Expression of the sweet protein brazzein in maize for production of a new commercial sweetener

Brazzein and Monellin: Chemical Analysis, Food Industry Applications, Safety and Quality Control, Nutritional Profile and Health Impacts

Recently, customers have been keener to buy products manufactured using all-natural ingredients with positive health properties, but without losing flavor. In this regard, the objective of the current study is to review the consumption of brazzein and monellin, their nutritional profiles and health effects, and their potential applications in the food industry. This poses challenges with sustainability and important quality and safety indicators, as well as the chemical processes used to determine them. To better understand the utilization of brazzein and monellin, the chemical analysis of these two natural sweet proteins was also reviewed by placing particular emphasis on their extraction methods, purification and structural characterization. Protein engineering is considered a means to improve the thermal stability of brazzein and monellin to enhance their application in food processing, especially where high temperatures are applied. When the quality and safety of these sweet proteins are well-investigated and the approval from safety authorities is secured, the market for brazzein and monellin as food ingredient substitutes for free sugar will be guaranteed in the future. Ultimately, the review on these two natural peptide sweeteners increases the body of knowledge on alleviating problems of obesity, diabetes and other non-communicable diseases.

Expression of a triple mutational des-pGlu brazzein in transgenic mouse milk

Brazzein has excellent potential for use as a sweetener because of its high level of sweetening potency and stability against extreme temperature and pH. It is extracted from the tropical and difficult—to‐cultivate African plant Pentadiplandra brazzeana, which hampers its commercial viability. Here we report the mammary‐specific expression of wildtype or triple mutational (H31R/E36D/E41A) des‐pGlu brazzeins in the milk of transgenic mice. Using enzyme‐linked immunoassay (ELISA), western blot, and sweetness intensity testing, we confirmed that the triple mutation made the des‐pGlu brazzein molecule 10,000 times sweeter than sucrose in a weight base, even after 10 min of incubation at 100 °C; in addition, the triple mutant was also significantly sweeter than the wildtype des‐pGlu brazzein. This study provides new insights for producing brazzein or brazzein‐sweetened milk from animals for use in food and healthcare applications.

Bioprospecting and biotechnological insights into sweet-tasting proteins by microbial hosts-a review

Owing to various undesirable health effects of sugar overconsumption, joint efforts are being made by industrial sectors and regulatory authorities to reduce sugar consumption practices, worldwide. Artificial sweeteners are considered potential substitutes in several products, e.g., sugar alcohols (polyols), high-fructose corn syrup, powdered drink mixes, and other beverages. Nevertheless, their long-standing health effects continue to be debatable. Consequently, growing interest has been shifted in producing non-caloric sweetenersfrom renewable resources to meet consumers' dietary requirements. Except for the lysozyme protein, various sweet proteins including thaumatin, mabinlin, brazzein, monellin, miraculin, pentadin, and curculin have been identified in tropical plants. Given the high cost and challenging extortion of natural resources, producing these sweet proteins using engineered microbial hosts, such as Yarrowia lipolytica, Pichia pastoris, Hansenula polymorpha, Candida boidinii, Arxula adeninivorans, Pichia methanolica, Saccharomyces cerevisiae, and Kluyveromyces lactis represents an appealing choice. Engineering techniques can be applied for large-scale biosynthesis of proteins, which can be used in biopharmaceutical, food, diagnostic, and medicine industries. Nevertheless, extensive work needs to be undertaken to address technical challenges in microbial production of sweet-tasting proteins in bulk. This review spotlights historical aspects, physicochemical properties (taste, safety, stability, solubility, and cost), and recombinant biosynthesis of sweet proteins. Moreover, future opportunities for process improvement based on metabolic engineering strategies are also discussed.

Efficient brazzein production in yeast (Kluyveromyces lactis) using a chemically defined medium

The sweet-tasting protein brazzein offers considerable potential as a functional sweetener with antioxidant, anti-inflammatory, and anti-allergic properties. Here, we optimized a chemically defined medium to produce secretory recombinant brazzein in Kluyveromyces lactis, with applications in mass production. Compositions of defined media were investigated for two phases of fermentation: the first phase for cell growth, and the second for maximum brazzein secretory production. Secretory brazzein expressed in the optimized defined medium exhibited higher purity than in the complex medium; purification was by ultrafiltration using a molecular weight cutoff, yielding approximately 107 mg L^-1. Moreover, the total media cost in this defined medium system was approximately 11% of that in the optimized complex medium to generate equal amounts of brazzein. Therefore, the K. lactis expression system is useful for mass-producing recombinant brazzein with high purity and yield at low production cost and indicates a promising potential for applications in the food industry.

