Genetically engineered mutant of the cyanobacterium Synechocystis 6803 lacks the photosystem II chlorophyll-binding protein CP-47

CP-47 is absent in a genetically engineered mutant of cyanobacterium Synechocystis 6803, in which the psbB gene [encoding the chlorophyll-binding photosystem II (PSII) protein CP-47] was interrupted. Another chlorophyll-binding PSII protein, CP-43, is present in the mutant, and functionally inactive PSII-enriched particles can be isolated from mutant thylakoids. We interpret these data as indicating that the PSII core complex of the mutant still assembles in the absence of CP-47. The mutant lacks a 77 K fluorescence emission maximum at 695 nm, suggesting that the PSII reaction center is not functional. The absence of primary photochemistry was indicated by EPR and optical measurements: no chlorophyll triplet originating from charge recombination between P680(+) and Pheo(-) was observed in the mutant, and there were no flash-induced absorption changes at 820 nm attributable to chlorophyll P680 oxidation. These observations lead us to conclude that CP-47 plays an essential role in the activity of the PSII reaction center.

Triazine herbicide resistance in the photosynthetic bacterium Rhodopseudomonas sphaeroides

The photoaffinity herbicide azidoatrazine (2-azido-4-ethylamino-6-isopropylamino-s-triazine) selectively labels the L subunit of the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides. Herbicide-resistant mutants retain the L subunit and have altered binding properties for methylthio- and chloro-substituted triazines as well as altered equilibrium constants for electron transfer between primary and secondary electron acceptors. We suggest that a subtle alteration in the L subunit is responsible for herbicide resistance and that the L subunit is the functional analog of the 32-kDa Q(B) protein of chloroplast membranes.

Identification of the triazine receptor protein as a chloroplast gene product

The triazine herbicides inhibit photosynthesis by blocking electron transport at the second stable electron acceptor of photosystem II. This electron transport component of chloroplast thylakoid membranes is a protein-plastoquinone complex termed "B." The polypeptide that is believed to be a component of the B complex has recently been identified as a 32- to 34-kilo-dalton polypeptide by using a photoaffinity labeling probe, azido-[(14)C]atrazine. A 34-kilodalton polypeptide of pea chloroplasts rapidly incorporates [(35)S]methionine in vivo and is also a rapidly labeled product of chloroplast-directed protein synthesis. Trypsin treatment of membranes tagged with azido-[(14)C]atrazine, [(35)S]methionine in vivo, or [(35)S]methionine in isolated intact chloroplasts results in identical, sequential alterations of the 34-kilo-dalton polypeptide to species of 32, then 18 and 16 kilodaltons. From the identical pattern of susceptibility to trypsin we conclude that the rapidly synthesized 34-kilodalton polypeptide that is a product of chloroplast-directed protein synthesis is identical to the triazine herbicide-binding protein of photosystem II. Chloroplasts of both triazine-susceptible and triazine-resistant biotypes of Amaranthus hybridus synthesize the 34-kilodalton polypeptide, but that of the resistant biotype does not bind the herbicide.

Membrane protein damage and repair: Selective loss of a quinone-protein function in chloroplast membranes

A loss of electron transport capacity in chloroplast membranes was induced by high-light intensities (photoinhibition). The primary site of inhibition was at the reducing side of photosystem II (PSII) with little damage to the oxidizing side or to the reaction center core of PSII. Addition of herbicides (atrazine or diuron) partially protected the membrane from photoinhibition; these compounds displace the bound plastoquinone (designated as Q(B)), which functions as the secondary electron acceptor on the reducing side of PSII. Loss of function of the 32-kilodalton Q(B) apoprotein was demonstrated by a loss of binding sites for [(14)C]atrazine. We suggest that quinone anions, which may interact with molecular oxygen to produce an oxygen radical, selectively damage the apoprotein of the secondary acceptor of PSII, thus rendering it inactive and thereby blocking photosynthetic electron flow under conditions of high photon flux densities.

Photoaffinity labeling of an herbicide receptor protein in chloroplast membranes

2-Azido-4-ethylamino-6-isopropylamino-s-triazine (azido-atrazine) inhibits photosynthetic electron transport at a site identical to that affected by atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine). The latter is a well-characterized inhibitor of photosystem II reactions. Azido-atrazine was used as a photoaffinity label to identify the herbicide receptor protein; UV irradiation of chloroplast thylakoids in the presence of azido[(14)C]atrazine resulted in the covalent attachment of radioactive inhibitor to thylakoid membranes isolated from pea seedlings and from a triazine-susceptible biotype of the weed Amaranthus hybridus. No covalent binding of azido-atrazine was observed for thylakoid membranes isolated from a naturally occurring triazine-resistant biotype of A. hybridus. Analysis of thylakoid polypeptides from both the susceptible and resistant A. hybridus biotypes by sodium dodecyl sulfate/polyacrylamide gel electrophoresis, followed by fluorography to locate (14)C label, demonstrated specific association of the azido[(14)C]atrazine with polypeptides of the 34- to 32-kilodalton size class in susceptible but not in resistant membranes.

Chloroplast membrane alterations in triazine-resistant Amaranthus retroflexus biotypes

The effectiveness of diuron, atrazine, procyazine, and cyanazine were compared in controlling growth of redroot pigweed (Amaranthus retroflexus L.) in hydroponic culture. A very marked differential inhibition response was observed for atrazine between resistant and susceptible biotypes. Procyazine and cyanazine exhibited less dramatic differential responses, whereas diuron was equally effective in controlling growth in both biotypes. Photosystem II activity of chloroplasts from both triazine-resistant and triazine-susceptible biotypes was inhibited by diuron but only the chloroplasts from triazine-susceptible biotypes were inhibited significantly by atrazine. The photochemical activity of chloroplasts from triazine-resistant biotypes was partially resistant to procyazine or cyanazine inhibition. The parallel lack of diuron differential effects, partial procyazine and cyanazine differential response, and very marked atrazine differential response in both whole plant and chloroplast assays indicates that the chloroplast is the site of selective herbicide tolerance in these triazine-resistant redroot pigweed biotypes.Photosystem II photochemical properties were characterized by analysis of chlorophyll fluorescence transients in the presence or absence of herbicides. Data with susceptible chloroplasts indicated that both diuron and atrazine inhibit electron flow very near the primary electron acceptor of photosystem II. Only diuron altered the fluorescence transient in resistant chloroplasts. In untreated preparations there were marked differences in the fast phases of the fluorescence increase in resistant vs. susceptible chloroplasts; these data are interpreted as showing that the resistant plastids have an alteration in the rate of reoxidation of the primary photosystem II electron acceptor. Electrophoretic analysis of chloroplast membrane proteins of the two biotypes showed small changes in the electrophoretic mobilities of two polypeptide species. The data provide evidence for the following herbicide resistance mechanism: genetically controlled modification of the herbicide target site.

