The major biotinyl protein from Pisum sativum seeds covalently binds biotin at a novel site

TMT proteomics analysis of a pseudocereal crop, quinoa (Chenopodium quinoa Willd.), during seed maturation

Quinoa (Chenopodium quinoa Willd.), an Andean native crop, is increasingly popular around the world due to its high nutritional content and stress tolerance. The production and the popularity of this strategic global food are greatly restricted by many limiting factors, such as seed pre-harvest sprouting, bitter saponin, etc. To solve these problems, the underlying mechanism of seed maturation in quinoa needs to be investigated. In this study, based on the investigation of morphological characteristics, a quantitative analysis of its global proteome was conducted using the combinational proteomics of tandem mass tag (TMT) labeling and parallel reaction monitoring (PRM). The proteome changes related to quinoa seed maturation conversion were monitored to aid its genetic improvement. Typical changes of morphological characteristics were discovered during seed maturation, including mean grain diameter, mean grain thickness, mean hundred-grain weight, palea, episperm color, etc. With TMT proteomics analysis, 581 differentially accumulated proteins (DAPs) were identified. Functional classification analysis and Gene Ontology enrichment analysis showed that most DAPs involved in photosynthesis were downregulated, indicating low levels of photosynthesis. DAPs that participated in glycolysis, such as glyceraldehyde-3-phosphate dehydrogenase, pyruvate decarboxylase, and alcohol dehydrogenase, were upregulated to fulfill the increasing requirement of energy consumption during maturation conversion. The storage proteins, such as globulins, legumins, vicilins, and oleosin, were also increased significantly during maturation conversion. Protein-protein interaction analysis and function annotation revealed that the upregulation of oleosin, oil body-associated proteins, and acyl-coenzyme A oxidase 2 resulted in the accumulation of oil in quinoa seeds. The downregulation of β-amyrin 28-oxidase was observed, indicating the decreasing saponin content, during maturation, which makes the quinoa "sweet". By the PRM and qRT-PCR analysis, the expression patterns of most selected DAPs were consistent with the result of TMT proteomics. Our study enhanced the understanding of the maturation conversion in quinoa. This might be the first and most important step toward the genetic improvement of quinoa.

Quantitative proteomics reveals dual effects of calcium on radicle protrusion in soybean

To reveal calcium-mediated germination in soybean, a gel-free/label-free proteomics was performed in radicle of seed imbibed with CaCl2. Morphological analysis presented promoting and suppressing performance of seed growth under 5 and 50 mM CaCl2, respectively. A total of 106 and 581 proteins were identified in response to 5 and 50 mM CaCl2, respectively. Among 33 proteins, which were simultaneously affected by 5 and 50 mM CaCl2 imbibition, proteins related to protein metabolism, cell, development, and stress showed reversed abundance in response to CaCl2 on dose-dependent manner. Notably, protein abundance of late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin decreased and increased by 5 and 50 mM CaCl2, respectively, consistent with the transcript level. Moreover, inhibited biosynthesis of gibberellic acid repressed growth of 5 mM CaCl2-imbibed soybean, while inhibition of abscisic acid biosynthesis released the suppressing effects of 50 mM CaCl2. Taken together, these results suggest that decreased or increased protein abundance of LEA4-5, LEA4, and dehydrin might determine promoting or suppressing effects of low or high level of calcium on soybean through enhancing seed sensitivity to gibberellic acid or abscisic acid during radicle protrusion. SIGNIFICANCE: Calcium serves as a versatile signal in plant growth; however, calcium-mediated germination on dose-dependent manner remains elusive. In this study, dual effects of calcium on radicle protrusion in soybean were investigated using proteomic approach. Radicle growth of germinating seed was improved by 5 mM CaCl2; however, it was retarded by 50 mM CaCl2. Late embryogenesis abundant (LEA) 4-5, LEA4, and dehydrin displayed converse profiles in response to low and high concentrations of CaCl2 at both protein abundance and gene expression level. Inhibited biosynthesis of gibberellic acid (GA) significantly impeded radicle protrusion in presence of low concentration of CaCl2, while inhibiting of abscisic acid (ABA) biosynthesis released suppression induced by high concentration of CaCl2. These findings suggest that LEA proteins are associated with calcium-mediated radicle protrusion on dose-dependent manner, and seed sensitivity to GA and ABA might determine promoting and suppressing effects of calcium on radicle protrusion in soybean.