Comparison of Different Signal Peptides for the Efficient Secretion of the Sweet-Tasting Plant Protein Brazzein in Pichia pastoris

Brazzein is a small sweet-tasting protein found in the red berries of a West African evergreen shrub, Pentadiplandra brazzeana Baillon. Brazzein is highly soluble and stable over a large pH range and at high temperatures, which are characteristics that suggest its use as a natural sweetener. However, Pentadiplandra brazzeana culture is difficult at a large scale, limiting the natural source of brazzein. Heterologous expression of brazzein has been established in numerous systems, including bacteria, yeast, and transgenic plants. Brazzein requires four disulfide bonds to be active in eliciting an intense sweet taste, and the yeast Pichia pastoris appears to be one of the best options for obtaining functional brazzein in high quantities. Employing yeast secretion in the culture medium allows us to obtain fully active brazzein and facilitate purification later. To increase yeast secretion, we compared seven different signal peptides to successfully achieve brazzein secretion using the yeast P. pastoris. The brazzein proteins corresponding to these signal peptides elicited activation of the sweet taste receptor functionally expressed in a cellular assay. Among these tested signal peptides, three resulted in the secretion of brazzein at high levels.

Expression of Brazzein, a Small Sweet-Tasting Protein in Saccharomyces cerevisiae: An Introduction for Production of Sweet Yeasts

BACKGROUND

The replacement of carbohydrate sweeteners with protein sweeteners from plants has attracted the interest of researchers because these proteins don't trigger the insulin response and are more nutritive for consumption in food. Brazzein (Braz) is a small and heat- stable sweet protein that has been originally derived from African plant Pentadiplandra brazzeana. In the present work the solubility, sweetness and yield of recombinant forms of Braz in two expression hosts, E. coli and S. cerevisiae were comprised.

METHODS

The codon-optimized gene of Braz was cloned in expression vectors pET28a and pET41a and GPD. The resulted vectors pET28a-Braz and pEt41a-Braz were transformed into Escherichia coli strain Rosetta (DE3) and the vector GPD-Braz was transformd to S. cerevisiae. The expression of Braz in different systems was analyzed by SDS-PAGE and western blotting.

RESULTS

The results verified the heterologous expression of Braz in S. cerevisiae carrying GPDBraz. Also the expression of Braz as carboxy-terminal extensions of His-tag and Glutathione-STransferase (GST) were verified in transgenic E. coli containing pET28a-Braz and pET41a-Braz, respectively.

CONCLUSION

Although the yield of GST-Braz was higher than His-Braz and Braz expressed in S. cerevisiae, but the higher solubility, sweetness, safety (GRAS) are important advantages of the use of S. cerevisiae as expression host for production of Braz. Therefore the result of present work opens new insights for providing the new sweet yeasts that can be used as food additives.

Sugar reduction in bakery products: Current strategies and sourdough technology as a potential novel approach

The world is facing a big problem of non-communicable diseases, such as obesity, cardiovascular disease and diabetes. An excessive sugar consumption is considered as a main factor, which triggers these diseases. The two main sources of sugar in processed products on the market are sugar-sweetened beverages and sweet bakery products. Sugar reduction is challenging, especially in baked goods, since it interacts significantly with all ingredients. These interactions cause an increase in gelatinization temperature, a delay in gluten network development, an increase or decrease in yeast activity depending on the sugar concentration, as well as an enhancement of emulsification. Reflecting the molecular interactions on the product quality characteristics of different types of baked goods, sugar also contributes to browning reactions and extension of microbial shelf life. During cake preparation, sugar supports the batter aeration which results in the typical soft cake crumb. Furthermore, it contributes to the spreading process of biscuits during baking and enhances surface cracking due to recrystallization. Sugar reduction requires the development of different strategies; Two well-known strategies are the replacement of added sugar by the combination of bulking agents and high-intensive sweeteners, or by sweet bulking ingredients, such as polyols. The in-situ production of polyols to enhance sweetness, and exopolysaccharides to improve texture, in a sourdough system shows high potential as sugar replacement. Lactobacillus sanfranciscensis, Leuconostoc mesenteroides and Leuconostoc citreum are high mannitol producing lactic acid bacteria (LAB) strains with yields of 70-98% and Leuconostoc oenos was found to produce erythritol. Furthermore, the yeast strain Candida milleri isolated from sourdough produces xylitol in the presence of xylose. Exopolysaccharides produced by LAB and/or yeasts are known to improve the texture and structure of bakery products and, thus, have high potential as natural functional ingredients to compensate quality loss in sweet bakery goods.