Plant-derived recombinant F1, V, and F1-V fusion antigens of Yersinia pestis activate human cells of the innate and adaptive immune system

Plague is still endemic in different regions of the world. Current vaccines raise concern for their side effects and limited protection, highlighting the need for an efficacious and rapidly producible vaccine. F1 and V antigens of Yersinia pestis, and F1-V fusion protein produced in Nicotiana benthamiana administered to guinea pigs resulted in immunity and protection against an aerosol challenge of virulent Y. pestis. We examined the effects of plant-derived F1, V, and F1-V on human cells of the innate immunity. F1, V, and F1-V proteins engaged TLR2 signalling and activated IL-6 and CXCL-8 production by monocytes, without affecting the expression of TNF-alpha, IL-12, IL-10, IL-1beta, and CXCL10. Native F1 antigen and recombinant plant-derived F1 (rF1) and rF1-V all induced similar specific T-cell responses, as shown by their recognition by T-cells from subjects who recovered from Y. pestis infection. Native F1 and rF1 were equally well recognized by serum antibodies of Y. pestis-primed donors, whereas serological reactivity to rF1-V hybrid was lower, and that to rV was virtually absent. In conclusion, plant-derived F1, V, and F1-V antigens are weakly reactogenic for human monocytes and elicit cell-mediated and humoral responses similar to those raised by Y. pestis infection.

Production of a fusion protein consisting of the enterotoxigenic Escherichia coli heat-labile toxin B subunit and a tuberculosis antigen in Arabidopsis thaliana

Transgenic plants are potentially safe and inexpensive vehicles to produce and mucosally deliver protective antigens. However, the application of this technology is limited by the poor response of the immune system to non-particulate, subunit vaccines. Co-delivery of therapeutic proteins with carrier proteins could increase the effectiveness of the antigen. This paper reports the ability of transgenic Arabidopsis thaliana plants to produce a fusion protein consisting of the B subunit of the Escherichia coli heat-labile enterotoxin and a 6 kDa tuberculosis antigen, the early secretory antigenic target ESAT-6. Both components of the fusion protein were detected using GM1-ganglioside-dependent enzyme-linked immunosorbant assay. This suggested the fusion protein retained both its native antigenicity and the ability to form pentamers.

Expression of the B subunit of Escherichia coli heat-labile enterotoxin as a fusion protein in transgenic tomato

Epitopes often require co-delivery with an adjuvant or targeting protein to enable recognition by the immune system. This paper reports the ability of transgenic tomato plants to express a fusion protein consisting of the B subunit of the Escherichia coli heat-labile enterotoxin (LTB) and an immunocontraceptive epitope. The fusion protein was found to assemble into pentamers, as evidenced by its ability to bind to gangliosides, and had an average expression level of 37.8 microg g(-1) in freeze-dried transgenic tissues. Processing of selected transgenic fruit resulted in a 16-fold increase in concentration of the antigen with minimal loss in detectable antigen. The species-specific nature of this epitope was shown by the inability of antibodies raised against non-target species to detect the LTB fusion protein. The immunocontraceptive ability of this vaccine will be tested in future pilot mice studies.

Expression of recombinant human acetylcholinesterase in transgenic tomato plants

Enzyme therapy for the prevention and treatment of organophosphate poisoning depends on the availability of large amounts of cholinesterases. Transgenic plants are being evaluated for their efficiency and cost-effectiveness as a system for the bioproduction of therapeutically valuable proteins. Here we report production of a recombinant isoform of human acetylcholinesterase in transgenic tomato plants. Active and stable acetylcholinesterase, which retains the kinetic characteristics of the human enzyme, accumulated in tomato plants. High levels of specific activity were registered in leaves (up to 25 nmol min(-1) mg protein(-1)) and fruits (up to 250 nmol min(-1) mg protein(-1)).

Avicins, a family of triterpenoid saponins from Acacia victoriae (Bentham), inhibit activation of nuclear factor-kappaB by inhibiting both its nuclear localization and ability to bind DNA

Triterpenoid saponins, which are present in leguminous plants and some marine animals, possess a broad range of biological actions. We have earlier reported the extraction of avicins, a family of triterpenoid saponins obtained from the Australian desert tree Acacia victoriae (Leguminosae: Mimosoideae) that inhibit tumor cell growth and induce apoptosis, in part, by perturbing mitochondrial function. These saponins have also been found to prevent chemical-induced carcinogenesis in mice. This study examines the effect of a triterpene mixture (F094) and a single molecular species (avicin G) isolated from the mixture on tumor necrosis factor (TNF)-induced activation of nuclear transcription factor-kappaB (NF-kappaB) in Jurkat cells (human T cell leukemia). Both F094 and avicin G were found to be potent inhibitors of TNF-induced NF-kappaB. Treatment of Jurkat cells with avicin G resulted in a much slower accumulation of the p65 subunit of NF-kappaB into the nucleus whereas the degradation of IkappaBalpha was unaffected. Avicin G also impaired the binding of NF-kappaB to DNA in in vitro binding assays. Treatment of cells with DTT totally reversed the avicin G-induced inhibition of NF-kappaB activity, suggesting that sulfhydryl groups critical for NF-kappaB activation were being affected. Avicin G treatment resulted in decreased expression of NF-kappaB-regulated proteins such as inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2). Thus, the avicins may prove important for reducing both oxidative and nitrosative cellular stress and thereby suppressing the development of malignancies and related diseases.