Modification of the endogenous NO level influences apple embryos dormancy by alterations of nitrated and biotinylated protein patterns

NO donors and Arg remove dormancy of apple embryos and stimulate germination. Compounds lowering NO level (cPTIO, L -NAME, CAN) strengthen dormancy. Embryo transition from dormancy state to germination is linked to increased nitric oxide synthase (NOS)-like activity. Germination of embryos is associated with declined level of biotin containing proteins and nitrated proteins in soluble protein fraction of root axis. Pattern of nitrated proteins suggest that storage proteins are putative targets of nitration. Nitric oxide (NO) acts as a key regulatory factor in removal of seed dormancy and is a signal necessary for seed transition from dormant state into germination. Modulation of NO concentration in apple (Malus domestica Borkh.) embryos by NO fumigation, treatment with NO donor (S-nitroso-N-acetyl-D,L-penicillamine, SNAP), application of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), N ω-nitro-L-arginine methyl ester (L-NAME), canavanine (CAN) or arginine (Arg) allowed us to investigate the NO impact on seed dormancy status. Arg analogs and NO scavenger strengthened embryo dormancy by lowering reactive nitrogen species level in embryonic axes. This effect was accompanied by strong inhibition of NOS-like activity, without significant influence on tissue NO2 (-) concentration. Germination sensu stricto of apple embryos initiated by dormancy breakage via short term NO treatment or Arg supplementation were linked to a reduced level of biotinylated proteins in root axis. Decrease of total soluble nitrated proteins was observed at the termination of germination sensu stricto. Also modulation of NO tissue status leads to modification in nitrated protein pattern. Among protein bands that correspond to molecular mass of approximately 95 kDa, storage proteins (legumin A-like and seed biotin-containing protein) were identified, and can be considered as good markers for seed dormancy status. Moreover, pattern of nitrated proteins suggest that biotin containing proteins are also targets of nitration.

The Seed Biotinylated Protein of Soybean (Glycine max): A Boiling-Resistant New Allergen (Gly m 7) with the Capacity To Induce IgE-Mediated Allergic Responses

Soybean is a common allergenic food; thus, a comprehensive characterization of all the proteins that cause allergy is crucial to the development of effective diagnostic and immunotherapeutic strategies. A cDNA library was constructed from seven stages of developing soybean seeds to investigate candidate allergens. We searched the library for cDNAs encoding a seed-specific biotinylated protein (SBP) based on its allergenicity in boiled lentils. A full-length cDNA clone was retrieved and expressed as a 75.6-kDa His-tagged recombinant protein (rSBP) in Escherichia coli. Western immunoblotting of boiled bacterial extracts demonstrated specific IgE binding to rSBP, which was further purified by metal affinity and anion exchange chromatographies. Of the 23 allergic sera screened by ELISA, 12 contained IgEs specific to the purified rSBP. Circular dichroism spectroscopy revealed a predominantly unordered structure consistent with SBP's heat stability. The natural homologues (nSBP) were the main proteins isolated from soybean and peanut embryos after streptavidin affinity purification, yet they remained low-abundance proteins in the seed as confirmed by LC-MS/MS. Using capture ELISAs, the soybean and peanut nSBPs were bound by IgEs in 78 and 87% of the allergic sera tested. The soybean nSBP was purified to homogeneity and treatments with different denaturing agents before immunoblotting highlighted the diversity of its IgE epitopes. In vitro activation of basophils was assessed by flow cytometry in a cohort of peanut-allergic children sensitized to soybean. Stronger and more frequent (38%) activations were induced by nSBP-soy compared to the major soybean allergen, Gly m 5. SBPs may represent a novel class of biologically active legume allergens with the structural resilience to withstand many food-manufacturing processes.

