The effects of inhibitors of RNA and protein synthesis on chloroplast structure and function in wild-type Chlamydomonas reinhardi

Coexpressed subunits of dual genetic origin define a conserved supercomplex mediating essential protein import into chloroplasts

In photosynthetic eukaryotes, thousands of proteins are translated in the cytosol and imported into the chloroplast through the concerted action of two translocons-termed TOC and TIC-located in the outer and inner membranes of the chloroplast envelope, respectively. The degree to which the molecular composition of the TOC and TIC complexes is conserved over phylogenetic distances has remained controversial. Here, we combine transcriptomic, biochemical, and genetic tools in the green alga Chlamydomonas (Chlamydomonas reinhardtii) to demonstrate that, despite a lack of evident sequence conservation for some of its components, the algal TIC complex mirrors the molecular composition of a TIC complex from Arabidopsis thaliana. The Chlamydomonas TIC complex contains three nuclear-encoded subunits, Tic20, Tic56, and Tic100, and one chloroplast-encoded subunit, Tic214, and interacts with the TOC complex, as well as with several uncharacterized proteins to form a stable supercomplex (TIC-TOC), indicating that protein import across both envelope membranes is mechanistically coupled. Expression of the nuclear and chloroplast genes encoding both known and uncharacterized TIC-TOC components is highly coordinated, suggesting that a mechanism for regulating its biogenesis across compartmental boundaries must exist. Conditional repression of Tic214, the only chloroplast-encoded subunit in the TIC-TOC complex, impairs the import of chloroplast proteins with essential roles in chloroplast ribosome biogenesis and protein folding and induces a pleiotropic stress response, including several proteins involved in the chloroplast unfolded protein response. These findings underscore the functional importance of the TIC-TOC supercomplex in maintaining chloroplast proteostasis.

The algal pyrenoid: key unanswered questions

The confinement of Rubisco in a chloroplast microcompartment, or pyrenoid, is a distinctive feature of most microalgae, and contributes to perhaps ~30 Pg of carbon fixed each year, yet our understanding of pyrenoid composition, regulation, and function remains fragmentary. Recently, significant progress in understanding the pyrenoid has arisen from studies using mutant lines, mass spectrometric analysis of isolated pyrenoids, and advanced ultrastructural imaging of the microcompartment in the model alga Chlamydomonas. The emergence of molecular details in other lineages provides a comparative framework for this review, and evidence that most pyrenoids function similarly, even in the absence of a common ancestry. The objective of this review is to explore pyrenoid diversity throughout key algal lineages and discuss whether common ultrastructural and cellular features are indicative of common functional processes. By characterizing pyrenoid origins in terms of mechanistic and structural parallels, we hope to provide key unanswered questions which will inform future research directions.

Monitoring dynamic expression of nuclear genes in Chlamydomonas reinhardtii by using a synthetic luciferase reporter gene

For monitoring the expression profile of selected nuclear genes in Chlamydomonas reinhardtii in response to altered environmental parameters or during cell cycle, in the past many RNA or protein samples had to be taken and analyzed by RNA hybridization or protein immunoblotting. Here we report the synthesis of a gene that codes for the luciferase of Renilla reniformis (RLuc) and is adapted to the nuclear codon usage of C. reinhardtii . This crluc gene was expressed alone or as a fusion to the zeocin resistance gene ble under control of different promoter variants. Luciferase activity was monitored in living cells, increased with the promoter strength and paralleled the amount of expressed protein. Under control of the Lhcb-1 promoter the Luc-activity in synchronized cultures was dependent on the dark-light cycle. Additionally, crluc was placed under control of the Chop-2 promoter and activity was measured under different light conditions. Chop-2 promoter activity was found to be most pronouced under low-light and dark conditions, further supporting that channelrhodopsin-2 is most active in dark-adapted cells. We conclude that crluc is a reliable tool for convenient monitoring of nuclear gene expression in C. reinhardtii .