Avicins, a family of triterpenoid saponins from Acacia victoriae (Bentham), suppress H-ras mutations and aneuploidy in a murine skin carcinogenesis model

We tested the ability of avicins, a family of triterpenoid saponins obtained from Acacia victoriae (Bentham) (Leguminosae: Mimosoideae), to inhibit chemically induced mouse skin carcinogenesis. Varying doses of avicins were applied to shaved dorsal skin of SENCAR mice 15 min before application of 100 nmol of 7,12-dimethylbenz[a]anthracene (DMBA) twice a week for 4 weeks (complete carcinogenesis model). The dorsal skin of a second group of mice was treated with one dose of 10 nmol of DMBA. Avicins were then applied 15 min before repetitive doses of 2 microg of phorbol 12-tetradecanoate 13-acetate (TPA) twice a week for 8 weeks (initiation/promotion model). At 12 weeks, avicins produced a 70% decrease in the number of mice with papillomas and a greater than 90% reduction in the number of papillomas per mouse in both protocols. We also observed a 62% and 74% reduction by avicins in H-ras mutations at codon 61 in the DMBA and DMBA/TPA models, respectively, as well as a significant inhibition of the modified DNA base formation (8-OH-dG) in both protocols. Marked suppression of aneuploidy occurred with treatment at 16 weeks in the initiation/promotion experiment. These findings, when combined with the proapoptotic property of these compounds and their ability to inhibit hydrogen peroxide (H(2)O(2)) generation, nuclear factor-kappaB (NF-kappaB) activation, and inducible nitric oxide synthase (iNOS) induction reported elsewhere, suggest that avicins could prove exciting in reducing oxidative and nitrosative stress and thereby suppressing the development of human skin cancer and other epithelial malignancies.

Oral immunization with hepatitis B surface antigen expressed in transgenic plants

Oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) derived from yeast (purified product) or in transgenic potatoes (uncooked unprocessed sample) was compared. An oral adjuvant, cholera toxin, was used to increase immune responses. Transgenic plant material containing HBsAg was the superior means of both inducing a primary immune response and priming the mice to respond to a subsequent parenteral injection of HBsAg. Electron microscopy of transgenic plant samples revealed evidence that the HBsAg accumulated intracellularly; we conclude that natural bioencapsulation of the antigen may provide protection from degradation in the digestive tract until plant cell degradation occurs near an immune effector site in the gut. The correlate of protection from hepatitis B virus infection is serum antibody titers induced by vaccination; the protective level in humans is 10 milliunits/ml or greater. Mice fed HBsAg-transgenic potatoes produced HBsAg-specific serum antibodies that exceeded the protective level and, on parenteral boosting, generated a strong long-lasting secondary antibody response. We have also shown the effectiveness of oral delivery by using a parenteral prime-oral boost immunization schedule. The demonstrated success of oral immunization for hepatitis B virus with an "edible vaccine" provides a strategy for contributing a means to achieve global immunization for hepatitis B prevention and eradication.

Avicins: triterpenoid saponins from Acacia victoriae (Bentham) induce apoptosis by mitochondrial perturbation

Anticancer agents target various subcellular components and trigger apoptosis in chemosensitive cells. We have recently reported the tumor cell growth inhibitory properties of a mixture of triterpenoid saponins obtained from an Australian desert tree (Leguminosae) Acacia victoriae (Bentham). Here we report the purification of this mixture into two biologically pure components called avicins that contain an acacic acid core with two acyclic monoterpene units connected by a quinovose sugar. We demonstrate that the mixture of triterpenoid saponins and avicins induce apoptosis in the Jurkat human T cell line by affecting the mitochondrial function. Avicin G induced cytochrome c release within 30-120 min in whole cells and within a minute in the cell-free system. Caspase inhibitors DEVD or zVAD-fmk had no effect on cytochrome c release, suggesting the direct action of avicin G on the mitochondria. Activation of caspase-3 and total cleavage of poly(ADP-ribose) polymerase (PARP) occurred between 2 and 6 h posttreatment with avicins by zVAD-fmk. Interestingly, in the treated cells no significant changes in the membrane potential preceded or accompanied cytochrome c release. A small decrease in the generation of reactive oxygen species (ROS) was measured. The study of these evolutionarily ancient compounds may represent an interesting paradigm for the application of chemical ecology and chemical biology to human health.

Agrobacterium -mediated transformation of embryogenic cell suspensions of the banana cultivar Rasthali (AAB)

 A protocol was developed for establishing embryogenic suspension cultures from in vitro-grown, thin shoot-tip sections of the banana cultivar Rasthali. The best medium for callus induction was an MS-based medium supplemented with 2 mg/l 2,4-D and 0.2 mg/l zeatin. The callus was transferred to liquid medium to establish embryogenic cell suspensions. These cultures were subsequently used for Agrobacterium-mediated transformation. The Agrobacterium tumefaciens strain EHA105 containing the binary vector pVGSUN with the als gene as a selectable marker and an intron-containing the gusA gene as a reporter gene was used for transformations. The herbicide Glean was used as a selection agent. Two hundred putative transformants were recovered, of which a set of 16 was tested by histochemical analysis for GUS expression and by Southern blot analysis with a probe for the gusA gene. The plants were positive for GUS expression and integration of the gusA gene. Two of the transformants were grown to maturity under greenhouse conditions. Bananas were harvested to test GUS expression by histochemical analysis. The fruit from both transgenics tested positive for GUS expression.

Production of hepatitis B surface antigen in transgenic plants for oral immunization

Here we present data showing oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) in preclinical animal trials. Mice fed transgenic HBsAg potato tubers showed a primary immune response (increases in HBsAg-specific serum antibody) that could be greatly boosted by intraperitoneal delivery of a single subimmunogenic dose of commercial HBsAg vaccine, indicating that plants expressing HBsAg in edible tissues may be a new means for oral hepatitis B immunization. However, attainment of such a goal will require higher HBsAg expression than was observed for the potatoes used in this study. We conducted a systematic analysis of factors influencing the accumulation of HBsAg in transgenic potato, including 5' and 3' flanking elements and protein targeting within plant cells. The most striking improvements resulted from (1) alternative polyadenylation signals, and (2) fusion proteins containing targeting signals designed to enhance integration or retention of HBsAg in the endoplasmic reticulum (ER) of plant cells.

