A conditional marker gene allowing both positive and negative selection in plants

A dual-function selection system enables positive selection of multigene CRISPR mutants and negative selection of Cas9-free progeny in Arabidopsis

The CRISPR/Cas9 technology revolutionizes targeted gene knockout in diverse organisms including plants. However, screening edited alleles, particularly those with multiplex editing, from herbicide- or antibiotic-resistant transgenic plants and segregating out the Cas9 transgene represent two laborious processes. Current solutions to facilitate these processes rely on different selection markers. Here, by taking advantage of the opposite functions of a d-amino acid oxidase (DAO) in detoxifying d-serine and in metabolizing non-toxic d-valine to a cytotoxic product, we develop a DAO-based selection system that simultaneously enables the enrichment of multigene edited alleles and elimination of Cas9-containing progeny in Arabidopsis thaliana. Among five DAOs tested in Escherichia coli, the one encoded by Trigonopsis variabilis (TvDAO) could confer slightly stronger d-serine resistance than other homologs. Transgenic expression of TvDAO in Arabidopsis allowed a clear distinction between transgenic and non-transgenic plants in both d-serine-conditioned positive selection and d-valine-conditioned negative selection. As a proof of concept, we combined CRISPR-induced single-strand annealing repair of a dead TvDAO with d-serine-based positive selection to help identify transgenic plants with multiplex editing, where d-serine-resistant plants exhibited considerably higher co-editing frequencies at three endogenous target genes than those selected by hygromycin. Subsequently, d-valine-based negative selection successfully removed Cas9 and TvDAO transgenes from the survival offspring carrying inherited mutations. Collectively, this work provides a novel strategy to ease CRISPR mutant identification and Cas9 transgene elimination using a single selection marker, which promises more efficient and simplified multiplex CRISPR editing in plants.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42994-023-00132-6.

d-Amino acids in biological systems

Investigations on the occurrence and biochemical roles of free D-amino acids and D-amino acid-containing peptides and proteins in living systems have increased in frequency and significance. Their occurrence and roles may vary substantially with progression from microbiotic to evermore advanced macrobiotic systems. We now understand many of the biosynthetic and regulatory pathways, which are outlined herein. Important uses for D-amino acids in plants, invertebrates, and vertebrates are reviewed. Given its importance, a separate section on the occurrence and role of D-amino acids in human disease is presented.

Design of an Arabidopsis thaliana reporter line to detect heat-sensing and signaling mutants

BACKGROUND

Global warming is a major challenge for plant survival and growth. Understanding the molecular mechanisms by which higher plants sense and adapt to upsurges in the ambient temperature is essential for developing strategies to enhance plant tolerance to heat stress. Here, we designed a heat-responsive Arabidopsis thaliana reporter line that allows an in-depth investigation of the mechanisms underlying the accumulation of protective heat-shock proteins (HSPs) in response to high temperature.

METHODS

A transgenic Arabidopsis thaliana reporter line named "Heat-Inducible Bioluminescence And Toxicity" (HIBAT) was designed to express from a conditional heat-inducible promoter, a fusion gene encoding for nanoluciferase and D-amino acid oxidase, whose expression is toxic in the presence of D-valine. HIBAT seedlings were exposed to different heat treatments in presence or absence of D-valine and analyzed for survival rate, bioluminescence and HSP gene expression.

RESULTS

Whereas at 22 °C, HIBAT seedlings grew unaffected by D-valine, and all survived iterative heat treatments without D-valine, 98% died following heat treatments on D-valine. The HSP17.3B promoter was highly specific to heat as it remained unresponsive to various plant hormones, Flagellin, H₂O₂, osmotic stress and high salt. RNAseq analysis of heat-treated HIBAT seedlings showed a strong correlation with expression profiles of two wild type lines, confirming that HIBAT does not significantly differ from its Col-0 parent. Using HIBAT, a forward genetic screen revealed candidate loss-of-function mutants, apparently defective either at accumulating HSPs at high temperature or at repressing HSP accumulation at non-heat-shock temperatures.

CONCLUSION

HIBAT is a valuable candidate tool to identify Arabidopsis mutants defective in the response to high temperature stress. It opens new avenues for future research on the regulation of HSP expression and for understanding the mechanisms of plant acquired thermotolerance.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition

BACKGROUND

For more than a century, crop N nutrition research has primarily focused on inorganic N (IN) dynamics, building the traditional model that agricultural plants predominantly take up N in the form of NO₃ ^- and NH₄ ⁺. However, results reported in the ecological and agricultural literature suggest that the traditional model of plant N nutrition is oversimplified.

SCOPE

We examine the role of organic N (ON) in plant N nutrition, first by reviewing the historical discoveries by ecologists of plant ON uptake, then by discussing the advancements of key analytical techniques that have furthered the cause (stable isotope and microdialysis techniques). The current state of knowledge on soil ON dynamics is analyzed concurrently with recent developments that show ON uptake and assimilation by agricultural plant species. Lastly, we consider the relationship between ON uptake and nitrogen use efficiency (NUE) in an agricultural context.

CONCLUSIONS

We propose several mechanisms by which ON uptake and assimilation may increase crop NUE, such as by reducing N assimilation costs, promoting root biomass growth, shaping N cycling microbial communities, recapturing exuded N compounds, and aligning the root uptake capacity to the soil N supply in highly fertilized systems. These hypothetical mechanisms should direct future research on the topic. Although the quantitative role remains unknown, ON compounds should be considered as significant contributors to plant N nutrition.

D-amino Acids in Plants: Sources, Metabolism, and Functions

Although plants are permanently exposed to d-amino acids (d-AAs) in the rhizosphere, these compounds were for a long time regarded as generally detrimental, due to their inhibitory effects on plant growth. Recent studies showed that this statement needs a critical revision. There were several reports of active uptake by and transport of d-AAs in plants, leading to the question whether these processes happened just as side reactions or even on purpose. The identification and characterization of various transporter proteins and enzymes in plants with considerable affinities or specificities for d-AAs also pointed in the direction of their targeted uptake and utilization. This attracted more interest, as d-AAs were shown to be involved in different physiological processes in plants. Especially, the recent characterization of d-AA stimulated ethylene production in Arabidopsis thaliana revealed for the first time a physiological function for a specific d-AA and its metabolizing enzyme in plants. This finding opened the question regarding the physiological or developmental contexts in which d-AA stimulated ethylene synthesis are involved in. This question and the ones about the transport characteristics of d-AAs, their metabolism, and their different physiological effects, are the focus of this review.

