In vitro synthesis of photosynthetic membranes: I. Development of photosystem I activity and cyclic photophosphorylation

Green giant-a tiny chloroplast genome with mighty power to produce high-value proteins: history and phylogeny

Free-living cyanobacteria were entrapped by eukaryotic cells ~2 billion years ago, ultimately giving rise to chloroplasts. After a century of debate, the presence of chloroplast DNA was demonstrated in the 1960s. The first chloroplast genomes were sequenced in the 1980s, followed by ~100 vegetable, fruit, cereal, beverage, oil and starch/sugar crop chloroplast genomes in the past three decades. Foreign genes were expressed in isolated chloroplasts or intact plant cells in the late 1980s and stably integrated into chloroplast genomes, with typically maternal inheritance shown in the 1990s. Since then, chloroplast genomes conferred the highest reported levels of tolerance or resistance to biotic or abiotic stress. Although launching products with agronomic traits in important crops using this concept has been elusive, commercial products developed include enzymes used in everyday life from processing fruit juice, to enhancing water absorption of cotton fibre or removal of stains as laundry detergents and in dye removal in the textile industry. Plastid genome sequences have revealed the framework of green plant phylogeny as well as the intricate history of plastid genome transfer events to other eukaryotes. Discordant historical signals among plastid genes suggest possible variable constraints across the plastome and further understanding and mitigation of these constraints may yield new opportunities for bioengineering. In this review, we trace the evolutionary history of chloroplasts, status of autonomy and recent advances in products developed for everyday use or those advanced to the clinic, including treatment of COVID-19 patients and SARS-CoV-2 vaccine.

Containment of herbicide resistance through genetic engineering of the chloroplast genome

Glyphosate is a potent herbicide. It works by competitive inhibition of the enzyme 5-enol-pyruvyl shikimate-3-phosphate synthase (EPSPS), which catalyzes an essential step in the aromatic amino acid biosynthetic pathway. We report the genetic engineering of herbicide resistance by stable integration of the petunia EPSPS gene into the tobacco chloroplast genome using the tobacco or universal vector. Southern blot analysis confirms stable integration of the EPSPS gene into all of the chloroplast genomes (5000-10,000 copies per cell) of transgenic plants. Seeds obtained after the first self-cross of transgenic plants germinated and grew normally in the presence of the selectable marker, whereas the control seedlings were bleached. While control plants were extremely sensitive to glyphosate, transgenic plants survived sprays of high concentrations of glyphosate. Chloroplast transformation provides containment of foreign genes because plastid transgenes are not transmitted by pollen. The escape of foreign genes via pollen is a serious environmental concern in nuclear transgenic plants because of the high rates of gene flow from crops to wild weedy relatives.

Uptake and expression of bacterial and cyanobacterial genes by isolated cucumber etioplasts

The uptake and expression by plastids isolated from dark-grown cucumber cotyledons (etioplasts) of two pUC derivatives, pCS75 and pUC9-CM, respectively carrying genes for the large small subunits of ribulose bisphosphate carboxylase/oxygenase of Anacystis nidulans or chloramphenicol acetyltransferase, is reported. Untreated etioplasts take up only 3% as much DNA as that taken up by EDTA-washed etioplasts after 2 hr of incubation with nick-translated [32P]-pCS75. The presence or absence of light does not affect DNA uptake, binding, or breakdown by etioplasts. Calcium or magnesium ions inhibit DNA uptake by 86% but enhance binding (23-200%) and breakdown (163-235%) of donor DNA by EDTA-treated etioplasts. Uncouplers that abolish membrane potential (delta psi), transmembrane proton gradient (delta pH), or both do not affect DNA uptake, binding, or breakdown by etioplast. However, both DNA uptake and binding are severely inhibited by ATP. Presumably this results from the hydrolysis of ATP, because the poorly hydrolyzable analog adenyl-5'-yl imidodiphosphate does not inhibit the uptake or binding of DNA by etioplasts. beta-Lactamase specified by the ampicillin resistance gene of pCS75 can be detected only in EDTA-treated etioplasts that have been incubated with the plasmid pCS75. After the incubation of EDTA-treated etioplasts with pCS75, immunoprecipitation using antiserum to the small subunit of ribulose bisphosphate carboxylase/oxygenase from A. nidulans reveals the synthesis of small subunits; these are smaller by 2 kDa than the cucumber small subunit encoded by the nuclear genome. Treatment of etioplasts with 10 mM EDTA shows a 10-min duration to be optimal for the expression of chloramphenicol acetyltransferase encoded by pUC9-CM. A progressive increase in the expression of this enzyme is observed with an increase in the concentration of pUC9-CM in the DNA uptake medium. The plasmid-dependent incorporation of [35S]methionine by EDTA-treated organelles declines markedly during cotyledon greening in vivo.