Optimized production and quantification of the tryptophan-deficient sweet-tasting protein brazzein in Kluyveromyces lactis

The sweet-tasting protein brazzein is a candidate sugar substitute owing to its sweet, sugar-like taste and good stability. To commercialize brazzein as a sweetener, optimization of fermentation and purification procedure is necessary. Here, we report the expression conditions of brazzein in the yeast Kluyveromices lactis and purification method for maximum yield. Transformed K. lactis was cultured in YPGlu (pH 7.0) at 25 °C and induced by adding glucose:galactose at a weight ratio of 1:2 (%/%) during the stationary phase, which increased brazzein expression 2.5 fold compared to the previous conditions. Cultures were subjected to heat treatment at 80 °C for 1 h, and brazzein containing supernatant was purified using carboxymethyl-sepharose cation exchange chromatography using 50 mM NaCl in 50 mM sodium acetate buffer (pH 4.0) as a wash buffer and 400 mM NaCl (pH 7.0) for elution. The yield of purified brazzein under these conditions was 2.0-fold higher than that from previous purification methods. We also determined that the NanoOrange assay was a suitable method for quantifying tryptophan-deficient brazzein. Thus, it is possible to obtain pure recombinant brazzein with high yield in K. lactis using our optimized expression, purification, and quantification protocols, which has potential applications in the food industry.

High-level production of single chain monellin mutants with enhanced sweetness and stability in tobacco chloroplasts

MAIN CONCLUSION

Plastid-based MNEI protein mutants retain the structure, stability and sweetness of their bacterial counterparts, confirming the attractiveness of the plastid transformation technology for high-yield production of recombinant proteins. The prevalence of obesity and diabetes has dramatically increased the industrial demand for the development and use of alternatives to sugar and traditional sweeteners. Sweet proteins, such as MNEI, a single chain derivative of monellin, are the most promising candidates for industrial applications. In this work, we describe the use of tobacco chloroplasts as a stable plant expression platform to produce three MNEI protein mutants with improved taste profile and stability. All plant-based proteins were correctly expressed in tobacco chloroplasts, purified and subjected to in-depth chemical and sensory analyses. Recombinant MNEI mutants showed a protein yield ranging from 5% to more than 50% of total soluble proteins, which, to date, represents the highest accumulation level of MNEI mutants in plants. Comparative analyses demonstrated the high similarity, in terms of structure, stability and function, of the proteins produced in plant chloroplasts and bacteria. The high yield and the extreme sweetness perceived for the plant-derived proteins prove that plastid transformation technology is a safe, stable and cost-effective production platform for low-calorie sweeteners, with an estimated production of up to 25-30 mg of pure protein/plant.

Natural product modulators of human sensations and mood: molecular mechanisms and therapeutic potential

Humans perceive physical information about the surrounding environment through their senses. This physical information is registered by a collection of highly evolved and finely tuned molecular sensory receptors. A multitude of bioactive, structurally diverse ligands have evolved in nature that bind these molecular receptors. The complex, dynamic interactions between the ligands and the receptors lead to changes in our sensory perception or mood. Here, we review our current knowledge of natural products and their derived analogues that interact specifically with human G protein-coupled receptors, ion channels, and nuclear hormone receptors to modulate the sensations of taste, smell, temperature, pain, and itch, as well as mood and its associated behaviour. We discuss the molecular and structural mechanisms underlying such interactions and highlight cases where subtle differences in natural product chemistry produce drastic changes in functional outcome. We also discuss cases where a single compound triggers complex sensory or behavioural changes in humans through multiple mechanistic targets. Finally, we comment on the therapeutic potential of the reviewed area of research and draw attention to recent technological developments in genomics, metabolomics, and metabolic engineering that allow us to tap the medicinal properties of natural product chemistry without taxing nature.