Human immune responses to a novel norwalk virus vaccine delivered in transgenic potatoes

A new approach for delivering vaccine antigens is the use of inexpensive, plentiful, plant-based oral vaccines. Norwalk virus capsid protein (NVCP), assembled into virus-like particles, was used as a test antigen, to determine whether immune responses could be generated in volunteers who ingested transgenic potatoes. Twenty-four healthy adult volunteers received 2 or 3 doses of transgenic potato (n=20) or 3 doses of wild-type potato (n=4). Each dose consisted of 150 g of raw, peeled, diced potato that contained 215-751 microgram of NVCP. Nineteen (95%) of 20 volunteers who ingested transgenic potatoes developed significant increases in the numbers of specific IgA antibody-secreting cells. Four (20%) of 20 volunteers developed specific serum IgG, and 6 (30%) of 20 volunteers developed specific stool IgA. Overall, 19 of 20 volunteers developed an immune response of some kind, although the level of serum antibody increases was modest.

Plants for delivery of edible vaccines

Over the past decade, scientific advances in molecular biology and immunology have improved understanding of many diseases and led to the development of novel strategies for vaccination. The development of plants expressing vaccine antigens is a particularly promising approach. Plant-derived antigenic proteins have delayed or prevented the onset of disease in animals and have proven to be safe and functional in human clinical trials. Future areas of research should further characterize the induction of the mucosal immune system and appropriate crop species for delivery of animal and human vaccines.

A tool for functional plant genomics: chimeric RNA/DNA oligonucleotides cause in vivo gene-specific mutations

Self-complementary chimeric oligonucleotides (COs) composed of DNA and modified RNA residues were evaluated as a means to (i) create stable, site-specific base substitutions in a nuclear gene and (ii) introduce a frameshift in a nuclear transgene in plant cells. To demonstrate the creation of allele-specific mutations in a member of a gene family, COs were designed to target the codon for Pro-196 of SuRA, a tobacco acetolactate synthase (ALS) gene. An amino acid substitution at Pro-196 of ALS confers a herbicide-resistance phenotype that can be used as a selectable marker in plant cells. COs were designed to contain a 25-nt homology domain comprised of a five-deoxyribonucleotide region (harboring a single base mismatch to the native ALS sequence) flanked by regions each composed of 10 ribonucleotides. After recovery of herbicide-resistant tobacco cells on selective medium, DNA sequence analyses identified base conversions in the ALS gene at the codon for Pro-196. To demonstrate a site-specific insertion of a single base into a targeted gene, COs were used to restore expression of an inactive green fluorescent protein transgene that had been designed to contain a single base deletion. Recovery of fluorescent cells confirmed the deletion correction. Our results demonstrate the application of a technology to modify individual genetic loci by catalyzing either a base substitution or a base addition to specific nuclear genes; this approach should have great utility in the area of plant functional genomics.

Edible vaccine protects mice against Escherichia coli heat-labile enterotoxin (LT): potatoes expressing a synthetic LT-B gene

The authors have designed and constructed a plant-optimize synthetic gene encoding the Escherichia coli heat-labile enterotoxin B subunit (LT-B), for use in transgenic plants as an edible vaccine against enterotoxigenic E. coli. Expression of the synthetic LT-B gene in potato plants under the control of a constitutive promoter yielded increased accumulation of LT-B in leaves and tubers, as compared to the bacterial LT-B gene. The plant-derived LT-B assembled into native pentameric structures as evidenced by its ability to bind ganglioside. The authors demonstrated immunogenicity by feeding mice the raw tubers and comparing the anti-LT-B serum IgG and faecal IgA to that produced in mice gavaged with bacterial LT-B. Mice were fed three weekly doses of 5 g tuber tissue containing either 20 or 50 micrograms LT-B, or gavaged weekly with 5 micrograms of LT-B from recombinant E. coli. One week after the third dose, mice immunized with potato LT-B had higher levels of serum and mucosal anti-LT-B than those gavaged with bacterial LT-B. Mice were challenged by oral administration of 25 micrograms LT, and protection assessed by comparing the gut/carcass mass ratios. Although none of the mice were completely protected, the higher dose potato vaccine compared favourably with the bacterial vaccine. These findings show that an edible vaccine against E. coli LT-B is feasible.

Immunogenicity in humans of a recombinant bacterial antigen delivered in a transgenic potato

Compared with vaccine delivery by injection, oral vaccines offer the hope of more convenient immunization strategies and a more practical means of implementing universal vaccination programs throughout the world. Oral vaccines act by stimulating the immune system at effector sites (lymphoid tissue) located in the gut. Genetic engineering has been used with variable success to design living and non-living systems as a means to deliver antigens to these sites and to stimulate a desired immune response. More recently, plant biotechnology techniques have been used to create plants which contain a gene derived from a human pathogen; the resultant plant tissues will accumulate an antigenic protein encoded by the foreign DNA. In pre-clinical trials, we found that antigenic proteins produced in transgenic plants retained immunogenic properties when purified; if injected into mice the antigen caused production of protein-specific antibodies. Moreover, in some experiments, if the plant tissues were simply fed to mice, a mucosal immune response occurred. The present study was conducted as a proof of principle to determine if humans would also develop a serum and/or mucosal immune response to an antigen delivered in an uncooked foodstuff.