An efficient and prolonged in vitro translational system from isolated cucumber etioplasts

Etioplasts were isolated from dark grown cucumber cotyledons pretreated with kinetin and gibberellic acid. When incubated in a cofactor enriched medium these etioplasts incorporated [35S] methionine into a hot trichloroacetic acid-insoluble fraction; this incorporation was linear for 8 h of incubation and was inhibited by chloramphenicol but not by cycloheximide. Over the same time period, the etioplasts showed continued linear synthesis of the chlorophyll precursors protochlorophyllide, Mg-protoporphyrin and protoporphyrin IX. Analysis of products of in vitro protein synthesis by etioplasts and cotyledons showed the thylakoid membrane polypeptide profiles to be identical. Continued incorporation of [35S] methionine into the large subunit of ribulose bisphosphate carboxylase/oxygenase (RuBisCO) for 8 h has been confirmed further by immunoprecipitation with anti-spinach RuBisCO. This competent in vitro translation system should be useful for future studies of chloroplast protein synthesis and gene expression.

Chlorophyll biosynthesis from glutamate or 5-aminolevulinate in intact Euglena chloroplasts

Chloroplasts observed, by electron microscopy, to be intact and uncontaminated, with high rates of light-dependent protein synthesis and CO2 fixation were isolated from cells grown on low-vitamin-B12 medium in the light or from cells grown in the same medium in the dark and then exposed to light for 36 h. Both types of chloroplasts were active but less variability was encountered with developing chloroplasts from 36-h cells. The 36-h chloroplasts showed good light-dependent incorporation of 5-amino-levulinic acid (ALA) or L-glutamic acid into chlorophyll (Chl) a which was linear for approx. 1 h. The specific activity of the Chl a remained the same after conversion to pheophytin a, methylpheophorbide a or pyromethylpheophorbide a and rechromatography, indicating that the label was in the tetrapyrrole. Incorporation of ALA was inhibited by levulinic acid, and by chloramphenicol and other inhibitors of translation of 70S-type chloroplast ribosomes at concentrations which did not appreciably inhibit photosynthesis but which blocked plastid protein synthesis nearly completely. Cycloheximide, an inhibitor of translation on 87S cytoplasmic ribosomes of Euglena, was without effect. The 70S inhibitors did not block uptake of labeled ALA. Although labeled glycine was taken up by the plastids, no incorporation into Chl a was observed. Thus the developing chloroplasts appear to contain all of the enzymatic machinery necessary to convert glutamic acid to Chl via the C5 pathway of ALA formation but the Shemin pathway from succinyl coenzyme A and glycine to ALA appears to be absent. The requirement for plastid protein synthesis concomitant with Chl synthesis indicates a regulatory interaction and also indicates that at least one protein influencing Chl synthesis is synthesized on 70S-type plastid ribosomes and is subject to metabolic turnover.

Oxygenic photoreduction of methyl viologen and nicotinamide adenine dinucleotide phosphate without the involvement of photosystem I during plastid development

Studies on the appearance of various electron transport functions were followed during greening of etiolated cucumber cotyledons. Appearance of dichlorodimethoxy-p-benzoquinone, dimethyl quinone, tetramethyl-p-phenylenediamine, dichlorophenol indophenol and ferricyanide Hill reactions were observed after 8h of greening. However, photoreduction of methyl viologen (MV) and nicotinamide adenine dinucleotide phosphate (NADP) was observed from 2h of greening. Variable fluorescence, which is a direct indication of water-splitting function, was observed from 2h of greening in cotyledons, thylakoid membranes and photosystem II (PSII) particles. The decrease in variable fluorescence in the presence of MV (due to rapid reoxidation of Q-) observed from early stages of greening confirmed the photoreduction of MV by PSII. The early development of water-splitting function was further confirmed by the abolition of variable fluorescence in thylakoid membranes and PSII particles by heat treatment and concomittant loss of light dependent oxygen uptake in the presence of MV in heat treated chloroplasts. However, the photoreduction of MV and NADP was insensitive to intersystem electron transport inhibitors, dichlorophenyl dimethylurea or dibromomethyl isopropyl-p-benzoquinone till 8h of greening. Though the oxidation of intersystem electron carrier cytochrome f was observed from early stages of greening, the reduction of cytochrome f was not observed till 8h of greening. All these observations confirm that during early stages of greening MV and NADP are photoreduced by PSII without the involvement of intersystem electron carriers or the collaboration of PSI. Since these observations are at variance with the currently prevalent concept (Z-Scheme) of the photosynthetic generation of reducing power, which requires definite collaboration of the two photosystems, an alternate electron flow pathway is proposed.

Radioisotopic evidence for the polypeptides associated with photosystem II

Direct radioisotopic evidence for the polypeptides associated with oxygen evolution in a non-destructive approach by comparing the polypeptides of photosystem II (PS II) particles in etioplasts that lack O2 evolution with that of chloroplasts that exhibit high rates of oxygen evolution is reported. Polypeptide analysis by coomassie blue staining revealed that 32, 23 and 15 kDa proteins were absent in etioplast particles but developed in chloroplast PS II particles after illumination. However, a strikingly different picture was obtained when labelled polypeptides were analyzed from the fluorograph. While the new proteins identified by coomassie blue staining were also labelled, intensely labelled major polypeptides of molecular range 40, 41, 43 kD and minor polypeptides of molecular range 47, 48, 51 kD were observed in chloroplast PS II particles but not in etioplasts or etioplast particles; these labelled polypeptides were so prominent that they could be identified even in thylakoid membranes.