A Brief History of Promoter Development for Use in Transgenic Maize Applications

Promoters regulate gene expression, and are essential biotechnology tools. Since its introduction in the mid-1990s, biotechnology has greatly enhanced maize productivity primarily through the development of insect control and herbicide tolerance traits. Additional biotechnology applications include improving seed nutrient composition, industrial protein production, therapeutic production, disease resistance, abiotic stress resistance, and yield enhancement. Biotechnology has also greatly expanded basic research into important mechanisms that govern plant growth and reproduction. Many novel promoters have been developed to facilitate this work, but only a few are widely used. Transgene optimization includes a variety of strategies some of which effect promoter structure. Recent reviews examine the state of the art with respect to transgene design for biotechnology applications. This chapter examines the use of transgene technology in maize, focusing on the way promoters are selected and used. The impact of new developments in genomic technology on promoter structure is also discussed.

Improved Secretory Production of the Sweet-Tasting Protein, Brazzein, in Kluyveromyces lactis

Brazzein is an intensely sweet protein with high stability over a wide range of pH values and temperatures, due to its four disulfide bridges. Recombinant brazzein production through secretory expression in Kluyveromyces lactis is reported, but is inefficient due to incorrect disulfide formation, which is crucial for achieving the final protein structure and stability. Protein disulfide bond formation requires protein disulfide isomerase (PDI) and Ero1p. Here, we overexpressed KlPDI in K. lactis or treated the cells with dithiothreitol to overexpress KlERO1 and improve brazzein secretion. KlPDI and KlERO1 overexpression independently increased brazzein secretion in K. lactis by 1.7-2.2- and 1.3-1.6-fold, respectively. Simultaneous overexpression of KlPDI and KlERO1 accelerated des-pE1M-brazzein secretion by approximately 2.6-fold compared to the previous system. Moreover, intracellular misfolded/unfolded recombinant des-pE1M-brazzein was significantly decreased. In conclusion, increased KlPDI and KlERO1 expression favors brazzein secretion, suggesting that correct protein folding may be crucial to brazzein secretion in K. lactis.

Expression of plant sweet protein brazzein in the milk of transgenic mice

Sugar, the most popular sweetener, is essential in daily food. However, excessive sugar intake has been associated with several lifestyle-related diseases. Finding healthier and more economical alternatives to sugars and artificial sweeteners has received increasing attention to fulfill the growing demand. Brazzein, which comes from the pulp of the edible fruit of the African plant Pentadiplandra brazzeana Baill, is a protein that is 2,000 times sweeter than sucrose by weight. Here we report the production of transgenic mice that carry the optimized brazzein gene driven by the goat Beta-casein promoter, which specifically directs gene expression in the mammary glands. Using western blot analysis and immunohistochemistry, we confirmed that brazzein could be efficiently expressed in mammalian milk, while retaining its sweetness. This study presents the possibility of producing plant protein–sweetened milk from large animals such as cattle and goats.

Efficient production and characterization of the sweet-tasting brazzein secreted by the yeast Pichia pastoris

Brazzein is a small, heat-, and pH-stable sweet protein present in the fruits of the West African plant Pentadiplandra brazzeana Baillon. It exists in two forms differing in sweetness intensity. The major form, called pyrE-bra, contains a pyroglutamic acid at its N-terminus, while the minor form, called des-pyrE-bra, lacks this residue. Here we describe the heterologous expression in the methylotrophic yeast Pichia pastoris of two natural forms of brazzein, pyrE-bra and des-pyrE-bra, and an additional form, called Q1-bra, which is not naturally occurring in the fruit. Q1-bra differs from pyrE-bra in having a glutamine residue instead of pyrE at its N-terminus. Over an expression period of 6 days, we obtained approximately 90, 30, and 90 mg/L of purified recombinant pyrE-bra, Q1-bra, and des-pyrE-bra brazzein forms, respectively. Recombinant proteins were purified and submitted to mass spectrometry and (1)H NMR spectroscopy. The data indicate that the recombinant brazzein forms were properly folded. Moreover, they activated the human sweet receptor in vitro and evoked sweetness in vivo with properties similar to those of the two natural brazzein forms.

A BioBrick compatible strategy for genetic modification of plants

Background

Plant biotechnology can be leveraged to produce food, fuel, medicine, and materials. Standardized methods advocated by the synthetic biology community can accelerate the plant design cycle, ultimately making plant engineering more widely accessible to bioengineers who can contribute diverse creative input to the design process.