In vivo Reporters for Visualizing Alternative Splicing of Hormonal Genes

Rapid progress in plant molecular biology in recent years has uncovered the main players in hormonal pathways and characterized transcriptomic networks associated with hormonal response. However, the role of RNA processing, in particular alternative splicing (AS), remains largely unexplored. Here, using example genes involved in cytokinin signaling, brassinosteroid synthesis and auxin transport, we present a set of reporters devised to visualize their AS events in vivo. These reporters show a differential tissue-specific expression of certain transcripts and reveal that expression of some of the them can be changed by the application of the exogenous hormone. Finally, based on the characterized AS event of the PIN7 auxin efflux carrier, we designed a system that allows a rapid genetic screening for the factors upstream of this AS event. Our innovative toolset can be therefore highly useful for exploring novel regulatory nodes of hormonal pathways and potentially helpful for plant researchers focusing on developmental aspects of AS.

Natural Occurrence, Biological Functions, and Analysis of D-Amino Acids

This review covers the recent development on the natural occurrence, functional elucidations, and analysis of amino acids of the D (dextro) configuration. In the pharmaceutical field, amino acids are not only used directly as clinical drugs and nutriments, but also widely applied as starting materials, catalysts, or chiral ligands for the synthesis of active pharmaceutical ingredients. Earler belief hold that only L-amino acids exist in nature and D-amino acids were artificial products. However, increasing evidence indicates that D-amino acids are naturally occurring in living organisms including human beings, plants, and microorganisms, playing important roles in biological processes. While D-amino acids have similar physical and chemical characteristics with their respective L-enantiomers in an achiral measurement, the biological functions of D-amino acids are remarkably different from those of L-ones. With the rapid development of chiral analytical techniques for D-amino acids, studies on the existence, formation mechanisms, biological functions as well as relevant physiology and pathology of D-amino acids have achieved great progress; however, they are far from being sufficiently explored.

Distribution and evolution of the serine/aspartate racemase family in plants

Previous studies have shown that several d-amino acids are widely present in plants, and serine racemase (SerR), which synthesizes d-serine in vivo, has already been identified from three plant species. However, the full picture of the d-amino acid synthesis pathway in plants is not well understood. To clarify the distribution of amino acid racemases in plants, we have cloned, expressed and characterized eight SerR homologous genes from five plant species, including green alga. These SerR homologs exhibited racemase activity towards serine or aspartate and were identified on the basis of their maximum activity as SerR or aspartate racemase (AspR). The plant AspR gene is identified for the first time from Medicago truncatula, Manihot esculenta, Solanum lycopersicum, Sphagnum girgensohnii and Spirogyra pratensis. In addition to the AspR gene, three SerR genes are identified in the former three species. Phylogenetic tree analysis showed that SerR and AspR are widely distributed in plants and form a serine/aspartate racemase family cluster. The catalytic efficiency (k_cat/K_m) of plant AspRs was more than 100 times higher than that of plant SerRs, suggesting that d-aspartate, as well as d-serine, can be synthesized in vivo by AspR. The amino acid sequence alignment and comparison of the chromosomal gene arrangement have revealed that plant AspR genes independently evolved from SerR in each ancestral lineage of plant species by gene duplication and acquisition of two serine residues at position 150 to 152.

Genome Editing in Agriculture: Technical and Practical Considerations

The advent of precise genome-editing tools has revolutionized the way we create new plant varieties. Three groups of tools are now available, classified according to their mechanism of action: Programmable sequence-specific nucleases, base-editing enzymes, and oligonucleotides. The corresponding techniques not only lead to different outcomes, but also have implications for the public acceptance and regulatory approval of genome-edited plants. Despite the high efficiency and precision of the tools, there are still major bottlenecks in the generation of new and improved varieties, including the efficient delivery of the genome-editing reagents, the selection of desired events, and the regeneration of intact plants. In this review, we evaluate current delivery and regeneration methods, discuss their suitability for important crop species, and consider the practical aspects of applying the different genome-editing techniques in agriculture.

Stereo-Specific Transcript Regulation of the Polyamine Biosynthesis Genes by Enantiomers of Ornithine in Tobacco Cell Culture

Background

Ornithine (Orn) plays an essential role in the metabolism of plant cells through incorporation in polyamines biosynthesis, the urea cycle and nitrogen metabolism. Physiological response of the plant cells to its two enantiomers have not been widely investigated yet.

Objectives

This study aimed to evaluate effect of ornithine enantiomers on expression of certain polyamine (PAs) biosynthetic genes in tobacco cells.

Materials and Methods

Suspension-cultured tobacco cells were treated with different concentrations of L- and D- Orn for 24 h. Cell viability was assayed by Evans Blue and hydrogen peroxide content. The changes of gene expression were analyzed by semi-quantitative RT-PCR.

Results

Exogenous D-Orn resulted in enhancement of expression of genes involved in Orn, arginine and S-adenosyl methionine metabolism. Additionally, exogenous D-Orn treatment resulted in sustained viability of cultured tobacco cells and normal levels of hydrogen peroxide were maintained. Supplied L-Orn increased the hydrogen peroxide level and lowered viability of cells. Treatment with L-Orn had a negative effect on the transcript levels for most analyzed PA-related genes. It was also illustrated that transcription of putrescine methyl transferase, key enzyme for nicotine production, was highly upregulated by L-Orn.

Conclusions

Based on the results, D-Orn was shown to have a stereo-selective function in regulation of the PAs-related genes.