The abundant 31-kilodalton banana pulp protein is homologous to class-III acidic chitinases

We have identified and characterized the abundant protein from the pulp of banana fruit (Musa acuminata cv. Grand Nain), and have isolated a cDNA clone encoding this protein. Comparison of the amino terminal sequence of the purified 31 kDa protein (P31) suggests that it is related to plant chitinases. Western analyses utilizing rabbit anti-P31 antiserum demonstrate that this protein is pulp-specific in banana. A full-length cDNA clone homologous to class III acidic chitinase genes has been isolated from a pulp cDNA library by differential screening. The identity of this clone as encoding P31 was verified by comparisons between the amino-terminal peptide sequence and the cDNA sequence and cross-hybridization of the translation product of the cDNA clone with P31 antiserum. Northern and western blot analyses of RNA and protein isolated from banana pulp at different stages of ripening indicate that the cDNA and protein are expressed at high levels in the pulp of unripe fruit, and that their abundance decreases as the fruit ripens. Based on its expression pattern and deduced amino acid sequence and composition, we hypothesize that the physiological role of P31 is not for plant protection, but as a storage protein in banana pulp.

Expression of Norwalk virus capsid protein in transgenic tobacco and potato and its oral immunogenicity in mice

Alternatives to cell culture systems for production of recombinant proteins could make very safe vaccines at a lower cost. We have used genetically engineered plants for expression of candidate vaccine antigens with the goal of using the edible plant organs for economical delivery of oral vaccines. Transgenic tobacco and potato plants were created that express the capsid protein of Norwalk virus, a calicivirus that causes epidemic acute gastroenteritis in humans. The capsid protein could be extracted from tobacco leaves in the form of 38-nm Norwalk virus-like particles. Recombinant Norwalk virus-like particle (rNV) was previously recovered when the same gene was expressed in recombinant baculovirus-infected insect cells. The capsid protein expressed in tobacco leaves and potato tubers cosedimented in sucrose gradients with insect cell-derived rNV and appeared identical to insect cell-derived rNV on immunoblots of SDS/polyacrylamide gels. The plant-expressed rNV was orally immunogenic in mice. Extracts of tobacco leaf expressing rNV were given to CD1 mice by gavage, and the treated mice developed both serum IgG and secretory IgA specific for rNV. Furthermore, when potato tubers expressing rNV were fed directly to mice, they developed serum IgG specific for rNV. These results indicate the potential usefulness of plants for production and delivery of edible vaccines. This is an appropriate technology for developing countries where vaccines are urgently needed.

Transgenic Plants Created for Oral Immunization Against Diarrheal Diseases

We have created transgenic plants that express subunit antigens of infectious bacteria and viruses. These proteins have been isolated and characterized to demonstrate that they retain immunogenic properties. When raw potatoes containing the recombinant immunogens were fed to mice, the animals were orally immunized. Research completed to date has demonstrated the feasibility of using a genetically-engineered food as an inexpensive oral vaccine production and delivery system for diarrheal disease. The system could be a convenient means to deliver vaccines to travelers. It also has great potential as an appropriate technology for producing vaccines "in country" in the developing world. This review focuses on work from our group on recombinant subunit vaccines in potato tubers against enterotoxigenic Escherichia coli and Norwalk virus.

Transgenic plants as vaccine production systems

Transgenic plants that express foreign proteins with industrial or pharmaceutical value represent an economical alternative to fermentation-based production systems. Specific vaccines have been produced in plants as a result of the transient or stable expression of foreign genes. It has recently been shown that genes encoding antigens of bacterial and viral pathogens can be expressed in plants in a form in which they retain native immunogenic properties. Transgenic potato tubers expressing a bacterial antigen stimulated humoral and mucosal immune responses when they were provided as food. These results provide 'proof of concept' for the use of plants as a vehicle to produce vaccines.

Oral immunization with a recombinant bacterial antigen produced in transgenic plants

The binding subunit of Escherichia coli heat-labile enterotoxin (LT-B) is a highly active oral immunogen. Transgenic tobacco and potato plants were made with the use of genes encoding LT-B or an LT-B fusion protein with a microsomal retention sequence. The plants expressed the foreign peptides, both of which formed oligomers that bound the natural ligand. Mice immunized by gavage produced serum and gut mucosal anti-LT-B immunoglobulins that neutralized the enterotoxin in cell protection assays. Feeding mice fresh transgenic potato tubers also caused oral immunization.

Immunophysiology of the gut: a research frontier for integrative studies of the common mucosal immune system

This review highlights work that, within the past decade, transformed mucosal immunophysiology from a hypothetical concept to a fully recognized interdiscipline. The regulation of epithelial and smooth muscle functions by the mucosal immune system represents an exquisitely sensitive adaptation to local antigenic challenge. Furthermore, immunologic cells communicate with nerves via paracrine secretions to rapidly transduce antigenic signals into panmucosal changes in function. These local immunocyte-nerve interactions are modulated by the autonomic and central nervous systems. Because of the common mucosal immune system, antigen-induced changes similar to those occurring in the intestine and colon are predicted to occur in mucosa of all hollow organs. The drawing together of fields as diverse as medicine and agriculture underscores the scope of areas encompassed by immunophysiology. Newly acquired knowledge has positioned the field to advance rapidly in both basic and applied directions. Forces that will remodel the field in the next decade will be derived from public concerns about human health maintenance and the explosive and novel use of new research tools stemming from molecular biology. These forces will draw on and advance our knowledge in areas as diverse as vaccine development and prevention of allergic reactions to foods, bioengineered foods in particular.

Expression of hepatitis B surface antigen in transgenic plants

Tobacco plants were genetically transformed with the gene encoding hepatitis B surface antigen (HBsAg) linked to a nominally constitutive promoter. Enzyme-linked immunoassays using a monoclonal antibody directed against human serum-derived HBsAg revealed the presence of HBsAg in extracts of transformed leaves at levels that correlated with mRNA abundance. This suggests that there were no major inherent limitations of transcription or translation of this foreign gene in plants. Recombinant HBsAg was purified from transgenic plants by immunoaffinity chromatography and examined by electron microscopy. Spherical particles with an average diameter of 22 nm were observed in negatively stained preparations. Sedimentation of transgenic plant extracts in sucrose and cesium chloride density gradients showed that the recombinant HBsAg and human serum-derived HBsAg had similar physical properties. Because the HBsAg produced in transgenic plants is antigenically and physically similar to the HBsAg particles derived from human serum and recombinant yeast, which are used as vaccines, we conclude that transgenic plants hold promise as low-cost vaccine production systems.