Results

This paper presents work done largely by undergraduate students participating in the 2010 International Genetically Engineered Machines (iGEM) competition. Described here is a framework for engineering the model plant Arabidopsis thaliana with standardized, BioBrick compatible vectors and parts available through the Registry of Standard Biological Parts (http://www.partsregistry.org). This system was used to engineer a proof-of-concept plant that exogenously expresses the taste-inverting protein miraculin.

Conclusions

Our work is intended to encourage future iGEM teams and other synthetic biologists to use plants as a genetic chassis. Our workflow simplifies the use of standardized parts in plant systems, allowing the construction and expression of heterologous genes in plants within the timeframe allotted for typical iGEM projects.

From miracle fruit to transgenic tomato: mass production of the taste-modifying protein miraculin in transgenic plants

The utility of plants as biofactories has progressed in recent years. Some recombinant plant-derived pharmaceutical products have already reached the marketplace. However, with the exception of drugs and vaccines, a strong effort has not yet been made to bring recombinant products to market, as cost-effectiveness is critically important for commercialization. Sweet-tasting proteins and taste-modifying proteins have a great deal of potential in industry as substitutes for sugars and as artificial sweeteners. The taste-modifying protein, miraculin, functions to change the perception of a sour taste to a sweet one. This taste-modifying function can potentially be used not only as a low-calorie sweetener but also as a new seasoning that could be the basis of a new dietary lifestyle. However, miraculin is far from inexpensive, and its potential as a marketable product has not yet been fully developed. For the last several years, biotechnological production of this taste-modifying protein has progressed extensively. In this review, the characteristics of miraculin and recent advances in its production using transgenic plants are summarized, focusing on such topics as the suitability of plant species as expression hosts, the cultivation method for transgenic plants, the method of purifying miraculin and future advances required to achieve industrial use.

Hydroxylation of recombinant human collagen type I alpha 1 in transgenic maize co-expressed with a recombinant human prolyl 4-hydroxylase

Background

Collagens require the hydroxylation of proline (Pro) residues in their triple-helical domain repeating sequence Xaa-Pro-Gly to function properly as a main structural component of the extracellular matrix in animals at physiologically relevant conditions. The regioselective proline hydroxylation is catalyzed by a specific prolyl 4-hydroxylase (P4H) as a posttranslational processing step.

Results

A recombinant human collagen type I α-1 (rCIα1) with high percentage of hydroxylated prolines (Hyp) was produced in transgenic maize seeds when co-expressed with both the α- and β- subunits of a recombinant human P4H (rP4H). Germ-specific expression of rCIα1 using maize globulin-1 gene promoter resulted in an average yield of 12 mg/kg seed for the full-length rCIα1 in seeds without co-expression of rP4H and 4 mg/kg seed for the rCIα1 (rCIα1-OH) in seeds with co-expression of rP4H. High-resolution mass spectrometry (HRMS) analysis revealed that nearly half of the collagenous repeating triplets in rCIα1 isolated from rP4H co-expressing maize line had the Pro residues changed to Hyp residues. The HRMS analysis determined the Hyp content of maize-derived rCIα1-OH as 18.11%, which is comparable to the Hyp level of yeast-derived rCIα1-OH (17.47%) and the native human CIa1 (14.59%), respectively. The increased Hyp percentage was correlated with a markedly enhanced thermal stability of maize-derived rCIα1-OH when compared to the non-hydroxylated rCIα1.

Conclusions

This work shows that maize has potential to produce adequately modified exogenous proteins with mammalian-like post-translational modifications that may be require for their use as pharmaceutical and industrial products.

Vaccine production in plant systems--an aid to the control of viral diseases in domestic animals: a review

Plants have been identified as promising expression systems for the commercial production of vaccines because of the possibility of introducing exogenous genes into them, which permits the development of a new generation of biological products called edible vaccines. The advantages of oral vaccines of this new type are that they induce mucosal, humoral, cellular and protective immunity, they are cheaper, easier to store, distribute and administer, they do not require cold chain management, and some species can be stored for long periods of time without any spoilage and may be administered as purified proteins. Owing to these benefits, plant-produced vaccines represent a valuable option for animal health. The aim of this paper is to present a review of plant-produced vaccines against viruses affecting domestic animals. Some aspects of the feasibility of their use and the immune response elicited by such vaccines are also discussed, as the balance between tolerance and immunogenicity is a major concern for the use of plant-based vaccines.