AtDAT1 Is a Key Enzyme of D-Amino Acid Stimulated Ethylene Production in Arabidopsis thaliana

D-Enantiomers of proteinogenic amino acids (D-AAs) are found ubiquitously, but the knowledge about their metabolism and functions in plants is scarce. A long forgotten phenomenon in this regard is the D-AA-stimulated ethylene production in plants. As a starting point to investigate this effect, the Arabidopsis accession Landsberg erecta (Ler) got into focus as it was found defective in metabolizing D-AAs. Combining genetics and molecular biology of T-DNA insertion lines and natural variants together with biochemical and physiological approaches, we could identify AtDAT1 as a major D-AA transaminase in Arabidopsis. Atdat1 loss-of-function mutants and Arabidopsis accessions with defective AtDAT1 alleles were unable to produce the metabolites of D-Met, D-Ala, D-Glu, and L-Met. This result corroborates the biochemical characterization, which showed highest activity of AtDAT1 using D-Met as a substrate. Germination of seedlings in light and dark led to enhanced growth inhibition of atdat1 mutants on D-Met. Ethylene measurements revealed an increased D-AA stimulated ethylene production in these mutants. According to initial working models of this phenomenon, D-Met is preferentially malonylated instead of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). This decrease of ACC degradation should then lead to the increase of ethylene production. We could observe a reciprocal relation of malonylated methionine and ACC upon D-Met application and significantly more malonyl-methionine in atdat1 mutants. Unexpectedly, the malonyl-ACC levels did not differ between mutants and wild type. With AtDAT1, the first central enzyme of plant D-AA metabolism was characterized biochemically and physiologically. The specific effects of D-Met on ACC metabolism, ethylene production, and plant development of dat1 mutants unraveled the impact of AtDAT1 on these processes; however, they are not in full accordance to previous working models. Instead, our results imply the influence of additional factors or processes on D-AA-stimulated ethylene production, which await to be uncovered.

d-Amino Acids Are Exuded by Arabidopsis thaliana Roots to the Rhizosphere

Proteinogenic l-amino acids (l-AAs) are essential in all kingdoms as building blocks of proteins. Their d-enantiomers are also known to fulfill important functions in microbes, fungi, and animals, but information about these molecules in plants is still sparse. Previously, it was shown that d-amino acids (d-AAs) are taken up and utilized by plants, but their ways to reduce excessive amounts of them still remained unclear. Analyses of plant d-AA content after d-Ala and d-Glu feeding opened the question if exudation of d-AAs into the rhizosphere takes place and plays a role in the reduction of d-AA content in plants. The exudation of d-Ala and d-Glu could be confirmed by amino acid analyses of growth media from plants treated with these d-AAs. Further tests revealed that other d-AAs were also secreted. Nevertheless, treatments with d-Ala and d-Glu showed that plants are still able to reduce their contents within the plant without exudation. Further exudation experiments with transport inhibitors revealed that d-AA root exudation is rather passive and comparable to the secretion of l-AAs. Altogether, these observations argued against a dominant role of exudation in the regulation of plant d-AA content, but may influence the composition of the rhizosphere.

New Insights Into the Mechanisms and Biological Roles of D-Amino Acids in Complex Eco-Systems

In the environment bacteria share their habitat with a great diversity of organisms, from microbes to humans, animals and plants. In these complex communities, the production of extracellular effectors is a common strategy to control the biodiversity by interfering with the growth and/or viability of nearby microbes. One of such effectors relies on the production and release of extracellular D-amino acids which regulate diverse cellular processes such as cell wall biogenesis, biofilm integrity, and spore germination. Non-canonical D-amino acids are mainly produced by broad spectrum racemases (Bsr). Bsr’s promiscuity allows it to generate high concentrations of D-amino acids in environments with variable compositions of L-amino acids. However, it was not clear until recent whether these molecules exhibit divergent functions. Here we review the distinctive biological roles of D-amino acids, their mechanisms of action and their modulatory properties of the biodiversity of complex eco-systems.

Amine oxidation by d-arginine dehydrogenase in Pseudomonas aeruginosa

d-Arginine dehydrogenase from Pseudomonas aeruginosa (PaDADH) is a flavin-dependent oxidoreductase, which is part of a novel two-enzyme racemization system that functions to convert d-arginine to l-arginine. PaDADH contains a noncovalently linked FAD that shows the highest activity with d-arginine. The enzyme exhibits broad substrate specificity towards d-amino acids, particularly with cationic and hydrophobic d-amino acids. Biochemical studies have established the structure and the mechanistic properties of the enzyme. The enzyme is a true dehydrogenase because it displays no reactivity towards molecular oxygen. As established through solvent and multiple kinetic isotope studies, PaDADH catalyzes an asynchronous CH and NH bond cleavage via a hydride transfer mechanism. Steady-state kinetic studies with d-arginine and d-histidine are consistent with the enzyme following a ping-pong bi-bi mechanism. As shown by a combination of crystallography, kinetic and computational data, the shape and flexibility of loop L1 in the active site of PaDADH are important for substrate capture and broad substrate specificity.

Moss Chloroplasts Are Surrounded by a Peptidoglycan Wall Containing D-Amino Acids

It is believed that the plastids in green plants lost peptidoglycan (i.e., a bacterial cell wall-containing d-amino acids) during their evolution from an endosymbiotic cyanobacterium. Although wall-like structures could not be detected in the plastids of green plants, the moss Physcomitrella patens has the genes required to generate peptidoglycan (Mur genes), and knocking out these genes causes defects in chloroplast division. Here, we generated P patens knockout lines (∆Pp-ddl) for a homolog of the bacterial peptidoglycan-synthetic gene encoding d-Ala:d-Ala ligase. ∆Pp-ddl had a macrochloroplast phenotype, similar to other Mur knockout lines. The addition of d-Ala-d-Ala (DA-DA) to the medium suppressed the appearance of giant chloroplasts in ∆Pp-ddl, but the addition of l-Ala-l-Ala (LA-LA), DA-LA, LA-DA, or d-Ala did not. Recently, a metabolic method for labeling bacterial peptidoglycan was established using ethynyl-DA-DA (EDA-DA) and click chemistry to attach an azide-modified fluorophore to the ethynyl group. The ∆Pp-ddl line complemented with EDA-DA showed that moss chloroplasts are completely surrounded by peptidoglycan. Our findings strongly suggest that the moss plastids have a peptidoglycan wall containing d-amino acids. By contrast, no plastid phenotypes were observed in the T-DNA tagged ddl mutant lines of Arabidopsis thaliana.

