Quantitative analysis of the accumulation of Zein mRNA during maize endosperm development

Micronutrient and functional compounds biofortification of maize grains

Maize, in addition to being the main staple food in many countries, is used in the production of hundreds of products. It is rich in compounds with potential benefits to health, such as carotenoids, phenolic compounds, vitamin E, and minerals that act as cofactors for antioxidant enzymes. Many of these compounds have been neglected thus far in the scientific literature. Nevertheless, deficiencies in the precursors of vitamin A and some minerals, such as iron and zinc, in maize, in association with the great genetic variability in its cultivars and our genomic, transcriptomic, and metabolomic knowledge of this species make targeted biofortification strategies for maize promising. This review discusses the potential of the main microconstituents found in maize with a focus on studies aimed at biofortification.

Expression of H3N2 nucleoprotein in maize seeds and immunogenicity in mice

Oral administration of maize-expressed H3N2 nucleoprotein induced antibody responses in mice showing the immunogenicity of plant-derived antigen and its potential to be utilized as a universal flu vaccine. Influenza A viruses cause influenza epidemics that are devastating to humans and livestock. The vaccine for influenza needs to be reformulated every year to match the circulating strains due to virus mutation. Influenza virus nucleoprotein (NP) is a multifunctional RNA-binding protein that is highly conserved among strains, making it a potential candidate for a universal vaccine. In this study, the NP gene of H3N2 swine origin influenza virus was expressed in maize endosperm. Twelve transgenic maize lines were generated and analyzed for recombinant NP (rNP) expression. Transcript analysis showed the main accumulation of rNP in seed. Protein level of rNP in T1 transgenic maize seeds ranged from 8.0 to 35 µg of NP/g of corn seed. The level increased up to 70 µg of NP/g in T3 seeds. A mouse study was performed to test the immunogenicity of one line of maize-derived rNP (MNP). Mice were immunized with MNP in a prime-boost design. Oral gavage administration showed that a humoral immune response was elicited in the mice treated with MNP indicating the immunogenicity of MNP. NP-specific antibody responses in the MNP group showed comparable antibody titer with the groups receiving positive controls such as Vero cell-derived NP (VNP) or alphavirus replicon particle-derived NP (ANP). Cytokine analysis showed antigen-specific stimulation of IL-4 cytokine elicited in splenocytes from mice treated with MNP further confirming a TH2 humoral immune response induced by MNP administration.

Dynamic transcriptome landscape of maize embryo and endosperm development

Maize (Zea mays) is an excellent cereal model for research on seed development because of its relatively large size for both embryo and endosperm. Despite the importance of seed in agriculture, the genome-wide transcriptome pattern throughout seed development has not been well characterized. Using high-throughput RNA sequencing, we developed a spatiotemporal transcriptome atlas of B73 maize seed development based on 53 samples from fertilization to maturity for embryo, endosperm, and whole seed tissues. A total of 26,105 genes were found to be involved in programming seed development, including 1,614 transcription factors. Global comparisons of gene expression highlighted the fundamental transcriptomic reprogramming and the phases of development. Coexpression analysis provided further insight into the dynamic reprogramming of the transcriptome by revealing functional transitions during maturation. Combined with the published nonseed high-throughput RNA sequencing data, we identified 91 transcription factors and 1,167 other seed-specific genes, which should help elucidate key mechanisms and regulatory networks that underlie seed development. In addition, correlation of gene expression with the pattern of DNA methylation revealed that hypomethylation of the gene body region should be an important factor for the expressional activation of seed-specific genes, especially for extremely highly expressed genes such as zeins. This study provides a valuable resource for understanding the genetic control of seed development of monocotyledon plants.

Expression of wheat high molecular weight glutenin subunit 1Bx is affected by large insertions and deletions located in the upstream flanking sequences

To better understand the transcriptional regulation of high molecular weight glutenin subunit (HMW-GS) expression, we isolated four Glu-1Bx promoters from six wheat cultivars exhibiting diverse protein expression levels. The activities of the diverse Glu-1Bx promoters were tested and compared with β-glucuronidase (GUS) reporter fusions. Although all the full-length Glu-1Bx promoters showed endosperm-specific activities, the strongest GUS activity was observed with the 1Bx7OE promoter in both transient expression assays and stable transgenic rice lines. A 43 bp insertion in the 1Bx7OE promoter, which is absent in the 1Bx7 promoter, led to enhanced expression. Analysis of promoter deletion constructs confirmed that a 185 bp MITE (miniature inverted-repeat transposable element) in the 1Bx14 promoter had a weak positive effect on Glu-1Bx expression, and a 54 bp deletion in the 1Bx13 promoter reduced endosperm-specific activity. To investigate the effect of the 43 bp insertion in the 1Bx7OE promoter, a functional marker was developed to screen 505 Chinese varieties and 160 European varieties, and only 1Bx7-type varieties harboring the 43 bp insertion in their promoters showed similar overexpression patterns. Hence, the 1Bx7OE promoter should be important tool in crop genetic engineering as well as in molecular assisted breeding.