Purification and characterization of a transgenic corn grain-derived recombinant collagen type I alpha 1

Corn offers advantages as a transgenic host for producing recombinant proteins required at large volumes (1,000's of tons per year) and low cost (less than US$50/kg) by generating them as co-products of biorefining. We describe the purification and characterization of a corn grain-derived mammalian structural protein having such market characteristics: a full length recombinant collagen type I alpha 1 (rCI alpha 1) chain. Material properties of interest are gelation behavior, which would depend on as yet unverified ability of corn to carry out post-translational prolyl hydroxylation and formation of triple helical conformation. The starting material was grain where the expression of rCI alpha 1 had been directed by an embryo-specific promoter. Purification consisted of extraction at low pH followed by membrane and chromatographic steps to isolate rCI alpha 1 for characterization. The amino acid composition and immunoreactivity of CI alpha 1 was similar to that of an analogous native human CI alpha 1 and to rCI alpha 1 produced by the yeast Pichia pastoris. Tandem mass spectrometry confirmed the primary sequence of the corn-derived rCI alpha 1 with 46% coverage. Fragments of the rCI alpha 1 chains were also observed, possibly caused by endogenous plant proteases. The corn-derived rCI alpha 1 had a low level of prolyl hydroxylation (approximately 1% versus 11%) relative to animal-derived CI alpha 1 and folded into its characteristic triple-helical structure as indicated by its resistance to pepsin digestion below its melting temperature of 26(o)C. The 29 amino acid foldon fused to the C-terminus to initiate triple helix formation was not cleaved from the rCI alpha 1 chains, but could be removed by pepsin treatment.

Fractionation of transgenic corn seed by dry and wet milling to recover recombinant collagen-related proteins

Corn continues to be considered an attractive transgenic host for producing recombinant therapeutic and industrial proteins because of its potential for producing recombinant proteins at large volume and low cost as coproducts of corn seed-based biorefining. Efforts to reduce production costs have been primarily devoted to increasing accumulation level, optimizing protein extraction conditions, and simplifying the purification. In the present work, we evaluated two grain fractionation methods, dry milling and wet milling, to enrich two recombinant collagen-related proteins; thereby, reducing the amount and type of corn-derived impurities in subsequent protein extraction and purification steps. The two proteins were a full-length human recombinant collagen type I alpha 1(rCIalpha1) chain with telopeptides and peptide foldon to effect triple helix formation and a 44-kDa rCIalpha1 fragment. For each, approximately 60% of the rCIalpha1s in the seed was recovered in the dry-milled germ-rich fractions making up ca. 25% of the total kernel mass. For wet milling, approximately 60% of each was recovered in three fractions accounting for 20-25% of the total kernel mass. The rCIalpha1s in the dry-milled germ-rich fractions were enriched three to six times compared with the whole corn kernel, whereas the rCIalpha1s were enriched 4-10 times in selected wet-milled fractions. The recovered starch from wet milling was almost free of rCIalpha1. Therefore, it was possible to generate rCIalpha1-enriched fractions by both dry and wet milling along with rCIalpha1-free starch using wet milling. Because of its simplicity, the dry milling procedure could be accomplished on-farm thus minimizing the risk of inadvertent release of viable transgenic seeds.

[Establishment of high efficiency genetic transformation system of maize mediated by Agrobacterium tumefaciens]

In order to establish high-frequency regeneration and high-efficiency genetic transformation system in maize, the significance of the 11 factors influencing maize embryonic callus induction and 9 factors affecting embryonic callus differentiation was researched by orthogonal experiment. The results showed that genotype had highly significant impact on induction of embryonic callus. The concentration of 6-BA, AgNO3, 2,4-D, ABA, and medium are the significant factors. The Multi-comparison showed that ABA 2 mg/L has a significant influence. Among the callus differentiation factors, the genotype and 6-BA concentration showed a strong main effect, the concentrations of NAA, medium, KT and 2,4-D had significant impacts on callus differentiation. Southern blotting analysis demonstrated that the resistant callus rate under the selection pressure of 25 mg/L hygromycin was a reliable indicator for system optimization in resistance screening. The concentration of acetosyringone (AS) showed sensitive differences among genotypes. The highest transformation rate was found with the optimized combination of 24-25 degrees C for co-culture temperature, 0.7 ODx15 min for Agrobacterium tumefa-ciens concentration and incubation-time, and pH 5.5-6.2. By this optimized combination, the survival rate of resistant calli as an index for the stable transformation rates of inbred lines Huangzao 4 and Zong 31 by introducing GUS gene into maize inbred lines was as high as 48.6% and 46.2%, respectively.