Deletion Mutagenesis of the Cytochrome b559 Protein Inactivates the Reaction Center of Photosystem II

In green plant-like photosynthesis, oxygen evolution is catalyzed by a thylakoid membrane-bound protein complex, photosystem II. Cytochrome b559, a protein component of the reaction center of this complex, is absent in a genetically engineered mutant of the cyanobacterium, Synechocystis 6803 [Pakrasi, H.B., Williams, J.G.K., and Arntzen, C.J. (1988). EMBO J. 7, 325-332]. In this mutant, the genes psbE and psbF, encoding cytochrome b559, were deleted by targeted mutagenesis. Two other protein components, D1 and D2 of the photosystem II reaction center, are also absent in this mutant. However, two chlorophyll-binding proteins, CP47 and CP43, as well as a manganese-stabilizing extrinsic protein component of photosystem II are stably assembled in the thylakoids of this mutant. Thus, this deletion mutation destabilizes the reaction center of photosystem II only. The mutant also lacks a fluorescence maximum peak at 695 nm (at 77 K) even though the CP47 protein, considered to be the origin of this fluorescence peak, is present in this mutant. We propose that the fluorescence at 695 nm originates from an interaction between the reaction center of photosystem II and CP47. The deletion mutant shows the absence of variable fluorescence at room temperature, indicating that its photosystem II complex is photochemically inactive. Also, photoreduction of QA, the primary acceptor quinone in photosystem II, could not be detected in the mutant. We conclude that cytochrome b559 plays at least an essential structural role in the reaction center of photosystem II.

Deletion Mutagenesis of the Cytochrome b559 Protein Inactivates the Reaction Center of Photosystem II

In green plant-like photosynthesis, oxygen evolution is catalyzed by a thylakoid membrane-bound protein complex, photosystem II. Cytochrome b559, a protein component of the reaction center of this complex, is absent in a genetically engineered mutant of the cyanobacterium, Synechocystis 6803 [Pakrasi, H.B., Williams, J.G.K., and Arntzen, C.J. (1988). EMBO J. 7, 325-332]. In this mutant, the genes psbE and psbF, encoding cytochrome b559, were deleted by targeted mutagenesis. Two other protein components, D1 and D2 of the photosystem II reaction center, are also absent in this mutant. However, two chlorophyll-binding proteins, CP47 and CP43, as well as a manganese-stabilizing extrinsic protein component of photosystem II are stably assembled in the thylakoids of this mutant. Thus, this deletion mutation destabilizes the reaction center of photosystem II only. The mutant also lacks a fluorescence maximum peak at 695 nm (at 77 K) even though the CP47 protein, considered to be the origin of this fluorescence peak, is present in this mutant. We propose that the fluorescence at 695 nm originates from an interaction between the reaction center of photosystem II and CP47. The deletion mutant shows the absence of variable fluorescence at room temperature, indicating that its photosystem II complex is photochemically inactive. Also, photoreduction of QA, the primary acceptor quinone in photosystem II, could not be detected in the mutant. We conclude that cytochrome b559 plays at least an essential structural role in the reaction center of photosystem II.

Targeted mutagenesis of the psbE and psbF genes blocks photosynthetic electron transport: evidence for a functional role of cytochrome b559 in photosystem II

The genes encoding the two subunits (alpha and beta) of the cytochrome b559 (cyt b559) protein, psbE and psbF, were cloned from the unicellular, transformable cyanobacterium, Synechocystis 6803. Cyt b559, an intrinsic membrane protein, is a component of photosystem II, a membrane-protein complex that catalyzes photosynthetic oxygen evolution. However, the role of cyt b559 in photosynthetic electron transport is yet to be determined. A high degree of homology was found between the cyanobacterial and green plant chloroplastidic psbE and psbE genes and in the amino acid sequences of their corresponding protein products. Cartridge mutagenesis techniques were used to generate a deletion mutant of Synechocystis 6803 in which the psbE and psbF genes were replaced by a kanamycin-resistance gene cartridge. Physiological analyses indicated that the PSII complexes of the mutant were inactivated. We conclude that cyt b559 is an essential component of PSII.

Sequencing and modification of psbB, the gene encoding the CP-47 protein of Photosystem II, in the cyanobacterium Synechocystis 6803

The Photosystem II protein CP-47 has been hypothesized to be involved in binding the reaction center chlorophyll. The psbB gene, encoding this protein, was cloned from the genome of the cyanobacterium Synechocystis 6803, and sequenced. The DNA sequence is 68% homologous with that of the psbB gene from spinach, whereas the predicted amino acid sequence is 76% homologous. The hydropathy patterns of Synechocystis and spinach CP-47 are almost indistinguishable, indicating the same general CP-47 folding pattern in the thylakoid membrane in the two species. There are five pairs of histidine residues in CP-47 that are spaced by 13 or 14 amino acids and that are located in hydrophobic regions of the protein; these histidine residues may be involved in chlorophyll binding. Interruption of the psbB gene by a DNA fragment carrying a gene conferring kanamycin resistance results in a loss of Photosystem II activity. This indicates that an intact CP-47 is required for a functional Photosystem II complex, but does not necessarily indicate that this protein would house the reaction center.

Evidence for two-step processing of nuclear-encoded chloroplast proteins during membrane assembly

A plastome (chloroplast genome) mutant of tobacco, lutescens-1, displays abnormal degradation of the chloroplast-encoded polypeptides which form the core complex of photosystem II (PSII). Two nuclear-encoded proteins (present in polymorphic forms), which normally function in the water oxidation process of PSII, accumulate as larger size-class polypeptides in mutant thylakoid membranes. These accumulated proteins are intermediate in size between the full-length primary protein synthesized in the cytoplasm and the proteolytically processed mature polypeptides. Trypsin treatment of unstacked mutant thylakoids and of inside-out vesicle (PSII-enriched) preparations indicated that the intermediate size forms were correctly localized on the inner surface of the thylakoid membrane, but not surface-exposed in the same way as the mature proteins. Only one of the intermediate size-class proteins could be extracted by salt washes. We interpret these data to be consistent with the idea that the two imported proteins that function in the water oxidation step of photosynthesis and are localized in the loculus (the space within the thylakoid vesicles) undergo two-step processing. The second step in proteolytic processing may be related to transport through a second membrane (the first transport step through the chloroplast envelope having been completed); this step may be arrested in the mutant due to the absence of the PSII core complex.