Staying Alive: Molecular Aspects of Seed Longevity

Mature seeds are an ultimate physiological status that enables plants to endure extreme conditions such as high and low temperature, freezing and desiccation. Seed longevity, the period over which seed remains viable, is an important trait not only for plant adaptation to changing environments, but also, for example, for agriculture and conservation of biodiversity. Reduction of seed longevity is often associated with oxidation of cellular macromolecules such as nucleic acids, proteins and lipids. Seeds possess two main strategies to combat these stressful conditions: protection and repair. The protective mechanism includes the formation of glassy cytoplasm to reduce cellular metabolic activities and the production of antioxidants that prevent accumulation of oxidized macromolecules during seed storage. The repair system removes damage accumulated in DNA, RNA and proteins upon seed imbibition through enzymes such as DNA glycosylase and methionine sulfoxide reductase. In addition to longevity, dormancy is also an important adaptive trait that contributes to seed lifespan. Studies in Arabidopsis have shown that the seed-specific transcription factor ABSCISIC ACID-INSENSITIVE3 (ABI3) plays a central role in ABA-mediated seed dormancy and longevity. Seed longevity largely relies on the viability of embryos. Nevertheless, characterization of mutants with altered seed coat structure and constituents has demonstrated that although the maternally derived cell layers surrounding the embryos are dead, they have a significant impact on longevity.

Legume adaptation to sulfur deficiency revealed by comparing nutrient allocation and seed traits in Medicago truncatula

Reductions in sulfur dioxide emissions and the use of sulfur-free mineral fertilizers are decreasing soil sulfur levels and threaten the adequate fertilization of most crops. To provide knowledge regarding legume adaptation to sulfur restriction, we subjected Medicago truncatula, a model legume species, to sulfur deficiency at various developmental stages, and compared the yield, nutrient allocation and seed traits. This comparative analysis revealed that sulfur deficiency at the mid-vegetative stage decreased yield and altered the allocation of nitrogen and carbon to seeds, leading to reduced levels of major oligosaccharides in mature seeds, whose germination was dramatically affected. In contrast, during the reproductive period, sulfur deficiency had little influence on yield and nutrient allocation, but the seeds germinated slowly and were characterized by low levels of a biotinylated protein, a putative indicator of germination vigor that has not been previously related to sulfur nutrition. Significantly, plants deprived of sulfur at an intermediary stage (flowering) adapted well by remobilizing nutrients from source organs to seeds, ensuring adequate quantities of carbon and nitrogen in seeds. This efficient remobilization of photosynthates may be explained by vacuolar sulfate efflux to maintain leaf metabolism throughout reproductive growth, as suggested by transcript and metabolite profiling. The seeds from these plants, deprived of sulfur at the floral transition, contained normal levels of major oligosaccharides but their germination was delayed, consistent with low levels of sucrose and the glycolytic enzymes required to restart seed metabolism during imbibition. Overall, our findings provide an integrative view of the legume response to sulfur deficiency.

Proteomic analysis of embryonic axis of Pisum sativum seeds during germination and identification of proteins associated with loss of desiccation tolerance

Seed germination is an important stage in life cycle of higher plants. The germination processes and its associated loss of desiccation tolerance, however, are still poorly understood. In present study, pea seeds were used to study changes in embryonic axis proteome during germination by 2-DE and mass spectrometry. We identified a total of 139 protein spots showing a significant (>2-fold) change during germination. The results show that seed germination is not only the activation of a series of metabolic processes, but also involves reorganization of cellular structure and activation of protective systems. To uncouple the physiological processes of germination and its associated loss of desiccation tolerance, we used the fact that pea seeds have different desiccation tolerance when imbibed in water, CaCl(2) and methylviologen at the same germination stage. We compared the proteome amongst these seeds to identify the candidate proteins associated with the loss of desiccation tolerance and found a total of seven proteins - tubulin alpha-1 chain, seed biotin-containing protein SBP65, P54 protein, vicilin, vicilin-like antimicrobial peptides 2-3, convicilin and TCP-1/cpn60 chaperonin family protein. The metabolic function of these proteins indicates that seed desiccation tolerance is related to pathogen defense, protein conformation conservation and cell structure stabilization.