Ectopic expression of bacterial amylopullulanase enhances bioethanol production from maize grain

Heterologous expression of amylopullulanase in maize seeds leads to partial starch degradation into fermentable sugars, which enhances direct bioethanol production from maize grain. Utilization of maize in bioethanol industry in the United States reached ±13.3 billion gallons in 2012, most of which was derived from maize grain. Starch hydrolysis for bioethanol industry requires the addition of thermostable alpha amylase and amyloglucosidase (AMG) enzymes to break down the α-1,4 and α-1,6 glucosidic bonds of starch that limits the cost effectiveness of the process on an industrial scale due to its high cost. Transgenic plants expressing a thermostable starch-degrading enzyme can overcome this problem by omitting the addition of exogenous enzymes during the starch hydrolysis process. In this study, we generated transgenic maize plants expressing an amylopullulanase (APU) enzyme from the bacterium Thermoanaerobacter thermohydrosulfuricus. A truncated version of the dual functional APU (TrAPU) that possesses both alpha amylase and pullulanase activities was produced in maize endosperm tissue using a seed-specific promoter of 27-kD gamma zein. A number of analyses were performed at 85 °C, a temperature typically used for starch processing. Firstly, enzymatic assay and thin layer chromatography analysis showed direct starch hydrolysis into glucose. In addition, scanning electron microscopy illustrated porous and broken granules, suggesting starch autohydrolysis. Finally, bioethanol assay demonstrated that a 40.2 ± 2.63 % (14.7 ± 0.90 g ethanol per 100 g seed) maize starch to ethanol conversion was achieved from the TrAPU seeds. Conversion efficiency was improved to reach 90.5 % (33.1 ± 0.66 g ethanol per 100 g seed) when commercial amyloglucosidase was added after direct hydrolysis of TrAPU maize seeds. Our results provide evidence that enzymes for starch hydrolysis can be produced in maize seeds to enhance bioethanol production.

Expression of the cholera toxin B subunit (CT-B) in maize seeds and a combined mucosal treatment against cholera and traveler's diarrhea

The non-toxic B subunit (CT-B) of cholera toxin from Vibrio cholerae is a strong immunogen and amplifies the immune reaction to conjugated antigens. In this work, a synthetic gene encoding for CT-B was expressed under control of a γ-zein promoter in maize seeds. Levels of CT-B in maize plants were determined via ganglioside dependent ELISA. The highest expression level recorded in T(1) generation seeds was 0.0014% of total aqueous soluble protein (TASP). Expression level of the same event in the T(2) generation was significantly increased to 0.0197% of TASP. Immunogenicity of maize derived CT-B was evaluated in mice with an oral immunization trial. Anti-CTB IgG and anti-CTB IgA were detected in the sera and fecal samples of the orally immunized mice, respectively. The mice were protected against holotoxin challenge with CT. An additional group of mice was administrated with an equal amount (5 μg per dose each) of mixed maize-derived CT-B and LT-B (B subunit of E. coli heat labile toxin). In the sera and fecal samples obtained from this group, the specific antibody levels were enhanced compared to either the same or a higher amount of CT-B alone. These results suggest that a synergistic action may be achieved using a CT-B and LT-B mixture that can lead to a more efficacious combined vaccine to target diarrhea induced by both cholera and enterotoxigenic strains of Escherichia coli.