Monoclonal antibodies to the light-harvesting chlorophyll a/b protein complex of photosystem II

A collection of 17 monoclonal antibodies elicited against the light-harvesting chlorophyll a/b protein complex which serves photosystem II (LHC-II) of Pisum sativum shows six classes of binding specificity. Antibodies of two of the classes recognize a single polypeptide (the 28- or the 26- kD polypeptides), thereby suggesting that the two proteins are not derived from a common precursor. Other classes of antibodies cross-react with several polypeptides of LHC-II or with polypeptides of both LHC-II and the light-harvesting chlorophyll a/b polypeptides of photosystem I (LHC-I), indicating that there are structural similarities among the polypeptides of LHC-II and LHC-I. The evidence for protein processing by which the 26-, 25.5-, and 24.5-kD polypeptides are derived from a common precursor polypeptide is discussed. Binding studies using antibodies specific for individual LHC-II polypeptides were used to quantify the number of antigenic polypeptides in the thylakoid membrane. 27 copies of the 26-kD polypeptide and two copies of the 28-kD polypeptide were found per 400 chlorophylls. In the chlorina f2 mutant of barley, and in intermittent light-treated barley seedlings, the amount of the 26-kD polypeptide in the thylakoid membranes was greatly reduced, while the amount of 28-kD polypeptide was apparently not affected. We propose that stable insertion and assembly of the 28-kD polypeptide, unlike the 26-kD polypeptide, is not regulated by the presence of chlorophyll b.

Developmental Loss of Photosystem II Activity and Structure in a Chloroplast-Encoded Tobacco Mutant, Lutescens-1

Lutescens-1, a tobacco mutant with a maternally inherited dysfunction, displayed an unusual developmental phenotype. In vivo measurement of chlorophyll fluorescence revealed deterioration in photosystem II (PSII) function as leaves expanded. Analysis of thylakoid membrane proteins by polyacrylamide gel electrophoresis indicated the physical loss of nuclear- and chloroplast-encoded polypeptides comprising the PSII core complex concomitant with loss of activity. Freeze fracture electron micrographs of mutant thylakoids showed a reduced density, compared to wild type, of the EF(s) particles which have been shown previously to be the structural entity containing PSII core complexes and associated pigment-proteins. The selective loss of PSII cores from thylakoids resulted in a higher ratio of antenna chlorophyll to reaction centers and an altered 77 K chlorophyll fluorescence emission spectra; these data are interpreted to indicate functional isolation of light-harvesting chlorophyll a/b complexes in the absence of PSII centers. Examination of PSII reaction centers (which were present at lower levels in mutant membranes) by monitoring the light-dependent phosphorylation of PSII polypeptides and flash-induced O(2) evolution patterns demonstrated that the PSII cores which were assembled in mutant thylakoids were functionally identical to those of wild type. We conclude that the lutescens-1 mutation affected the correct stoichiometry of PSII centers, in relation to other membrane constituents, by disrupting the proper assembly and maintenance of PSII complexes in lutescens-1 thylakoid membranes.

Reconstitution of the Light Harvesting Chlorophyll a/b Pigment-Protein Complex into Developing Chloroplast Membranes Using a Dialyzable Detergent

Conditions were developed to isolate the light-harvesting chlorophyll-protein complex serving photosystem II (LHC-II) using a dialyzable detergent, octylpolyoxyethylene. This LHC-II was successfully reconstituted into partially developed chloroplast thylakoids of Hordeum vulgare var Morex (barley) seedlings which were deficient in LHC-II. Functional association of LHC-II with the photosystem II (PSII) core complex was measured by two independent functional assays of PSII sensitization by LHC-II. A 3-fold excess of reconstituted LHC-II was required to equal the activity of LHC developing in vivo. We suggest that a linker component may be absent in the partially developed membranes which is required for specific association of the PSII core complex and LHC-II.

Stability of chloroplastic triazine resistance in rutabaga backcross generations

Triazine resistance originally observed in a weed biotype of birdsrape (Brassica campestris L.) has been transferred through cytoplasmic substitution into rutabaga (Brassica napus ssp. Rapifera [Metzg.] Minsk.) by conventional backcrossing. Photosynthetic function and resistance to triazines were examined in six backcross generations of rutabaga as well as in the original parents. Chloroplast thylakoid membranes were isolated and their sensitivity to atrazine, metribuzin, and diuron assayed by measuring the inhibition of photoreduction of 1,6-dichlorophenol indophenol as well as the alteration of in vitro chlorophyll fluorescence rise characteristics. Both assay methods indicated that triazine resistance persisted in all rutabaga backcross generations, and that it involved triazine binding sites in chloroplasts. There was little resistance to diuron. In vivo chlorophyll fluorescence was also monitored, in the absence of herbicides, as an indicator of the electron transfer properties of the chloroplast photosystem II complex. The results indicated that electron transport from Q(A) to Q(B) was slower (as indicated by a larger intermediate level fluorescence during the transient rise) in the triazine resistant parents as well as in all the rutabaga backcross generations.

Analysis of Photosynthetic Antenna Function in a Mutant of Arabidopsis thaliana (L.) Lacking trans-Hexadecenoic Acid

Several lines of evidence support the proposal that the unusual chloroplast-specific lipid acyl group Delta3,trans-hexadecenoic acid (trans-C(16:1)) stimulates the formation or maintenance of the oligomeric form of the light-harvesting chlorophyll a/b complex (LHCP). To assess the functional significance of this apparent association we have analyzed LHCP structure and function in a mutant of Arabidopsis thaliana (L.) which lacks trans-C(16:1) by electrophoretic analysis of the protein-chlorophyll complexes and by measurements of chlorophyll fluorescence under a variety of conditions. By these criteria the putative oligomeric form of LHCP appears to be slightly more labile to detergent-mediated dissociation in the mutant. The oligomeric PSI chlorophyll-protein complex, associated with PSI, was also more labile to detergent-mediated dissociation in the mutant, suggesting a previously unsuspected association of trans-C(16:1) with the PSI complex. However, no significant effect of the mutation on the efficiency of energy transfer from LHCP to the photochemical reaction centers was observed under any of the various conditions imposed. Also, the stability of the chlorophyll-protein complexes to temperature-induced dissociation was unaffected in the mutant. The role of trans-C(16:1) is very subtle or is only conditionally expressed.