Plastid ultrastructure and photosynthesis in greening petaloid hypsophylls

The ultrastructural and biochemicalphysiological aspects of postfloral greening have been studied in hypsophylls of Heliconia aurantiaca Ghiesbr., Guzmania cf. x magnifica Richter and Spathiphyllum wallisii Regel. In all three species the greening of the hypsophylls is due to plastid transformation, chloroplast formation proceeding from the initially different types of plastids. The degradation process of the original plastid structures and the mode of thylakoid formation are distinct in each case. In none of the species do the transformed plastids look identical to the chloroplasts of the corresponding foliage leaves. On a chlorophyll basis, the rate of photosynthesis of the greened hypsophylls surpasses the rate of the leaves considerably in Spathiphyllum, but is much lower in Heliconia (no data for Guzmania). In all species, anatomy, plastid structure, pigments, 77° K-fluorescence emission, ribulose-1,5-bis-phosphate carboxylase activities and short-term photosynthesis (14)CO2-assimilation patterns prove the greened hypsophylls to be capable of providing additional carbon to the developing fruits, thus supplementing the import of organic matter from the foliage leaves.

Synthesis of chloroplast ribonucleic acid in Chlamydomonas reinhardtii toluene-treated cells

Chlamydomonas reinhardtii cells treated with toluene at 0 degrees C and 25 degrees C incorporate ribonucleoside triphosphates (NTPs) into chloroplast RNA at 25 degrees C and also at 35 degrees C. The incorporation requires all four NTPs and Mg2+, and is completely inhibited by DNase, RNase, actinomycin D (40 microgram/ml) and rifampicin (350 microgram/ml). However, the incorporation is almost totally insensitive to both alpha-amanitin and streptolydigin at 200 microgram/ml.

Photoreactivation and dark repair of ultraviolet light-induced pyrimidine dimers in chloroplast DNA

A UV-specific endonuclease was used to detect ultraviolet light-induced pyrimidine dimers in chloroplast DNA of Chlamydomonas reinhardi that was specifically labeled with tritiated thymidine. All of the dimers induced by 100 J/m2 of 254 nm light are removed by photoreaction. Wild-type cells exposed to 50 J/m2 of UF light removed over 80% of the dimers from chloroplast DNA after 24 h of incubation in growth medium in the dark. A UV- sensitive mutant, UVS1, defective in the excision of pyrimidine dimers from nuclear DNA is capable of removing pyrimidine dimers from chloroplast DNA nearly as well as wild-type, suggesting that nuclear and chloroplast DNA dark-repair systems are under separate genetic control.

Identification and Precipitation of the Polyribosomes in Chlamydomonas reinhardi Involved in the Synthesis of the Large Subunit of d-ribulose-1,5-bisphosphate Carboxylase

The size classes of polyribosomes involved in the synthesis of ribulose-1,5-bisphosphate carboxylase large subunit were determined by binding radioiodinated specific antibodies to polyribosomal preparations from Chlamydomonas reinhardi. Antibodies specific to the denatured large subunit and to the native enzyme bound primarily to small polyribosomes (N = two to five ribosomes). The binding of antibodies to small polyribosomes was unexpected since the large subunit is a large polypeptide (molecular weight 55,000) coded for by a corresponding large mRNA (12-14S). Control experiments showed that this unexpected pattern of antibody binding was not a result of messenger RNA degradation, "run-off" of ribosomes from polyribosomes, or adventitious binding of the completed enzyme to a selected class of polyribosomes. In addition, polyribosomes bearing nascent large subunit chains have been immunoprecipitated from small polyribosome fractions. A large RNA species that can direct the synthesis of large subunit in vitro was extracted from small polyribosomes.