Seed germination and vigor

Germination vigor is driven by the ability of the plant embryo, embedded within the seed, to resume its metabolic activity in a coordinated and sequential manner. Studies using "-omics" approaches support the finding that a main contributor of seed germination success is the quality of the messenger RNAs stored during embryo maturation on the mother plant. In addition, proteostasis and DNA integrity play a major role in the germination phenotype. Because of its pivotal role in cell metabolism and its close relationships with hormone signaling pathways regulating seed germination, the sulfur amino acid metabolism pathway represents a key biochemical determinant of the commitment of the seed to initiate its development toward germination. This review highlights that germination vigor depends on multiple biochemical and molecular variables. Their characterization is expected to deliver new markers of seed quality that can be used in breeding programs and/or in biotechnological approaches to improve crop yields.

Proteome-wide characterization of seed aging in Arabidopsis: a comparison between artificial and natural aging protocols

A variety of mechanisms have been proposed to account for the extension of life span in seeds (seed longevity). In this work, we used Arabidopsis (Arabidopsis thaliana) seeds as a model and carried out differential proteomics to investigate this trait, which is of both ecological and agricultural importance. In our system based on a controlled deterioration treatment (CDT), we compared seed samples treated for different periods of time up to 7 d. Germination tests showed a progressive decrease of germination vigor depending on the duration of CDT. Proteomic analyses revealed that this loss in seed vigor can be accounted for by protein changes in the dry seeds and by an inability of the low-vigor seeds to display a normal proteome during germination. Furthermore, CDT strongly increased the extent of protein oxidation (carbonylation), which might induce a loss of functional properties of seed proteins and enzymes and/or enhance their susceptibility toward proteolysis. These results revealed essential mechanisms for seed vigor, such as translational capacity, mobilization of seed storage reserves, and detoxification efficiency. Finally, this work shows that similar molecular events accompany artificial and natural seed aging.

Biotin Sensing: Universal Influence of Biotin Status on Transcription

Although the role of biotin in metabolic reactions has long been recognized, its influence on transcription has only recently been discovered. A key protein in biotin-mediated transcription regulation is the biotin protein ligase, the enzyme responsible for catalyzing covalent linkage of the vitamin to biotin-dependent carboxylases. In the biotin regulatory system of Escherichia coli, the best characterized of the biotin-sensing systems, the biotin protein ligase functions both as the biotinylating enzyme and as a transcription repressor. Detailed mechanistic studies of this system are reviewed. In addition, recent studies have revealed other biotin-sensing systems in organisms ranging from bacteria to humans. These systems and the central role of the biotin protein ligase in each are also reviewed.

Plant biotin-containing carboxylases

Biotin-containing proteins are found in all forms of life, and they catalyze carboxylation, decarboxylation, or transcarboxylation reactions that are central to metabolism. In plants, five biotin-containing proteins have been characterized. Of these, four are catalysts, namely the two structurally distinct acetyl-CoA carboxylases (heteromeric and homomeric), 3-methylcrotonyl-CoA carboxylase and geranoyl-CoA carboxylase. In addition, plants contain a noncatalytic biotin protein that accumulates in seeds and is thought to play a role in storing biotin. Acetyl-CoA carboxylases generate two pools of malonyl-CoA, one in plastids that is the precursor for de novo fatty acid biosynthesis and the other in the cytosol that is the precursor for fatty acid elongation and a large number of secondary metabolites. 3-Methylcrotonyl-CoA carboxylase catalyzes a reaction in the mitochondrial pathway for leucine catabolism. The exact metabolic function of geranoyl-CoA carboxylase is as yet unknown, but it may be involved in isoprenoid metabolism. This minireview summarizes the recent developments in our understanding of the structure, regulation, and metabolic functions of these proteins in plants.

The role of biotin in regulating 3-methylcrotonyl-coenzyme a carboxylase expression in Arabidopsis