Expression of a Codon-Optimized dsdA Gene in Tobacco Plastids and Rice Nucleus Confers D-Serine Tolerance

D-serine is toxic to plants. D-serine ammonia lyase, which is encoded by the dsdA gene, can attenuate this toxicity with high specificity. In the present study, we explored the function of codon-optimized dsdA with tobacco plastids and rice nuclear transformation system. It was shown that dsdA gene was site-specifically integrated into the tobacco plastid genome and displayed a high level of expression. Genetic analysis of the progenies showed that dsdA gene is maternally inherited and confers sufficient D-serine resistance in tobacco. The effective screening concentrations of D-serine for seed germination, callus regeneration and foliar spray were 10, 30, and 75 mM, respectively. In addition, calluses from homozygous transgenic rice lines also showed significant tolerance to D-serine (up to 75 mM). Our study proves the feasibility of using dsdA gene as a selectable marker in both plastid and nuclear transformation systems.

Enhancement of the solubility and stability of D-amino acid oxidase by fusion to an elastin like polypeptide

An elastin-like polypeptide (ELP) was fused to D-amino acid oxidases (DAAO). ELP-DAAO exhibited a better solubility in aqueous solutions than DAAO, and its enzymatic activity is about 1.6 times that of DAAO. The stability of the proteins was investigated by interacting with urea at various concentrations. The circular dichroism and fluorescence spectra were measured. The results demonstrated that that ELP-DAAO exhibited a much better stability than DAAO, and ELP-DAAO has retained the α-helix content with a high percentage even at a high urea concentration. The results of this work have demonstrated that the ELP tag can be utilized to purify DAAO, in the meantime the solubility and stability of the enzyme are improved.

Maize transformation technology development for commercial event generation

Maize is an important food and feed crop in many countries. It is also one of the most important target crops for the application of biotechnology. Currently, there are more biotech traits available on the market in maize than in any other crop. Generation of transgenic events is a crucial step in the development of biotech traits. For commercial applications, a high throughput transformation system producing a large number of high quality events in an elite genetic background is highly desirable. There has been tremendous progress in Agrobacterium-mediated maize transformation since the publication of the Ishida et al. (1996) paper and the technology has been widely adopted for transgenic event production by many labs around the world. We will review general efforts in establishing efficient maize transformation technologies useful for transgenic event production in trait research and development. The review will also discuss transformation systems used for generating commercial maize trait events currently on the market. As the number of traits is increasing steadily and two or more modes of action are used to control key pests, new tools are needed to efficiently transform vectors containing multiple trait genes. We will review general guidelines for assembling binary vectors for commercial transformation. Approaches to increase transformation efficiency and gene expression of large gene stack vectors will be discussed. Finally, recent studies of targeted genome modification and transgene insertion using different site-directed nuclease technologies will be reviewed.

A new F131V mutation in Chlamydomonas phytoene desaturase locates a cluster of norflurazon resistance mutations near the FAD-binding site in 3D protein models

The green alga Chlamydomonas reinhardtii provides a tractable genetic model to study herbicide mode of action using forward genetics. The herbicide norflurazon inhibits phytoene desaturase, which is required for carotenoid synthesis. Locating amino acid substitutions in mutant phytoene desaturases conferring norflurazon resistance provides a genetic approach to map the herbicide binding site. We isolated a UV-induced mutant able to grow in very high concentrations of norflurazon (150 µM). The phytoene desaturase gene in the mutant strain contained the first resistance mutation to be localised to the dinucleotide-binding Rossmann-likedomain. A highly conserved phenylalanine amino acid at position 131 of the 564 amino acid precursor protein was changed to a valine in the mutant protein. F131, and two other amino acids whose substitution confers norflurazon resistance in homologous phytoene desaturase proteins, map to distant regions in the primary sequence of the C. reinhardtii protein (V472, L505) but in tertiary models these residues cluster together to a region close to the predicted FAD binding site. The mutant gene allowed direct 5 µM norflurazon based selection of transformants, which were tolerant to other bleaching herbicides including fluridone, flurtamone, and diflufenican but were more sensitive to beflubutamid than wild type cells. Norflurazon resistance and beflubutamid sensitivity allow either positive or negative selection against transformants expressing the mutant phytoene desaturase gene.

GreenGate---a novel, versatile, and efficient cloning system for plant transgenesis

Building expression constructs for transgenesis is one of the fundamental day-to-day tasks in modern biology. Traditionally it is based on a multitude of type II restriction endonucleases and T4 DNA ligase. Especially in case of long inserts and applications requiring high-throughput, this approach is limited by the number of available unique restriction sites and the need for designing individual cloning strategies for each project. Several alternative cloning systems have been developed in recent years to overcome these issues, including the type IIS enzyme based Golden Gate technique. Here we introduce our GreenGate system for rapidly assembling plant transformation constructs, which is based on the Golden Gate method. GreenGate cloning is simple and efficient since it uses only one type IIS restriction endonuclease, depends on only six types of insert modules (plant promoter, N-terminal tag, coding sequence, C-terminal tag, plant terminator and plant resistance cassette), but at the same time allows assembling several expression cassettes in one binary destination vector from a collection of pre-cloned building blocks. The system is cheap and reliable and when combined with a library of modules considerably speeds up cloning and transgene stacking for plant transformation.

Analyses of Arabidopsis ecotypes reveal metabolic diversity to convert D-amino acids

For a long time D-enantiomers of proteinogenic L-amino acids were assumed to be physiologically irrelevant for plants. But there is growing evidence that D-amino acids (D-AAs) also fulfil important physiological functions in these organisms. However, the knowledge about the metabolic fate of D-AAs in plants is still scarce and more information about it is needed. To close this gap we established an optimized protocol for the processing and analysis of D- and L-AAs from large numbers of Arabidopsis lines. This included the application of 18 different D-AAs to seedlings, the extraction of free amino acids from the samples and the determination of 16 L-AAs and their corresponding D-enantiomers. To validate our approach we searched for genetic accessions with aberrant amino acid metabolism. Therefore we applied D-AAs on 17 ecotypes of Arabidopsis thaliana and analysed their free amino acid contents. These analyses confirmed the suitability of the system for the analysis of large sets of plant samples with enhanced velocity and improved accuracy. Furthermore, the resulting data led to the definition of standard amino acid profiles in response to D-AAs of Arabidopsis seedlings. Within these analyses the ecotype Landsberg erecta was found with aberrant metabolic patterns like drastically reduced capabilities to convert different D-AAs to D-alanine and D-glutamate. The presented experimental setup and results of this study offer starting points to dissect the metabolic pathway of D-AAs in plants.