Paraquat resistance in conyza

A biotype of Conyza bonariensis (L.) Cronq. (identical to Conyza linefolia in other publications) originating in Egypt is resistant to the herbicide 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat). Penetration of the cuticle by [(14)C]paraquat was greater in the resistant biotype than the susceptible (wild) biotype; therefore, resistance was not due to differences in uptake. The resistant and susceptible biotypes were indistinguishable by measuring in vitro photosystem I partial reactions using paraquat, 6,7-dihydrodipyrido [1,2-alpha:2',1'-c] pyrazinediium ion (diquat), or 7,8-dihydro-6H-dipyrido [1,2-alpha:2',1'-c] [1,4] diazepinediium ion (triquat) as electron acceptors. Therefore, alteration at the electron acceptor level of photosystem I is not the basis for resistance. Chlorophyll fluorescence measured in vivo was quenched in the susceptible biotype by leaf treatment with the bipyridinium herbicides. Resistance to quenching of in vivo chlorophyll fluorescence was observed in the resistant biotype, indicating that the herbicide was excluded from the chloroplasts. Movement of [(14)C] paraquat was restricted in the resistant biotype when excised leaves were supplied [(14)C]paraquat through the petiole. We propose that the mechanism of resistance to paraquat is exclusion of paraquat from its site of action in the chloroplast by a rapid sequestration mechanism. No differential binding of paraquat to cell walls isolated from susceptible and resistant biotypes could be detected. The exact site and mechanism of paraquat binding to sequester the herbicide remains to be determined.

The origin of the long-wavelength fluorescence emission band (77 degrees K) from photosystem I

Isolated photosystem I (PSI)-110 particles, prepared using a minimal concentration of Triton X-100 [J. E. Mullet, J. J. Burke, and C. J. Arntzen (1980) Plant Physiol. 65, 814-822] and further subjected to short-term solubilization with sodium dodecyl sulfate (SDS), were resolved into four pigment-containing bands on polyacrylamide gel electrophoresis (PAGE). We have identified these in order of increasing electrophoretic mobility as being (a) CPIa, (b) CPI, (c) the light-harvesting complex of photosystem I (LHC-I), and (d) a free pigment-zone. LHC-I had an absorption maximum in the red at 668-669 nm and a shoulder at 650 nm, which was resolved by its first-derivative spectrum to indicate the presence of chlorophyll b. LHC-I exhibited a 77 degrees K fluorescence emission maximum at 729-730 nm. The 77 degrees K fluorescence emission maxima of CPIa and CPI, excised from the gel, were at 729 and 722 nm, respectively. The LHC-I band, excised from the gel and rerun on dissociating SDS-PAGE, was resolved into two polypeptide doublets of 24-22.5 and 21-20.5 kDa. The CPIa band under similar conditions was resolved into polypeptides of 68, 24, 22.5, 21, 20.5, 19, 15, and 14 kDa; on the contrary, CPI contained only the 68-kDa polypeptide. When intact thylakoids were subjected to "nondenaturing" SDS-PAGE, LHC-I comigrated with an oligomeric form (dimer) of the light-harvesting chlorophyll a/b pigment-protein that preferentially serves photosystem II (LHCP-II). When this combined LHC-I/LHCP-II pigment-protein band was prepared by SDS-PAGE from isolated stroma lamellae, it exhibited a long-wavelength fluorescence band near 730 nm at 77 degrees K. When a similar preparation was obtained from sucrose density gradients containing SDS [J. Argyroudi-Akoyunoglou and H. Thomou (1981) FEBS Lett. 135, 171-181], it was found to be enriched in a 21-kDa polypeptide. The data suggest that the 21-kDa polypeptide of LHC-I is the chlorophyll-containing polypeptide responsible for the long-wavelength fluorescence of LHC-I; other polypeptides in the complex (20.5, 22.5, and 24 kDa) presumably bind chlorophyll and also serve an antennae function.

Membrane protein damage and repair: removal and replacement of inactivated 32-kilodalton polypeptides in chloroplast membranes

Incubation of Chlamydomonas reinhardii cells at light levels that are several times more intense than those at which the cells were grown results in a loss of photosystem II function (termed photoinhibition). The loss of activity corresponded to the disappearance from the chloroplast membranes of a lysine-deficient, herbicide-binding protein of 32,000 daltons which is thought to be the apoprotein of the secondary quinone electron acceptor of photosystem II (the QB protein). In vivo recovery from the damage only occurred following de novo synthesis (replacement) of the chloroplast-encoded QB protein. We believe that the turnover of this protein is a normal consequence of its enzymatic function in vivo and is a physiological process that is necessary to maintain the photosynthetic integrity of the thylakoid membrane. Photoinhibition occurs when the rate of inactivation and subsequent removal exceeds the rate of resynthesis of the QB protein.

Characterization of chloroplast thylakoid polypeptides in the 32-kDa region: polypeptide extraction and protein phosphorylation affect binding of photosystem II-directed herbicides

In order to distinguish between two photosystem II proteins with apparent molecular weights of about 32 kDa, mild extraction procedures were used to remove several thylakoid membrane components. A 32-kDa protein that stained intensely with Coomassie brilliant blue could be extracted from the thylakoid membranes without removing the 32-kDa herbicide receptor protein, which stained poorly with Coomassie brilliant blue. The nonextracted protein was readily detectable after in vivo polypeptide labeling with [35S]methionine or after in vitro covalent tagging with [14C]azidoatrazine. The procedures used to extract the intensely stained, 32-kDa polypeptide resulted in changes in herbicide-binding characteristics, presumably due to conformational changes in the herbicide-binding environment. Alterations of membrane surface charge by protein phosphorylation also influenced herbicide binding.