As a catalytic cofactor, biotin has a critical role in the enzymological mechanism of a number of enzymes that are essential in both catabolic and anabolic metabolic processes. In this study we demonstrate that biotin has additional non-catalytic functions in regulating gene expression in plants, which are biotin autotrophic organisms. Biotin controls expression of the biotin-containing enzyme, methylcrotonyl-coenzyme A (CoA) carboxylase by modulating the transcriptional, translational and/or posttranslational regulation of the expression of this enzyme. The bio1 mutant of Arabidopsis, which is blocked in the de novo biosynthesis of biotin, was used to experimentally alter the biotin status of this organism. In response to the bio1-associated depletion of biotin, the normally biotinylated A-subunit of methylcrotonyl-CoA carboxylase (MCCase) accumulates in its inactive apo-form, and both MCCase subunits hyperaccumulate. This hyperaccumulation occurs because the translation of each subunit mRNA is enhanced and/or because the each protein subunit becomes more stable. In addition, biotin affects the accumulation of distinct charge isoforms of MCCase. In contrast, in response to metabolic signals arising from the alteration in the carbon status of the organism, biotin modulates the transcription of the MCCase genes. These experiments reveal that in addition to its catalytic role as an enzyme cofactor, biotin has multiple roles in regulating gene expression.

Molecular characterization of a second copy of holocarboxylase synthetase gene (hcs2) in Arabidopsis thaliana

Holocarboxylase synthetase (HCS), catalyzing the covalent attachment of biotin, is ubiquitously represented in living organisms. Indeed, the biotinylation is a post-translational modification that allows the transformation of inactive biotin-dependent carboxylases, which are committed in fundamental metabolisms such as fatty acid synthesis, into their active holo form. Among other living organisms, plants present a peculiarly complex situation. In pea, HCS activity has been detected in three subcellular compartments and the systematic sequencing of the Arabidopsis genome revealed the occurrence of two hcs genes (hcs1 and hcs2). Hcs1 gene product had been previously characterized at molecular and biochemical levels. Here, by PCR amplification, we cloned an hcs2 cDNA from Arabidopsis thaliana (Ws ecotype) mRNA. We observed the occurrence of multiple cDNA forms which resulted from the alternative splicing of hcs2 mRNA. Furthermore, we evidenced a nucleotide polymorphism at the hcs2 gene within the Ws ecotype, which affected splicing of hcs2 mRNA. This contrasted sharply with the situation at hcs1 locus. However, this polymorphism had no apparent effect on total HCS activity in planta. Finally, hcs2 mRNAs were found 4-fold less abundant than hcs1 mRNA and the most abundant hcs2 mRNA spliced variant should code for a truncated protein. We discuss the possible role of such a multiplicity of putative HCS proteins in plants and discuss the involvement of each of hcs genes in the correct realization of biotinylation.

Proteomic analysis of arabidopsis seed germination and priming

To better understand seed germination, a complex developmental process, we developed a proteome analysis of the model plant Arabidopsis for which complete genome sequence is now available. Among about 1,300 total seed proteins resolved in two-dimensional gels, changes in the abundance (up- and down-regulation) of 74 proteins were observed during germination sensu stricto (i.e. prior to radicle emergence) and the radicle protrusion step. This approach was also used to analyze protein changes occurring during industrial seed pretreatments such as priming that accelerate seed germination and improve seedling uniformity. Several proteins were identified by matrix-assisted laser-desorption ionization time of flight mass spectrometry. Some of them had previously been shown to play a role during germination and/or priming in several plant species, a finding that underlines the usefulness of using Arabidopsis as a model system for molecular analysis of seed quality. Furthermore, the present study, carried out at the protein level, validates previous results obtained at the level of gene expression (e.g. from quantitation of differentially expressed mRNAs or analyses of promoter/reporter constructs). Finally, this approach revealed new proteins associated with the different phases of seed germination and priming. Some of them are involved either in the imbibition process of the seeds (such as an actin isoform or a WD-40 repeat protein) or in the seed dehydration process (e.g. cytosolic glyceraldehyde-3-phosphate dehydrogenase). These facts highlight the power of proteomics to unravel specific features of complex developmental processes such as germination and to detect protein markers that can be used to characterize seed vigor of commercial seed lots and to develop and monitor priming treatments.

Swinging arms and swinging domains in multifunctional enzymes: catalytic machines for multistep reactions

Multistep chemical reactions are increasingly seen as important in a growing number of complex biotransformations. Covalently attached prosthetic groups or swinging arms, and their associated protein domains, are essential to the mechanisms of active-site coupling and substrate channeling in a number of the multifunctional enzyme systems responsible. The protein domains, for which the posttranslational machinery in the cell is highly specific, are crucially important, contributing to the processes of molecular recognition that define and protect the substrates and the catalytic intermediates. The domains have novel folds and move by virtue of conformationally flexible linker regions that tether them to other components of their respective multienzyme complexes. Structural and mechanistic imperatives are becoming apparent as the assembly pathways and the coupling of multistep reactions catalyzed by these dauntingly complex molecular machines are unraveled.