Differential effects of ornithine enantiomers on the activity of anti-oxidant enzymes, polyamines content, and growth of tobacco cells under osmotic stresses

Ornithine (Orn) plays an essential role in the metabolism of plant cells through incorporation in polyamines biosynthesis, the urea cycle and nitrogen metabolism. Herein, we show that Orn enantiomers have different effects on anti-oxidant enzymes activities, polyamines and proline biosynthesis and also an alleviation effect of osmotic stresses on tobacco cells. The type of stress has a significant impact on the function of L- and D-Orn for improvement of the stress effect on the cells. Under saline conditions, both enantiomers restored cell growth, though D-Orn was more beneficial to some extent. This was accompanied with a higher biosynthesis of putrescine, proline, and up-regulated activity of certain anti-oxidant enzymes by D-Orn. Under drought stress conditions, a distinct differential behavior emerged and only L-Orn showed an alleviative effect on the cell growth. Regulation of hydrogen peroxide content via the activity of catalase/peroxidase and production of osmolytes, e.g., proline and fructans, was dependent on the type of enantiomers. Activity of anti-oxidant enzymes and production of malondialdehyde from cell membranes were differently regulated following treatment with either Orn enantiomer. The results suggest that management of H2 O2 content is a determining feature of the function of Orn enantiomers in tobacco cells under salinity and drought stress conditions.

Enantiomeric separation of free L- and D-amino acids in hydrolyzed protein fertilizers by capillary electrophoresis tandem mass spectrometry

Two capillary electrophoresis-tandem mass spectrometry (CE-MS(2)) methods were optimized in this work using cyclodextrins (CDs) as chiral selectors in order to determine the degree of racemization of the free amino acids contained in different hydrolyzed protein fertilizers used as plant biostimulants. The methodologies developed were characterized by the specificity of MS(2) experiments enabling the identification of all protein amino acids, except for cysteine. The enantiomeric separation of up to 14 amino acids was achieved with resolutions above 1.0 and limits of detection between 0.02 and 0.8 μM. The methods were applied to the analysis of complex samples such as hydrolyzed protein fertilizers to evaluate the presence of d-amino acids after different kinds of hydrolysis treatments. The results corroborated the absence or almost negligible presence of enantiomeric conversions of the L-amino acids into D-amino acids in the case of fertilizers obtained by enzymatic hydrolysis, as well as the high racemization rate for those obtained through a chemical hydrolysis.

Selective regulation of nicotine and polyamines biosynthesis in tobacco cells by enantiomers of ornithine

L- and D-amino acids have diverse functions and effects on the metabolism, growth, and development of plants. Ornithine (Orn) plays a main role in the biosynthesis of many amino acids, nicotinic alkaloids, and polyamines in tobacco. This investigation describes the impact of Orn enantiomers on the production and distribution of free, conjugated, and bound polyamines, as well as nicotine in tobacco cells. It was recognized that the biosynthesis of metabolites was differently upregulated by each enantiomer. Putrescine was abundantly produced by exogenous L-ornithine (L-Orn), and both spermidine and spermine were significantly accumulated in D-ornithine (D-Orn)-supplied tobacco cells. Furthermore, nicotine production was highly upregulated by L-Orn, while the addition of D-Orn had no effect on the nicotine content of tobacco cells. It was observed that transcript expression of S-adenosylmethionine decarboxylase, as the key enzyme of spermidine/spermine biosynthesis, is coincident with their metabolic levels and is highly upregulated by D-Orn, as opposed to L-Orn. These results indicate that both enantiomers of Orn can trigger selected biosynthetic pathways in the cells, at the transcript level. Regarding these observations, it is proposed that L- and D-Orn function differently in the same biological pathways in which the latter, D-Orn specifically regulates important polyamines in the plant cells.

Growth of transplastomic cells expressing D-amino acid oxidase in chloroplasts is tolerant to D-alanine and inhibited by D-valine

Dual-conditional positive/negative selection markers are versatile genetic tools for manipulating genomes. Plastid genomes are relatively small and conserved DNA molecules that can be manipulated precisely by homologous recombination. High-yield expression of recombinant products and maternal inheritance of plastid-encoded traits make plastids attractive sites for modification. Here, we describe the cloning and expression of a dao gene encoding D-amino acid oxidase from Schizosaccharomyces pombe in tobacco (Nicotiana tabacum) plastids. The results provide genetic evidence for the uptake of D-amino acids into plastids, which contain a target that is inhibited by D-alanine. Importantly, this nonantibiotic-based selection system allows the use of cheap and widely available D-amino acids, which are relatively nontoxic to animals and microbes, to either select against (D-valine) or for (D-alanine) cells containing transgenic plastids. Positive/negative selection with d-amino acids was effective in vitro and against transplastomic seedlings grown in soil. The dual functionality of dao is highly suited to the polyploid plastid compartment, where it can be used to provide tolerance against potential D-alanine-based herbicides, control the timing of recombination events such as marker excision, influence the segregation of transgenic plastid genomes, identify loci affecting dao function in mutant screens, and develop D-valine-based methods to manage the spread of transgenic plastids tagged with dao.