Tissue-print immunodetection of transgene products in endosperm for high-throughput screening of seeds

This method allows high-throughput qualitative screening to identify seeds containing a transgene product in endosperm tissue. It is particularly useful for determining genetic segregation ratios or identifying seeds to be advanced in a breeding program. Tissue printing is used to avoid time-consuming extraction steps. Antibody-based detection of the transgene product makes this method suitable to any transgene product for which a specific antibody is available. It is possible to screen thousands of seeds per week using this method.

Purification and characterization of a 44-kDa recombinant collagen I alpha 1 fragment from corn grain

This paper demonstrates that a fibrous, repetitive amino acid sequence collagen-related protein, a 44-kDa fragment of human collagen I alpha 1 (CIalpha1), was expressed in corn grain molecularly equivalent to that produced in recombinant yeast. The recombinant CIalpha1 was extracted and purified from early generation plants having low levels of recombinant protein accumulation. It was selectively extracted at low pH and purified by ion exchange and gel filtration chromatography, resulting in a 44-kDa CIalpha1 with >70% purity and 60% recovery. The N-terminal sequence, amino acid composition, and immunoreactivity closely matched those of an analogous 44-kDa CIalpha1 fragment produced by the yeast Pichia . The corn-derived 44-kDa CIalpha1 had an intact protein mass of 44088 Da, which is within 0.2% of the mass calculated from the expected sequence. Tandem mass spectrometry confirmed the primary sequence with 78% coverage. The amino acid composition analysis indicated a low level of prolyl hydroxylation. Glycoprotein staining revealed no glycosylation.

Viral vectors for production of recombinant proteins in plants

Global demand for recombinant proteins has steadily accelerated for the last 20 years. These recombinant proteins have a wide range of important applications, including vaccines and therapeutics for human and animal health, industrial enzymes, new materials and components of novel nano-particles for various applications. The majority of recombinant proteins are produced by traditional biological "factories," that is, predominantly mammalian and microbial cell cultures along with yeast and insect cells. However, these traditional technologies cannot satisfy the increasing market demand due to prohibitive capital investment requirements. During the last two decades, plants have been under intensive investigation to provide an alternative system for cost-effective, highly scalable, and safe production of recombinant proteins. Although the genetic engineering of plant viral vectors for heterologous gene expression can be dated back to the early 1980s, recent understanding of plant virology and technical progress in molecular biology have allowed for significant improvements and fine tuning of these vectors. These breakthroughs enable the flourishing of a variety of new viral-based expression systems and their wide application by academic and industry groups. In this review, we describe the principal plant viral-based production strategies and the latest plant viral expression systems, with a particular focus on the variety of proteins produced and their applications. We will summarize the recent progress in the downstream processing of plant materials for efficient extraction and purification of recombinant proteins.

Optimization of fermentation conditions for the expression of sweet-tasting protein brazzein in Lactococcus lactis

AIMS

To improve the production of sweet-tasting protein brazzein in Lactococcus lactis using controlled fermentation conditions.

METHODS AND RESULTS

The nisin-controlled expression system was used for brazzein expression. The concentration of nisin for induction and the optical density (OD) at induction were therefore optimized, together with growth conditions (medium composition, pH, aerobic growth in the presence of hemin). Brazzein was assayed with ELISA on Ni-NTA plates and Western blot. Use of the M-17 medium, containing 2.5% glucose, anaerobic growth at pH 5.9 and induction with 40 ng ml(-1) nisin at OD 3.0 led to an approx. 17-fold increase in brazzein per cell production compared to non-optimized starting conditions. Aerobic growth in the presence of hemin did not increase the production.

CONCLUSIONS

Considerable increase in brazzein per cell production was obtained at optimized fermentation conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Optimized growth conditions could be used in application of brazzein expression in L. lactis. The importance of pH and OD at induction contributes to the body of knowledge of optimal recombinant protein expression in L. lactis. The new assay for brazzein quantification was introduced.

Endosperm tissue is good production platform for artificial recombinant proteins in transgenic rice

Transgenic rice plants expressing 7Crp peptide were generated by Agrobacterium-mediated transformation. The 7Crp peptide is the hybrid peptide of seven major human T-cell epitopes derived from Japanese cedar pollen allergens Cry j 1 and Cry j 2. When the 7Crp gene was expressed under the control of the rice AGPase large subunit or maize ubiquitin-1 promoters, it could only be detected in the endosperm of rice seed, although high levels of RNA transcript were observed in the leaf, stem, and seed embryo. It was demonstrated by confocal and electron microscopy analysis that the 7Crp peptide was mainly localized in the endoplasmic reticulum-derived protein bodies, designated protein body I (PB-I). Our results indicate that rice endosperm tissue has advantage over other tissues as a production platform for foreign recombinant proteins.