Isolation and characterization of relevant allergens from boiled lentils

BACKGROUND

Lentils seem to be the most common legume implicated in pediatric allergic patients in the Mediterranean area. However, no lentil allergen has been isolated and characterized.

OBJECTIVE

We sought to purify and characterize relevant IgE-binding proteins from boiled lentil extracts.

METHODS

IgE-binding proteins from crude and boiled lentil extracts were detected with a pool of sera from patients with lentil allergy. Allergens were isolated by gel-filtration chromatography followed by cation- and anion-exchange chromatography or by reverse-phase HPLC. Their characterization included N-terminal amino acid sequencing, complex asparagine-linked glycan detection, specific IgE immunodetection with 22 individual sera from allergic patients, and immunoblot and CAP inhibition assays.

RESULTS

Heat treatment of lentils produced substantial changes in the SDS-PAGE patterns of whole extracts, mainly a strong increase of 12- to 16-kd bands and a decrease of 25- to 45-kd components. Major IgE-binding proteins from the boiled lentil extract were located in the 12- to 16-kd and 45- to 70-kd ranges. Two allergens of 16 kd, proteins L1 and L2, and another one of 12 kd, protein L3, were purified. N-terminal sequencing indicated that all 3 were related and allowed their identification as gamma-vicilin subunits. Protein L1 was recognized by 68% of the individual sera tested and inhibited 64% of the IgE binding by commercial lentil CAPs. A second type of allergen of 66 kd, named protein H, was also isolated and identified as a seed-specific biotinylated protein. Protein H reacted with 41% of the individual sera and produced 45% inhibition in CAP inhibition assays.

CONCLUSIONS

Two different types of allergens have been identified in boiled lentils. Those of 12 to 16 kd, called Len c 1, correspond to gamma-vicilin subunits, and those of 66 kd, designated Len c 2, correspond to seed-specific biotinylated protein. Homology with proteins from other legume species can explain potential cross-reactions among these foods.

BIOTINMETABOLISM INPLANTS

Biotin is an essential cofactor for a small number of enzymes involved mainly in the transfer of CO2 during HCO-3-dependent carboxylation reactions. This review highlights progress in plant biotin research by focusing on the four major areas of recent investigation: the structure, enzymology, and localization of two important biotinylated proteins (methylcrotonoyl-CoA carboxylase involved in the catabolism of leucine and noncyclic isoprenoids; acetyl-CoA carboxylase isoforms involved in a number of biosynthetic pathways); the biosynthesis of biotin; the biotinylation of biotin-dependent carboxylases, including the characterization of biotin holocarboxylase synthetase isoforms; and the detailed characterization of a novel, seed-specific biotinylated protein. A central challenge for plant biotin research is to determine in molecular terms how plant cells regulate the flow of biotin to sustain the biotinylation of biotin-dependent carboxylases during biosynthetic reactions.

Structure and selectivity in post-translational modification: attaching the biotinyl-lysine and lipoyl-lysine swinging arms in multifunctional enzymes

The post-translational attachment of biotin and lipoic acid to specific lysine residues displayed in protruding beta-turns in homologous biotinyl and lipoyl domains of their parent enzymes is catalysed by two different ligases. We have expressed in Escherichia coli a sub-gene encoding the biotinyl domain of E.coli acetyl-CoA carboxylase, and by a series of mutations converted the protein from the target for biotinylation to one for lipoylation, in vivo and in vitro. The biotinylating enzyme, biotinyl protein ligase (BPL), and the lipoylating enzyme, LplA, exhibited major differences in the recognition process. LplA accepted the highly conserved MKM motif that houses the target lysine residue in the biotinyl domain beta-turn, but was responsive to structural cues in the flanking beta-strands. BPL was much less sensitive to changes in these beta-strands, but could not biotinylate a lysine residue placed in the DKA motif characteristic of the lipoyl domain beta-turn. The presence of a further protruding thumb between the beta2 and beta3 strands in the wild-type biotinyl domain, which has no counterpart in the lipoyl domain, is sufficient to prevent aberrant lipoylation in E.coli. The structural basis of this discrimination contrasts with other forms of post-translational modification, where the sequence motif surrounding the target residue can be the principal determinant.

