The active site is probably contained in the β (and α) subunit(s). The proposed functions for the γ and ε subunits are, however, still matters of controversy. Coupling factors from a wide variety of species including bacteria, algae, C3 and C4 plants, appear to be immunologically related. The β subunits are the most strongly related, although the α and γ subunits also show significant immunological cross-reactivity. DNA sequence analyses of the genes for the β subunit of CF1 have indicated that the primary sequence of this polypeptide is highly conserved. The genes for the polypeptides of CF1 appear to be located in two cellular compartments. The α, β and ε subunits are coded for on chloroplast DNA, whereas the γ and δ subunits are probably nuclear encoded. Experiments involving protein synthesis by isolated chloroplasts or protein synthesis in the presence of inhibitors specific for one or the other set of ribosomes in the cell suggest the existence of pools of unassembled CF1 subunits. These pools, if they do exist in vivo, probably make up no greater than 1% of the total CF1 content of the cell.

In vitro synthesis and assembly of the peripheral subunits of coupling factor CF1 (alpha and beta) by thylakoid-bound ribosomes

Bispecific antisera were prepared to a mixture of thylakoid membrane polypeptides 4.1 and 4.2. The identity of these polypeptides as the alpha and beta subunits of coupling factor (CF1) was established based on the cross-reactivity of the antisera toward CF1 from peas and by an analysis of the thm-24 mutant of Chlamydomonas which lacks the CF1 ATPase. Photochemical labeling of thylakoid membranes with hydrophobic and hydrophilic fluorescent probes indicated that these polypeptides did not significantly penetrate the membrane bilayer. Immunoprecipitation of the translation products of thylakoid-bound and soluble ribosomes showed the thylakoids to be the major site of synthesis of the polypeptides. Immunoprecipitation of the products of translation of total cellular RNA in a reticulocyte lysate showed no evidence for substantially higher molecular weight precursors. Further analysis of the thylakoid-bound synthesis of alpha and beta revealed that some of the in vitro synthesized polypeptides had been incorporated into the CF0-CF1 complex based on their release from membranes with trypsin and copurification with the CF0-CF1 ATPase.

Synthesis of the alpha and beta subunits of coupling factor 1 by polysomes from pea chloroplasts

Washed thylakoids of pea chloroplasts, containing tightly bound polysomes, incorporate radioactive amino acids into protein when supplied with soluble factors from Escherichia coli. Polyacrylamide gel electrophoresis with lithium dodecyl sulfate, followed by autoradiography of the labeled products, showed the synthesis of a number of different polypeptides. Two of the most heavily labeled products were in the region expected for the alpha and beta subunits of coupling factor 1, at 57 and 54 kDa. Positive identification of the subunits was made using monospecific antibodies. Furthermore, the same two polypeptides made by soluble polysomes located in the chloroplast stroma were found. While the major proportion of the newly formed alpha and beta subunits made by thylakoid-bound polysomes remained with the thylakoids after protein synthesis occurred, no evidence was found of incorporation into complete, EDTA-extractable coupling factor 1.

Thylakoid membranes: the translational site of chloroplast DNA-regulated thylakoid polypeptides

Stromal ribosomes and those bound to thylakoid membranes were prepared from intact spinach chloroplasts which were purified on Percoll gradients. The products of read-out translation of these ribosomes supplemented with an Escherichia coli extract were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Striking similarity was found between the polypeptides labeled in the read-out translation of the chloroplastic ribosomes and those synthesized in isolated chloroplasts. Among the polypeptides translated on thylakoid-bound ribosomes, apoprotein of chlorophyll-protein complex I, alpha and beta subunits of coupling factor 1, and 32,000-Da membrane polypeptide were identified from their mobility on the polyacrylamide gel. The large subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and other several stromal proteins were translated exclusively from stromal ribosomes. However, when the translation was programmed in cell-free systems from either E. coli, wheat germ, or rabbit reticulocytes by RNAs isolated separately from stroma and thylakoids, no qualitative difference was found between the products from those RNAs. These results suggest that thylakoid-bound ribosomes are the main sites of synthesis of thylakoid proteins and stromal-free ribosomes are that of stromal proteins, and that thylakoids and stroma contain mRNAs for the stromal and the thylakoid proteins, respectively, in a form not functioning in the chloroplasts.