Single molecule PCR reveals similar patterns of non-homologous DSB repair in tobacco and Arabidopsis

DNA double strand breaks (DSBs) occur constantly in eukaryotes. These potentially lethal DNA lesions are repaired efficiently by two major DSB repair pathways: homologous recombination and non-homologous end joining (NHEJ). We investigated NHEJ in Arabidopsis thaliana and tobacco (Nicotiana tabacum) by introducing DNA double-strand breaks through inducible expression of I-SceI, followed by amplification of individual repair junction sequences by single-molecule PCR. Using this process over 300 NHEJ repair junctions were analysed in each species. In contrast to previously published variation in DSB repair between Arabidopsis and tobacco, the two species displayed similar DSB repair profiles in our experiments. The majority of repair events resulted in no loss of sequence and small (1-20 bp) deletions occurred at a minority (25-45%) of repair junctions. Approximately ~1.5% of the observed repair events contained larger deletions (>20 bp) and a similar percentage contained insertions. Strikingly, insertion events in tobacco were associated with large genomic deletions at the site of the DSB that resulted in increased micro-homology at the sequence junctions suggesting the involvement of a non-classical NHEJ repair pathway. The generation of DSBs through inducible expression of I-SceI, in combination with single molecule PCR, provides an effective and efficient method for analysis of individual repair junctions and will prove a useful tool in the analysis of NHEJ.

Environmental stress increases the entry of cytoplasmic organellar DNA into the nucleus in plants

Mitochondria and chloroplasts (photosynthetic members of the plastid family of cytoplasmic organelles) in eukaryotic cells originated more than a billion years ago when an ancestor of the nucleated cell engulfed two different prokaryotes in separate sequential events. Extant cytoplasmic organellar genomes contain very few genes compared with their candidate free-living ancestors, as most have functionally relocated to the nucleus. The first step in functional relocation involves the integration of inactive DNA fragments into nuclear chromosomes, and this process continues at high frequency with attendant genetic, genomic, and evolutionary consequences. Using two different transplastomic tobacco lines, we show that DNA migration from chloroplasts to the nucleus is markedly increased by mild heat stress. In addition, we show that insertion of mitochondrial DNA fragments during the repair of induced double-strand breaks is increased by heat stress. The experiments demonstrate that the nuclear influx of organellar DNA is a potentially a source of mutation for nuclear genomes that is highly susceptible to temperature fluctuations that are well within the range experienced naturally.

A novel light-dependent selection marker system in plants

Photosensitizers are common in nature and play diverse roles as defense compounds and pathogenicity determinants and as important molecules in many biological processes. Toxoflavin, a photosensitizer produced by Burkholderia glumae, has been implicated as an essential virulence factor causing bacterial rice grain rot. Toxoflavin produces superoxide and H₂O₂ during redox cycles under oxygen and light, and these reactive oxygen species cause phytotoxic effects. To utilize toxoflavin as a selection agent in plant transformation, we identified a gene, tflA, which encodes a toxoflavin-degrading enzyme in the Paenibacillus polymyxa JH2 strain. TflA was estimated as 24.56 kDa in size based on the amino acid sequence and is similar to a ring-cleavage extradiol dioxygenase in the Exiguobacterium sp. 255-15; however, unlike other extradiol dioxygenases, Mn(2+) and dithiothreitol were required for toxoflavin degradation by TflA. Here, our results suggested toxoflavin is a photosensitizer and its degradation by TflA serves as a light-dependent selection marker system in diverse plant species. We examined the efficiencies of two different plant selection systems, toxoflavin/tflA and hygromycin/hygromycin phosphotransferase (hpt) in both rice and Arabidopsis. The toxoflavin/tflA selection was more remarkable than hygromycin/hpt selection in the high-density screening of transgenic Arabidopsis seeds. Based on these results, we propose the toxoflavin/tflA selection system, which is based on the degradation of the photosensitizer, provides a new robust nonantibiotic selection marker system for diverse plants.

D-glufosinate as a male sterility agent for hybrid seed production

A chemical male sterility system based on anther-localized conversion of the inactive D-enantiomer of the herbicide, glufosinate (2-amino-4-(methylphosphinyl)-butanoate) to the phytotoxic L is described. Highly pure D-glufosinate was isolated in >98% enantiomeric excess from the racemate via fermentation with a strain of Escherichia coli expressing the PAT (L-glufosinate N-acetyl transferase) gene and purification of the unreacted D-enantiomer from the broth by ion exchange. A modified (F58K, M213S) form of the D-amino acid oxidase (DAAO) (EC 1.4.3.3) from Rhodosporidium toruloides was designed, tested in vitro and found to efficiently oxidize D-glufosinate to its 2-oxo derivative [2-oxo-4-(methylphosphinyl)-butanoic acid]. Tobacco (Nicotiana tabacum) plants were transformed to express this modified oxidase under control of the TAP1 tapetum-specific promoter. A number of the resultant transgenic lines exhibited complete male sterility that persisted for two or more weeks immediately following foliar treatment with 75 or 200 g/ha of D-glufosinate without exhibiting obvious phytotoxic symptoms or any measurable decline in female fertility. Similarly, plants containing the same construct and, additionally, a PAT gene expressed from a plastocyanin promoter exhibited significantly reduced male fertility and no reduction in female fertility following foliar application of racemic glufosinate. Thus, foliar application of d-glufosinate either purified or as the commercial herbicide, combined with anther expression of a modified DAAO promises to provide a cost-effective conditional chemical male sterility system with the characteristics necessary for practical F₁ hybrid seed production.

New biotech applications from evolved D-amino acid oxidases

D-Amino acid oxidase (DAAO) is a well-known flavoenzyme that catalyzes the oxygen-dependent oxidative deamination of amino acid D-isomers with absolute stereospecificity, which results in α-keto acids, ammonia and hydrogen peroxide. Recently, the extraordinary functional plasticity of DAAO has become evident; in turn, boosting research on this flavoprotein. Protein engineering has allowed for a redesign of DAAO substrate specificity, oxygen affinity, cofactor binding, stability, and oligomeric state. We review recent developments in utilizing DAAO, including as a biocatalyst for resolving racemic amino acid mixtures, as a tool for biosensing, and as a new mechanism of herbicide resistance. Perspectives for future biotechnological applications of this oxidative biocatalyst are also outlined.