An embryo-defective mutant of arabidopsis disrupted in the final step of biotin synthesis

Auxotrophic mutants have played an important role in the genetic dissection of biosynthetic pathways in microorganisms. Equivalent mutants have been more difficult to identify in plants. The bio1 auxotroph of Arabidopsis thaliana was shown previously to be defective in the synthesis of the biotin precursor 7, 8-diaminopelargonic acid. A second biotin auxotroph of A. thaliana has now been identified. Arrested embryos from this bio2 mutant are defective in the final step of biotin synthesis, the conversion of dethiobiotin to biotin. This enzymatic reaction, catalyzed by the bioB product (biotin synthase) in Escherichia coli, has been studied extensively in plants and bacteria because it involves the unusual addition of sulfur to form a thiophene ring. Three lines of evidence indicate that bio2 is defective in biotin synthase production: mutant embryos are rescued by biotin but not dethiobiotin, the mutant allele maps to the same chromosomal location as the cloned biotin synthase gene, and gel-blot hybridizations and polymerase chain reaction amplifications revealed that homozygous mutant plants contain a deletion spanning the entire BIO2-coding region. Here we describe how the isolation and characterization of this null allele have provided valuable insights into biotin synthesis, auxotrophy, and gene redundancy in plants.

Complementation of an Arabidopsis thaliana biotin auxotroph with an Escherichia coli biotin biosynthetic gene

The Arabidopsis thaliana biotin auxotroph biol was rendered prototrophic by transformation with a chimeric transgene containing the Escherichia coli bio A gene driven by a constitutive promoter. The bio A gene encodes the biotin biosynthetic enzyme 7, 8-diaminopelargonic acid aminotransferase. Unlike the untransformed control plants, transgenic plants expressing the bacterial transgene synthesized biotin and grew to maturity without biotin-deficiency symptoms. These findings demonstrate that bio1/bio1 mutant plants are defective in the gene encoding 7,8-diaminopelargonic acid aminotransferase.

Co-purification, co-imniunoprecipitation, and coordinate expression of acetyl-coenzyme A carboxylase activity, biotin carboxylase, and biotin carboxyl carrier protein of higher plants

Acetyl-CoA carboxylase (ACCase; EC 6.4.1.2) is a regulatory enzyme of fatty acid synthesis, and in some higher-plant plastids is a multi-subunit complex consisting of biotin carboxylase (BC), biotin-carboxyl carrier protein (BCCP), and carboxyl transferase (CT). We recently described a Nicotiana tabacum L. (tobacco) cDNA with a deduced amino acid sequence similar to that of prokaryotic BC. We here provide further biochemical and immunological evidence that this higher-plant polypeptide is an authentic BC component of ACCase. The BC protein co-purified with ACCase activity and with BCCP during gel permeation chromatography of Pisum sativum L. (pea) chloroplast proteins. Antibodies to the Ricinus communis L. (castor) BC co-precipitated ACCase activity and BCCP. During castor seed development, ACCase activity and the levels of BC and BCCP increased and subsequently decreased in parallel, indicating their coordinate regulation. The BC protein comprised about 0.8% of the soluble protein in developing castor seed, and less than 0.05% of the protein in young leaf or root. Polypeptides cross-reacting with antibodies to castor BC were detected in several dicotyledons and in the monocotyledons Hemerocallis fulva L. (day lily), Iris L., and Allium cepa L. (onion), but not in the Gramineae species Hordeum vulgare L. (barley) and Panicum virgatum L. (switchgrass). The castor endosperm and pea chloroplast ACCases were not significantly inhibited by long-chain acyl-acyl carrier protein, free fatty acids or acyl carrier protein. The BC polypeptide was detected throughout Brassica napus L. (rapeseed) embryo development, in contrast to the multi-functional ACCase isoenzyme which was only detected early in development. These results firmly establish the identity of the BC polypeptide in plants and provide insight into the structure, regulation and roles of higherplant ACCases.