Delivery of prolamins to the protein storage vacuole in maize aleurone cells

Zeins, the prolamin storage proteins found in maize (Zea mays), accumulate in accretions called protein bodies inside the endoplasmic reticulum (ER) of starchy endosperm cells. We found that genes encoding zeins, α-globulin, and legumin-1 are transcribed not only in the starchy endosperm but also in aleurone cells. Unlike the starchy endosperm, aleurone cells accumulate these storage proteins inside protein storage vacuoles (PSVs) instead of the ER. Aleurone PSVs contain zein-rich protein inclusions, a matrix, and a large system of intravacuolar membranes. After being assembled in the ER, zeins are delivered to the aleurone PSVs in atypical prevacuolar compartments that seem to arise at least partially by autophagy and consist of multilayered membranes and engulfed cytoplasmic material. The zein-containing prevacuolar compartments are neither surrounded by a double membrane nor decorated by AUTOPHAGY RELATED8 protein, suggesting that they are not typical autophagosomes. The PSV matrix contains glycoproteins that are trafficked through a Golgi-multivesicular body (MVB) pathway. MVBs likely fuse with the multilayered, autophagic compartments before merging with the PSV. The presence of similar PSVs also containing prolamins and large systems of intravacuolar membranes in wheat (Triticum aestivum) and barley (Hordeum vulgare) starchy endosperm suggests that this trafficking mechanism may be common among cereals.

Maize: A Paramount Staple Crop in the Context of Global Nutrition

  The maize plant (Zea mays), characterized by an erect green stalk, is one of the 3 great grain crops of the world. Its kernels, like other seeds, are storage organs that contain essential components for plant growth and reproduction. Many of these kernel constituents, including starch, protein, and some micronutrients, are also required for human health. For this reason, and others, maize has become highly integrated into global agriculture, human diet, and cultural traditions. The nutritional quality and integrity of maize kernels are influenced by many factors including genetic background, environment, and kernel processing. Cooking procedures, including nixtamalization and fermentation, can increase accessibility of micronutrients such as niacin. However, man cannot live on maize alone. For one-third of the world's population, namely in sub-Saharan Africa, Southeast Asia, and Latin America, humans subsist on maize as a staple food but malnutrition pervades. Strategies to further improve kernel macronutrient and micronutrient quality and quantities are under intense investigation. The 2 most common routes to enhance grain nutritional value are exogenous and endogenous fortification. Although exogenous fortification, such as addition of multivitamin premixes to maize flour, has been successful, endogenous fortification, also known as "biofortification," may provide a more sustainable and practical solution for chronically undernourished communities. Recent accomplishments, such as low-phytate, high-lysine, and multivitamin maize varieties, have been created using novel genetic and agronomic approaches. Investigational studies related to biofortified maize are currently underway to determine nutrient absorption and efficacy related to human health improvement.

Seed-specific expression of the wheat puroindoline genes improves maize wet milling yields

The texture of maize (Zea mays L.) seeds is important to seed processing properties, and soft dent maize is preferred for both wet-milling and livestock feed applications. The puroindoline genes (Pina and Pinb) are the functional components of the wheat (Triticum aestivum L.) Hardness locus and together function to create soft grain texture in wheat. The PINs (PINA and PINB) are believed to act by binding to lipids on the surface of starch granules, preventing tight adhesion between starch granules and the surrounding protein matrix during seed maturation. Here, maize kernel structure and wet milling properties were successfully modified by the endosperm-specific expression of wheat Pins (Pina and Pinb). Pins were introduced into maize under the control of a maize gamma-Zein promoter. Three Pina/Pinb expression positive transgenic lines were evaluated over two growing seasons. Textural analysis of the maize seeds indicated that the expression of PINs decreased adhesion between starch and protein matrix and reduced maize grain hardness significantly. Reduction in pressure required to fracture kernels ranged from 15.65% to 36.86% compared with control seeds. Further, the PINs transgenic maize seeds had increased levels of extractable starch as characterized by a small scale wet milling method. Starch yield was increased by 4.86% on average without negatively impacting starch purity. The development of softer maize hybrids with higher starch extractability would be of value to maize processors.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.

Expressional profiling study revealed unique expressional patterns and dramatic expressional divergence of maize alpha-zein super gene family

The alpha-zein super gene family encodes the most predominant storage protein in maize (Zea mays) endosperm. In maize inbred line B73, it consists of four gene families with 41 member genes. In this study, we combined quantitative real-time PCR and random clone sequencing to successfully profile the expression of alpha-zein super gene family during endosperm development. We found that only 18 of the 41 member genes were expressed, and their expression levels diverge greatly. At the gene family level, all families had characteristic "up-and-down" oscillating expressional patterns that diverged into two major groups. At the individual gene level, member genes showed dramatic divergence of expression patterns, indicating fast differentiation of their expression regulation. A comparison study among different inbred lines revealed significantly different expressed gene sets, indicating the existence of highly diverged haplotypes. Large gene families containing long gene clusters, e.g. z1A or z1C, mainly contributed the highly divergent haplotypes. In addition, allelic genes also showed significant divergence in their expressional levels. These results indicated a highly dynamic and fast evolving nature to the maize alpha-zein super gene family, which might be a common feature for other large gene families.