Agrobacterium-mediated transformation of cereals: a promising approach crossing barriers

Cereal crops have been the primary targets for improvement by genetic transformation because of their worldwide importance for human consumption. For a long time, many of these important cereals were difficult to genetically engineer, mainly as a result of their inherent limitations associated with the resistance to Agrobacterium infection and their recalcitrance to in vitro regeneration. The delivery of foreign genes to rice plants via Agrobacterium tumefaciens has now become a routine technique. However, there are still serious handicaps with Agrobacterium-mediated transformation of other major cereals. In this paper, we review the pioneering efforts, existing problems and future prospects of Agrobacterium-mediated genetic transformation of major cereal crops, such as rice, maize, wheat, barley, sorghum and sugarcane.

Developments in biotechnological production of sweet proteins

Most proteins are tasteless and flavorless, while some proteins elicit a sweet-taste response on the human palate. Six proteins, thaumatin, monellin, mabinlin, brazzein, egg lysozyme, and neoculin (previously considered as curculin) have been identified as sweet-tasting proteins. However, no common features among them have been observed. Herein, recent advances in the research of sweet-tasting proteins and the production of such proteins by biotechnological approaches are reviewed. Information on the structure-sweetness relationship for these proteins would help not only in the clarification of the mechanism of interaction of sweet-tasting proteins with their receptors, but also in the design of more effective low-calorie sweeteners.

Expression of the sweet-tasting plant protein brazzein in Escherichia coli and Lactococcus lactis: a path toward sweet lactic acid bacteria

Brazzein is an intensely sweet-tasting plant protein with good stability, which makes it an attractive alternative to sucrose. A brazzein gene has been designed, synthesized, and expressed in Escherichia coli at 30 degrees C to yield brazzein in a soluble form and in considerable quantity. Antibodies have been produced using brazzein fused to His-tag. Brazzein without the tag was sweet and resembled closely the taste of its native counterpart. The brazzein gene was also expressed in Lactococcus lactis, using a nisin-controlled expression system, to produce sweet-tasting lactic acid bacteria. The low level of expression was detected with anti-brazzein antibodies. Secretion of brazzein into the medium has not led to significant yield increase. Surprisingly, optimizing the codon usage for Lactococcus lactis led to a decrease in the yield of brazzein.

Mucosal immunization using recombinant plant-based oral vaccines

The induction of mucosal immunity is very important in conferring protection against pathogens that typically invade via mucosal surfaces. Delivery of a vaccine to a mucosal surface optimizes the induction of mucosal immunity. The apparent linked nature of the mucosal immune system allows delivery to any mucosal surface to potentially induce immunity at others. Oral administration is a very straightforward and inexpensive approach to deliver a vaccine to the mucosal lining of the gut. However, vaccines administered by this route are subject to proteolysis in the gastrointestinal tract. Thus, dose levels for protein subunit vaccines are likely to be very high and the antigen may need to be protected from proteolysis for oral delivery to be efficacious. Expression of candidate vaccine antigens in edible recombinant plant material offers an inexpensive means to deliver large doses of vaccines in encapsulated forms. Certain plant tissues can also stably store antigens for extensive periods of time at ambient temperatures, obviating the need for a cold-chain during vaccine storage and distribution, and so further limiting costs. Antigens can be expressed from transgenes stably incorporated into a host plant's nuclear or plastid genome, or from engineered plant viruses infected into plant tissues. Molecular approaches can serve to boost expression levels and target the expressed protein for appropriate post-translational modification. There is a wide range of options for processing plant tissues to allow for oral delivery of a palatable product. Alternatively, the expressed antigen can be enriched or purified prior to formulation in a tablet or capsule for oral delivery. Fusions to carrier molecules can stabilize the expressed antigen, aid in antigen enrichment or purification strategies, and facilitate delivery to effector sites in the gastrointestinal tract. Many antigens have been expressed in plants. In a few cases, vaccine candidates have entered into early phase clinical trials, and in the case of farmed animal vaccines into relevant animal trials.