Sequence homology between the 32K dalton and the D2 chloroplast membrane polypeptides of Chlamydomonas reinhardii

The region of the chloroplast genome of Chlamydomonas reinhardii containing the gene of the thylakoid polypeptide D2 (psbD) has been sequenced. A unique open reading frame of 350 codons exists in this region. Because the first ATG is followed 11 codons downstream by a second one, the D2 polypeptide consists of either 339 or 350 amino acids. Comparison of the sequences of D2 and the 32K dalton polypeptides, both of which are associated with photosystem II, reveals partial homology. Although, the overall homology of these two polypeptides is only 27%, they contain several related regions and their hydropathic profiles are strikingly similar. These data suggest that the two polypeptides may have related functions and/or that their genes may have originated from a common ancestor. Alternatively, convergent evolution of these polypeptides may be due to structural constraints in the thylakoid membrane. Limited sequence homology is also observed between the D2 polypeptide and some of the subunits of the reaction centers of photosynthetic bacteria.

Localization and nucleotide sequence of the gene for the membrane polypeptide D2 from pea chloroplast DNA

The gene for the membrane polypeptide D2 has been mapped on the pea (Pisum sativum) chloroplast genome. The nucleotide sequence of the gene and its flanking regions is presented. The only large open reading frame in the sequence codes for a protein of MW 39.5 kD. A potential ribosome binding site is located 6 nucleotides upstream from the initiation codon and there are two sets of putative promotor sequences in the 5' flanking region. The polypeptide has a high content of hydrophobic amino acids, predominatly grouped in clusters of 20 or more residues. The 3' end of the D2 gene is overlapped by 50 nucleotides of a second open reading frame (UORF I) which is at least 369 nucleotides long. Based on current data we suggest the D2 polypeptide to be a constituent of photosystem II (PSII).

In-vitro translation of different mRNA-containing fractions of Chlamydomonas chloroplasts

Starting from isolated chloroplasts of the Chlamydomonas reinhardii cw 15 mutant, several mRNA-containing chloroplast subfractions, i.e. thylakoid-bound polysomes, detached polysomes or isolated RNA, were prepared and incubated in homologous and heterologous translation systems. In the reticulocyte lysate these fractions gave rise to strikingly different product patterns. A most prominent difference concerned the in-vivo rapidly labelled 32,000-dalton thylakoid polypeptide. Neither this membrane protein nor its 34,000-dalton precursor was formed when membrane-containing or free polysomes were translated, while the 34,000-dalton precursor was a main product of the RNA isolated from the same membranes. The influence of thylakoid membranes during translation was also observed in homologous translation systems with lysed chloroplasts supplemented with ATP. Membrane and soluble fractions, when translated separately, yielded product patterns which differed from each other, although the RNAs extracted from the respective fractions gave the same product patterns when translated in reticulocyte lysate; the latter included a soluble protein, the large subunit of ribulose-1,5-bisphosphate carboxylase, and a membrane protein, the 34,000-dalton precursor of the 32,000-dalton membrane protein, as major labelled translation products. These results point to a regulatory role of thylakoid membranes in the expression of chloroplast mRNA and argue against compartmentation of the chloroplast mRNAs between the soluble and membrane fractions.

Synthesis of photosynthetic membrane proteins directed by RNA from rough thylakoids of Chlamydomonas reinhardtii

Messenger RNA activities associated with thylakoids of Chlamydomonas reinhardtii were examined by translation of RNA of rough thylakoids (thylakoids with bound ribosomes) in a wheat germ protein-synthesis system. The RNA fraction, active in translation, did not bind to oligod (T)-cellulose indicating that the mRNA associated with the rough thylakoids contains little, if any, 3' polyadenylic acid. Assuming that 1% of the thylakoid RNA was mRNA, it was found to have a specific translation activity between that of globin mRNA and tobacco mosaic virus RNA. Translation products of thylakoid RNA were immunoprecipitated by crossed immunoelectrophoresis, using antisera to two polypeptides synthesized within the chloroplast, one of which specifically precipitates the beta subunit of chloroplast coupling factor 1. Each antiserum immunoprecipitated radioactive polypeptide from translates of thylakoid RNA, indicating that the presence of mRNA for both polypeptides is associated with the thylakoids. The results suggest that thylakoid-bound polyribosomes are involved in the synthesis of thylakoid polypeptides.