Capacity for chlorophyll synthesis in heat-bleached 70S ribosome-deficient rye leaves

The leaves of young rye plants (Secale cereale L.) grown at 32° were deficient in chlorophyll and in chloroplastic rRNA as compared to those grown at 22°, which developed normally. Both chlorophyll accumulation and the formation of plastidic rRNA were largely restored at 32° when the plants were transfered several times for 1 h per day to 22°. In the chlorotic 32°-grown rye leaves the in vivo activity of δ-aminolevulinate synthetase was very low. Aminolevulinate dehydratase however, exhibited high activity in extracts from 32°-grown leaves and was localized in the plastid fraction isolated from the chlorotic leaf tissue. After application of δ-aminolevulinic acid to chlorotic parts of leaves growing at 32°, protochlorophyll(ide) was formed and accumulated in the dark. In the light, the protochlorophyll(ide) was photooxidized at 32°. The results suggest a cytoplasmic site of synthesis for the series of enzymes converting δ-aminolevulinate to protochlorophyll(ide). It is concluded that an inhibition of δ-aminolevulinate synthetase and the photooxidation of protochlorophyll(ide) or chlorophyll are responsible for the chlorosis of the leaves at 32°.

The isolation and characterization of intact polyribosomes from a cell wall mutant of Chlamydomonas reinhardi

A technique is described for obtaining intact polyribosomes from a cell-wall mutant of Chlamydomonas reinhardi. When cells were lysed by nonionic detergent in buffers containing high salt and Mg2+-EDTA, polyribosomes of up to 25 ribosomes per polyribosome were obtained on sucrose density gradients. Under these conditions, nascent polypeptide radioactivity was associated with the polyribosomes and not with monoribosomes, and inactive monoribosomes were dissociated to ribosomal subunits. Whole cell lysates contain a mixture of cytoplasmic and chloroplastic polyribosomes. The relative contribution of the two types of polyribosomes was evaluated by polyacrylamide gel electrophoretic analysis of ribosomal RNA extracted from polyribosomes. This analysis showed that less than 15% of the polyribosomes from detergent-lysed cells were from chloroplasts. The contribution of chloroplast polyribosomes to total polyribosomes was increased to about 30% by incubation of the cells in chloramphenicol. When cells were disrupted mechanically (in a French pressure cell) only about 10% of the resulting free polyribosomes were chloroplastic.

Formation of chloroplast pigments and sterols in rye leaves deficient in plastid ribosomes

1. In rye (Secale cereale L.) leaves the formation of plastidic ribosomes is sensitive to elevated growth temperatures. Parallel to the loss of 70S ribosomes, in leaves growing at 32° chlorophyll accumulation was also prevented. Except for the tips of the first leaves which still contained some 70S ribosomes, the leaves were chlorotic. The amount of chlorophyll formed at 32° depended on the light intensity and decreased with higher intensities. After return to normal temperature (22°) chlorotic parts of the first leaves greened to a varying extent while those parts of most 2. or 3. leaves which had been formed in light at 32° remained permanently bleached until they died. Those parts of 2. and 3. leaves which were newly formed at 22° became normally green again. - 2. Formation and distribution of total and individual carotenoids were compared after development at 22° and 32°. In dark-grown leaves the higher growth temperature had no marked influence on the quantity or composition of carotenoids. At 22° the content of total carotenoids was 5fold and that of β-carotene 25fold increased by light. At 32° these light-induced increases were much lower. Only 41% of the total carotenoids and 18% of the β-carotene formed at 22° in light were found at 32°. Of the carotenoids present at 32°, 76% were located in the light green tips of the leaves. In plastids isolated from completely chlorotic leaf parts, carotenoids were still present and were even the predominant pigments. - 3. The contents of total sterols, the fractions of free sterols, sterol glycosides and esters, and the composition of individual sterols were compared in rye leaves grown at 22° and at 32°, in light or darkness. Light had little effect on the total sterol contents per leaf. However, more than 2fold higher sterol contents were observed in leaves grown at 32°, as compared to those from 22°. The amounts of most sterol fractions and individual sterols were similarly increased at the higher temperature but the sterol glycosides being relatively more increased than the total sterols.