Generation of transgenic maize with enhanced provitamin A content

Vitamin A deficiency (VAD) affects over 250 million people worldwide and is one of the most prevalent nutritional deficiencies in developing countries, resulting in significant socio-economic losses. Provitamin A carotenoids such as beta-carotene, are derived from plant foods and are a major source of vitamin A for the majority of the world's population. Several years of intense research has resulted in the production of 'Golden Rice 2' which contains sufficiently high levels of provitamin A carotenoids to combat VAD. In this report, the focus is on the generation of transgenic maize with enhanced provitamin A content in their kernels. Overexpression of the bacterial genes crtB (for phytoene synthase) and crtI (for the four desaturation steps of the carotenoid pathway catalysed by phytoene desaturase and zeta-carotene desaturase in plants), under the control of a 'super gamma-zein promoter' for endosperm-specific expression, resulted in an increase of total carotenoids of up to 34-fold with a preferential accumulation of beta-carotene in the maize endosperm. The levels attained approach those estimated to have a significant impact on the nutritional status of target populations in developing countries. The high beta-carotene trait was found to be reproducible over at least four generations. Gene expression analyses suggest that increased accumulation of beta-carotene is due to an up-regulation of the endogenous lycopene beta-cylase. These experiments set the stage for the design of transgenic approaches to generate provitamin A-rich maize that will help alleviate VAD.

The ethylene biosynthetic and perception machinery is differentially expressed during endosperm and embryo development in maize

The maize endosperm undergoes programmed cell death late in its development so that, with the exception of the aleurone layer, the tissue is dead by the time the kernel matures. Although ethylene is known to regulate the onset of endosperm cell death, the temporal and spatial control of the ethylene biosynthetic and perception machinery during maize endosperm development has not been examined. In this study, we report the isolation of the maize gene families for ACC synthase, ACC oxidase, the ethylene receptor, and EIN2 and EIL, which act downstream of the receptor. We show that ACC oxidase is expressed primarily in the endosperm, and only at low levels in the developing embryo late in its development. ACC synthase is expressed throughout endosperm development but, in contrast to ACC oxidase, it is transiently expressed to a significantly higher level in the developing embryo at a time that corresponds with the onset of endosperm cell death. Only two ethylene receptor gene families were identified in maize, in contrast to the five types previously identified in Arabidopsis. Members of both ethylene receptor families were expressed to substantially higher levels in the developing embryo than in the endosperm, as were members of the EIN2 and EIL gene families. These results suggest that the endosperm and embryo both contribute to the synthesis of ethylene, and they provide a basis for understanding why the developing endosperm is especially sensitive to ethylene-induced cell death while the embryo is protected.

Localization of a bacterial protein in starch granules of transgenic maize kernels

The B subunit of Escherichia coli heat labile enterotoxin (LT-B) is a potent oral immunogen with potential for use as a vaccine, a carrier molecule to deliver antigens to gut-associated lymphoid tissues, and possibly an adjuvant to make coadministered vaccines more effective. LT-B produced in plants was shown to be functional and immunogenic in animals and humans. In this work, we show that maize-derived LT-B is strongly associated with starch granules in endosperm. Using immunogold labeling/electron microscopy, cell fractionation, and protein analysis techniques, we observed that LT-B protein could be detected both internally and externally in starch granules. This strong association confers an effective copurification of the antigen with the starch fraction of maize kernels, thermostability desirable in maize processing, and resistance to peptic degradation in simulated gastric fluid digests, an important attribute for an orally delivered antigen.

Quantification of individual zein isoforms resolved by two-dimensional electrophoresis: genetic variability in 45 maize inbred lines

A two-dimensional (2-D) electrophoresis procedure was optimized to obtain well-resolved and reproducible patterns of zein polypeptides in maize. All zein isoforms obtained through zein-specific extraction were observed from whole meal extracted with a urea/Triton/2-mercaptoethanol solution. Loading the protein samples at the acidic side of the gradient, using an amino acid solution as catholyte and running for a short time period under high Vh reduced shrinking and instability at the basic side of the isoelectric focusing (IEF) gels. Good quality and reproducible 2-D patterns were thus obtained, allowing automatic spot quantification. A linear relationship between spot intensity and zein isoform amount was established for 20 of 22 zeins detected in a 5.5-8.5 pH range using colloidal Coomassie blue staining in one maize line. The analysis of 45 genetically diverse inbred lines allowed the detection of 59 isoforms belonging to the four classes of zeins, and revealed a large qualitative and quantitative variability of individual isoforms. The classical decrease in zein amount in o2 mutant genotype was observed, and could be quantified for every isoform. The improved technique will be useful to dissect the genetic control of zein expression in maize.