In planta function of compatible solute transporters of the AtProT family

The three proline transporters of Arabidopsis thaliana (AtProTs) transport the compatible solutes proline and glycine betaine and the stress-induced compound γ-aminobutyric acid when expressed in heterologous systems. The aim of the present study was to show transport and physiological relevance of these three AtProTs in planta. Using single, double, and triple knockout mutants and AtProT-overexpressing lines, proline content, growth on proline, transport of radiolabelled betaine, and expression of AtProT genes and enzymes of proline metabolism were analysed. AtProT2 was shown to facilitate uptake of L- and D-proline as well as [(14)C]glycine betaine in planta, indicating a role in the import of compatible solutes into the root. Toxic concentrations of L- and D-proline resulted in a drastic growth retardation of AtProT-overexpressing plants, demonstrating the need for a precise regulation of proline uptake and/or distribution. Furthermore evidence is provided that AtProT genes are highly expressed in tissues with elevated proline content--that is, pollen and leaf epidermis.

Aminoglycoside antibiotics: structure, functions and effects on in vitro plant culture and genetic transformation protocols

Plant transformation protocols generally involve the use of selectable marker genes for the screening of transgenic material. The bacterial gene nptII, coding for a neomycin phosphotransferase, and the hpt gene, coding for a hygromycin phosphotransferase, are frequently used. These enzymes detoxify aminoglycoside antibiotics by phosphorylation, thereby permitting cell growth in the presence of antibiotics. Nevertheless, the screening for transgenic regenerated shoots is often partial and difficult due to regeneration of escapes and chimeras. These difficulties can be caused, in part, by an incorrect assumption about the mode of action of antibiotics in bacterial and eukaryotic cells and in in vitro tissue culture. The information contained in this review could be useful to establish better selection strategies by taking into account factors such as explant complexity, transformation and selection protocols that allow better accessibility to cells of Agrobacterium and antibiotics, and faster regeneration methods that avoid collateral effects of antibiotics on recovered, putative transgenic shoots.

Amino acid homeostasis modulates salicylic acid-associated redox status and defense responses in Arabidopsis

The tight association between nitrogen status and pathogenesis has been broadly documented in plant-pathogen interactions. However, the interface between primary metabolism and disease responses remains largely unclear. Here, we show that knockout of a single amino acid transporter, LYSINE HISTIDINE TRANSPORTER1 (LHT1), is sufficient for Arabidopsis thaliana plants to confer a broad spectrum of disease resistance in a salicylic acid-dependent manner. We found that redox fine-tuning in photosynthetic cells was causally linked to the lht1 mutant-associated phenotypes. Furthermore, the enhanced resistance in lht1 could be attributed to a specific deficiency of its main physiological substrate, Gln, and not to a general nitrogen deficiency. Thus, by enabling nitrogen metabolism to moderate the cellular redox status, a plant primary metabolite, Gln, plays a crucial role in plant disease resistance.

Site-directed mutagenesis of rice serine racemase: evidence that Glu219 and Asp225 mediate the effects of Mg2+ on the activity

We succeeded in constructing the Glu219Ala/Asp225Ala (i.e., E219A/D225A) serine racemase (SerR) by site-directed mutagenesis, and the effects of Mg(2+) on the catalytic efficiency and the structure were compared between the E219A/D225A-SerR and the wild-type protein. This is the first example of a serine racemase whose amino acid residues in the Mg(2+)-binding site were replaced with other amino acids by site-directed mutagenesis. Neither the serine racemase nor the dehydratase activities of the E219A/D225A-SerR were affected by the addition of Mg(2+), and Glu219 and Asp225 of the SerR are the essential amino acid residues for Mg(2+) to affect both kinds of enzyme activities. Therefore, Glu219 and Asp225 mediate the effects of Mg(2+) on the activity and are important for the SerR to form the Mg(2+)-binding site. Judging from the difference of the K(eq) values between the E219A/D225A-SerR and the SerR, Mg(2+) might affect the equilibrium states in the racemase reaction. The fluorescence quenching analysis of the E219A/D225A-SerR showed that Mg(2+) bound to Glu219 and Asp225 of the SerR probably causes a conformational change in the ternary structure of the SerR.

Uptake and conversion of D-amino acids in Arabidopsis thaliana

The D-enantiomers of proteinogenic amino acids fulfill essential functions in bacteria, fungi and animals. Just in the plant kingdom, the metabolism and role of D-amino acids (D-AAs) still remains unclear, although plants have to cope with significant amounts of these compounds from microbial decay in the rhizosphere. To fill this gap of knowledge, we tested the inhibitory effects of D-AAs on plant growth and established a method to quantitate 16 out of 19 proteinogenic amino acids and their D-enantiomers in plant tissue extracts. Therefore, the amino acids in the extracts were derivatized with Marfey's reagent and separated by HPLC-MS. We used two ecotypes (Col-0 and C24) and a mutant (lht1) of the model plant Arabidopsis thaliana to determine the influence and fate of exogenously applied D-AAs. All of them were found in high concentrations in the plant extracts after application, even in lht1, which points to additional transporters facilitating the import of D-AAs. The addition of particular amino acids (D-Trp, D-Phe, D-Met and D-His) led to the accumulation of the corresponding L-amino acid. In almost all cases, the application of a D-AA resulted in the accumulation of D-Ala and D-Glu. The presented results indicate that soil borne D-AAs can actively be taken up and metabolized via central metabolic routes.

A novel mannose-based selection system for plant transformation using celery mannose-6-phosphate reductase gene

To investigate its potential application as a selectable marker for plant transformation, the mannitol producing, celery mannose-6-phosphate reductase gene (M6PR) was transformed into Arabidopsis and tobacco using Agrobacterium tumefaciens-mediated transformation. Mannose-tolerance assays in transgenic materials revealed that the M6PR can act as a selectable marker gene in either a positive or a negative selection mode depending on the plant species. For mannose sensitive species, such as Arabidopsis, expression of M6PR enhanced mannose tolerance and provided a positive selection for transgenic seeds. On medium containing 2 g/L mannose, transgenic seeds germinated, whereas wild type (WT) seeds did not. For mannose-tolerant species, expression of M6PR increased mannose sensitivity in tobacco and enabled a negative selection for transgenic leaves and seeds. Mannose at 30 g/L blanched leaf explants from all 29 transgenic tobacco events with M6PR. In contrast, 30 g/L mannose did not inhibit shoot regeneration from leaf explants of WT or transgenic plants with either an antisense M6PR or a plasmid control. Similarly, mannose at 30 g/L inhibited seed germination of transgenic tobacco seeds with M6PR but not that of WT or transgenic tobacco with either the antisense M6PR or the plasmid control. Northern blot confirmed transcripts of the M6PR in transgenic tobacco, and accumulation of mannitol verified activity of the M6PR in tobacco leaves. Either positive or negative selection using the celery M6PR is versatile for plant transformation. Additionally, the celery M6PR is a potential target gene for improving salt-tolerance in plants due to mannitol accumulation.