[Precursors of chloroplast ribosomal RNA and their maturation in Chlorella]

In Chlorella pyrenoidosa, tritiated uridine is incorporated specifically into the RNA of the chloroplast. The 16 S and 23 S ribosomal RNA become labeled after at least 15 min. Short pulse labeling of 5 min results in peaks of radioactivity in the 17 S region and at the heavy side of the 23 S peak, as shown by polyacrylamide electrophoresis.During chase treatment with unlabeled uridine after the pulse labeling, a shift of radioactivity from the 17 S to the 16 S region is observed. At the same time, the radioactivity over the 23 S speak becomes symmetrical. In the 17 S region, there are at least two peaks which appear and disappear during chase treatment. From data of specific radioactivity a precursor-end product relation can be deduced.After blocking of the chloroplast translation with spectinomycin, the RNA in the 17 S region is accumulated. This product is not stringently the same as that from pulse labeling experiments, because it migrates slightly faster than 17 S RNA. Removal of the antibiotic results in a shift of the radioactivity to the 16 S region. At the same time, the previously blocked chloroplast ribosome synthesis is reinitiated.Attempts have been made to localize the precursor molecules of 17 S and 23 S within the cell. By means of differential centrifugation it has been shown that the precursor RNA components are located in ribosomal particles. No free precursor molecules are found in the ribosome-free supernatant. This is the case in normal as well as in spectinomycin-treated cells.The results are discussed in view of the possible role of chloroplast ribosomal particles as processing agents for the maturation of chloroplast ribosomal RNA.

Chloroplast ribosome biogenesis in Chlamydomonas. Selection and characterization of mutants blocked in ribosome formation

Chloroplast protein synthesis in Chlamydomonas reinhardtii is dispensable when cells are provided acetate as a carbon source. Mutants defective in synthesis, assembly, or function of chloroplast ribosomes are therefore conditionally viable. Positive selection of nonphotosynthetic cells on arsenate has been combined with a simple screening procedure to isolate mutants with a broad spectrum of defects in chloroplast protein synthesis. Eight new mutants deficient in chloroplast ribosomes have been isolated. Three of these have been characterized genetically and phenotypically, and compared with two previously described ribosome mutants, ac-20 and cr-1. A working model of ribosome assembly is proposed which suggests possible biochemical roles for these five Mendelian gene loci.

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.

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

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

[Synthesis of chlorophyll and of chloroplast structure in Chlorella without participation of chloroplast ribosomes]

The unicellular green alga Chlorella pyrenoidosa shows normal heterotrophic growth and pigment synthesis in the presence of inhibitors of chloroplast transcription and translation.Cells treated with 100 μg/ml of rifampin, 25 μg/ml of lincomycin or 25 μg/ml of spectinomycin for 48 h do not contain any chloroplast ribosomes, 16 S or 23 S RNA as shown with cell fractionation and gel electrophoresis. Labeling experiments with [5(-3) H]uridine indicate that rifampin blocks completely the formation of 16 S and 23 S RNA whilst cytoplasmic rRNA synthesis is unaffected.Production of dry matter and of cell nitrogen are unaffected by treatment of the cells with antibiotics. Chlorophyll production is almost normal as compared with the untreated control culture. The release of daughter cells is partially inhibited by rifampin treatment. In such cultures, giant cells can be observed among cells of normal size.The O2-production of rifampin-treated cells remains unaffected up to 24 h after application of the drug. Later on, the oxygen evolution declines and, after 72 h, oxygen is consumed even during illumination. The O2-consumption in the dark is markedly enhanced after rifampin treatment.Electron microscopy of rifampin-treated Chlorella shows that apparently normal chloroplast membranes are formed. The thylakoids are arranged in grana-like structures whereas in untreated cells they form only stacks of two or three thylakoids. In the chloroplasts of rifampin-treated Chlorella cells, no more ribosomes are found.The cellular location of synthesis of some chloroplast constituents is discussed.