The bifactorial endosperm box of gamma-zein gene: characterisation and function of the Pb3 and GZM cis-acting elements

The proximal region of the gamma-zein promoter (gamma Z) has a functional bifactorial prolamin box element containing two cis-acting elements, a prolamin-box motif (Pb3) and a GCN4-like motif (GZM). By particle bombardment of maize endosperms with 5' deletions and internal deletions of gamma Z fused to the GUS gene, we have shown that a 135 bp region containing the bifactorial element is involved in the transcriptional activation of the gamma Z promoter. However, the 135 bp region was unable to activate the gamma Z promoter in the absence of a 84 bp downstream sequence. Using in vivo footprinting and gel mobility shift assays with 15 DAP endosperm nuclear extracts, we have demonstrated the presence of trans-acting factors that interact with Pb3 and GZM target sites. Base-substitution mutations within Pb3 and GZM decreased transcription activity of the gamma Z promoter suggesting a co-ordinated function between the two cis-acting elements. Two additional cis-motifs upstream of the bifactorial prolamin element have been identified: a motif with high homology to the AACA elements of rice glutelin genes and an AZM motif containing an ACGT core which binds nuclear proteins other than the Opaque 2 (O2).

Analysis of kafirin promoter activity in transgenic tobacco seeds

Sequences corresponding to 855 bp of 5' promoter region and the transit peptide from lambdaGK.1,a genomic clone encoding a 22 kDa alpha-kafirin seed protein from sorghum, were translationally fused to a cloned beta-glucuronidase (GUS) coding sequence from uidA and transferred to tobacco via Agrobacterium tumefaciens-mediated transformation. No GUS expression was detectable at any stage of growth in stems or leaves of these plants. However, GUS expression was detected in both embryo and endosperm tissues of resulting tobacco seeds 10-15 days after flowering. Dissected tissues indicate endosperm expression was localized within the bulk endosperm and not within the parenchyma cell layer underlying the integument. These studies also demonstrate that within dissected tobacco embryos, expression from the kafirin promoter was restricted to the mesocotyl region.

Differential stability of zein mRNA in developing corn kernel

The lifetime of the zein mRNA in a developing corn (Zea mays L.) kernel under genome transcription blockade with actinomycin D (in vivo) and in a cell-free system (in vitro) was studied. After a 10 h blockade of gene transcription with actinomycin D, only 55% of 19 kDa zein mRNA and 40% of 22 kDa mRNA were detected in a developing kernel of normal corn. In that of the opaque-2 mutant 80% of 19 kDa zein mRNA remained. To examine the relative stability of poly(A)-containing mRNA, cell-free systems from rabbit reticulocyte lysate and wheat-germ extract were used. In both cases only 40% of 19 kDa zein mRNA and 60% of 22 kDa zein mRNA decayed during a 30 min incubation. Differential mRNA degradation of poly(A)-containing zein mRNA was observed on affinity chromatography; poly(A)-containing 19 kDa zein mRNA from normal corn partially decayed by elution from poly(U)-Sepharose whereas that from opaque-2 remained stable. These data suggest that differential mRNA stability is an important factor in the regulation of the zein gene expression in a developing corn kernel.

The involvement of Opaque 2 on beta-prolamin gene regulation in maize and Coix suggests a more general role for this transcriptional activator

The maize opaque 2 (o2) mutation is known to have numerous pleiotropic effects. Some polypeptides have their expression depressed while others are enhanced. The best characterized effects of the o2 mutation are those exerted on endosperm genes encoding the storage protein class of the 22 kDa alpha-zeins and the ribosome inactivating protein b-32. The Opaque 2 (O2) locus encodes a basic domain-leucine zipper DNA-binding factor, O2, which transcriptionally regulates these genes. In the maize-related grass Coix lacryma-jobi, an O2-homologous protein regulates the 25 kDa alpha-coixin family. We show in this paper that O2 transcriptionally regulates the structurally and developmentally different class of the beta-prolamins. A new O2-binding box was identified in beta-prolamin genes from maize and Coix that, together with the boxes previously identified in other endosperm expressed genes, forms a curious collection of O2 cis elements. This may have regulatory implications on the role of O2 in the mechanism that controls coordinated gene expression in the developing endosperm. Considering that the O2 locus controls at least three distinct classes of genes in maize endosperm, we propose that the O2 protein may play a more general role in maize endosperm development than previously conceived.