Regulation of the dauBAR operon and characterization of d-amino acid dehydrogenase DauA in arginine and lysine catabolism of Pseudomonas aeruginosa PAO1

A unique d-to-l racemization of arginine by coupled arginine dehydrogenases DauA and DauB encoded by the dauBAR operon has been recently reported as a prerequisite for d-arginine utilization as the sole source of carbon and nitrogen through l-arginine catabolic pathways in P. aeruginosa. In this study, enzymic properties of the catabolic FAD-dependent d-amino acid dehydrogenase DauA and the physiological functions of the dauBAR operon were further characterized with other d-amino acids. These results establish DauA as a d-amino acid dehydrogenase of broad substrate specificity, with d-Arg and d-Lys as the two most effective substrates, based on the kinetic parameters. In addition, expression of dauBAR is specifically induced by exogenous d-Arg and d-Lys, and mutations in the dauBAR operon affect utilization of these two amino acids alone. The function of DauR as a repressor in the control of the dauBAR operon was demonstrated by dauB promoter activity measurements in vivo and mobility shift assays with purified His-tagged protein in vitro. The potential effect of 2-ketoarginine (2-KA) derived from d-Arg deamination by DauA as a signal molecule in dauBAR induction was first revealed by mutation analysis and further supported by its in vitro effect on alleviation of DauR–DNA interactions. Through sequence analysis, putative DauR operators were identified and confirmed by mutation analysis. Induction of the dauBAR operon to the maximal level was found to require the l-arginine-responsive regulator ArgR, as supported by the loss of inductive effect by l-Arg on dauBAR expression in the argR mutant and binding of purified ArgR to the dauB regulatory region in vitro. In summary, this study establishes that optimal induction of the dauBAR operon requires relief of DauR repression by 2-KA and activation of ArgR by l-Arg as a result of d-Arg racemization by the encoded DauA and DauB.

Lysine racemase: a novel non-antibiotic selectable marker for plant transformation

A non-antibiotic based selection system using L-lysine as selection agent and the lysine racemase (lyr) as selectable marker gene for plant transformation was established in this study. L-lysine was toxic to plants, and converted by Lyr into D-lysine which would subsequently be used by the transgenic plants as nitrogen source. Transgenic tobacco and Arabidopsis plants were successfully recovered on L-lysine medium at efficiencies of 23 and 2.4%, respectively. Phenotypic characterization of transgenic plants clearly revealed the expression of normal growth and developmental characteristics as that of wild-type plants, suggesting no pleiotropic effects associated with the lyr gene. The specific activity of Lyr in transgenic tobacco plants selected on L: -lysine ranged from 0.77 to 1.06 mU/mg protein, whereas no activity was virtually detectable in the wild-type plants. In addition, the composition of the free amino acids, except aspartic acid, was not affected by the expression of the lyr gene in the transgenic tobacco plants suggesting very limited interference with endogenous amino acid metabolism. Interestingly, our findings also suggested that the plant aspartate kinases may possess an ability to distinguish the enantiomers of lysine for feedback regulation. To our knowledge, this is the first report to demonstrate that the lysine racemase selectable marker system is novel, less controversial and inexpensive than the traditional selection systems.

Nitrate and glutamate as environmental cues for behavioural responses in plant roots

As roots explore the soil, they encounter a complex and fluctuating environment in which the different edaphic resources (water and nutrients) are heterogeneously distributed in space and time. Many plant species are able to respond to this heterogeneity by modifying their root system development, such that they colonize the most resource-rich patches of soil. The complexities of these responses, and their dependence on the implied ability to perceive and integrate multiple external signals, would seem to amply justify the term 'behaviour'. This review will consider the types of behaviour that are elicited in roots of Arabidopsis thaliana by exposure to variations in the external concentrations and distribution of two different N compounds, nitrate and glutamate. Molecular genetic studies have revealed an intricate N regulatory network at the root tip that is responsible for orchestrating changes in root growth rate and root architecture to accommodate variations in the extrinsic and intrinsic supply of N. The review will discuss what is known of the genetic basis for these responses and speculate on their physiological and ecological significance.

Molecular cloning and expression in Escherichia coli of an active fused Zea mays L. D-amino acid oxidase

D-Amino acid oxidase (DAAO) is an FAD-dependent enzyme that metabolizes D-amino acids in microbes and animals. However, such ability has not been identified in plants so far. We predicted a complete DAAO coding sequence consisting of 1158 bp and encoding a protein of 386 amino acids. We cloned this sequence from the leaf cDNA population of maize plants that could utilize D-alanine as a nitrogen source and grow normally on media containing D-Ala at the concentrations of 100 and 1000 ppm. For more understanding of DAAO ability in maize plant, we produced a recombinant plasmid by the insertion of isolated cDNA into the pMALc2X Escherichia coli expression vector, downstream of the maltose-binding protein coding sequence. The pMALc2X-DAAO vector was used to transform the TB1 strain of E. coli cells. Under normal growth conditions, fused DAAO (with molecular weight of about 78 kDa) was expressed up to 5 mg/liter of bacterial cells. The expressed product was purified by affinity chromatography and subjected to in vitro DAAO activity assay in the presence of five different D-amino acids. Fused DAAO could oxidize D-alanine and D-aspartate, but not D-leucine, D-isoleucine, and D-serine. The cDNA sequence reported in this paper has been submitted to EMBL databases under accession number AM407717.