Functional expression of the transcriptional activator Opaque-2 of Zea mays in transformed yeast

The aim of this research was to determine whether the structural homology between the O2 gene, a maize transcriptional activator, and the GCN4 gene, a yeast transcriptional factor, is reflected at the level of function. The O2 cDNA was cloned in the yeast expression vector pEMBLyex4 under the control of a hybrid inducible promoter, and used to transform the yeast Saccharomyces cerevisiae. Transformed yeast cells produced O2 mRNA and a polypeptide immunoreactive with anti-O2 antibodies during growth in galactose. The heterologous protein was correctly translocated into the yeast nuclei, as demonstrated by immunofluorescence, indicating that the nuclear targeting sequences of maize are recognized by yeast cells. Further experiments demonstrated the ability of O2 to rescue a gcn4 mutant grown in the presence of aminotriazole, an inhibitor of the HIS3 gene product, suggesting that O2 activates the HIS3 gene, gene normally under control of GCN4. It was shown that the O2 protein is able to trans-activate the HIS4 promoter in yeast cells and binds to it in vitro. The sequence protected by O2, TGACTC, is also the binding site for GCN4. Finally, the expression of O2 protein in yeast did not produce alterations during batch growth at 30 degrees C, while transformants expressing O2 protein showed a conditionally lethal phenotype when grown in galactose at 36 degrees C; this phenotype mimics the behaviour of gcd mutants. The results support the idea that basic mechanisms of transcription control have been highly conserved in eukaryotes.

Genomic organization of an alpha-zein gene cluster in maize

The genes encoding the alpha-zein proteins of maize constitute a large multigene family of some 75 genes. This multigene family can be divided into four subfamilies based on the nucleotide sequences of their genes and the deduced amino acid sequences of their proteins. We describe for the first time evidence of a clustering of five alpha-zein subfamily 4 (SF4) genes that are members of one of the major alpha-zein subfamilies in a 56 kb region of the genome of the maize inbred line W22. None of the other three known alpha-zein gene subfamilies (SF1, SF2, or SF3) are present in this cluster. The genomic region was reconstructed using restriction endonuclease maps to identify and align three overlapping cosmid clones isolated from a genomic library. The alpha-zein genes are not evenly spaced; the minimum distance between genes is 3.5 kb; the maximum is 13 kb. All the alpha-zein genes in the cluster have the same transcriptional orientation. The location and sequences of some of the repetitive DNA elements in this gene cluster were determined. We estimate that there are a minimum of eight repetitive DNA elements in this region. The sequences of the repetitive elements (not functionally defined) are located between or among the alpha-zein genes. The regions containing two of these repetitive elements (Rep1 and Rep4) have been sequenced; they are about 15 kb apart in the genome. These repetitive elements have similar sequences for about 300 bp out of the 400 bp compared. The regions of sequence similarity, however, are in reverse orientation to one another.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the structure and transcription of an ubiquitin fusion gene from maize

We have identified a maize ubiquitin (Ubi) fusion gene (UBF9) by screening a maize W22 genomic phage lambda library with a short (16-nucleotide) oligodeoxyribonucleotide probe derived from the sequence for the extension sequence of a yeast UB13 fusion gene. UBF9 consists of an UB monomer sequence (228 bp long) joined to an extension sequence (237 bp long). The extension sequence encodes a protein of 79 amino acids which shares extensive identity with similar extension aa sequences found in yeast, humans, barley and Arabidopsis thaliana. UBF9 encodes a small-size class of Ubi mRNAs in the maize tissues investigated. The UBF9 transcript is present in high levels in maize endosperm tissues 22 days after pollination. Genomic Southern blots of maize inbred W22 DNA indicate that the fusion gene sequences are present in multiple copies in the maize genome. Primer extension experiments indicate that the transcription start point is located at 80 bp upstream from the translation start codon of UBF9. Two 37-bp tandem repeated A + T-rich sequences are found in the 5'-flanking region of UBF9. The A + T-rich sequences share the motif, AATATTTTATT, which is present in a diverse set of plant genes.

A homologous expression system for cloned zein genes

Expression of the genes encoding the 10-, 15-, and 27-kDa zeins is maintained in suspension cultures derived from developing endosperm tissue of maize (Zea mays L.). Although expression of these genes is reduced in endosperm cultures as compared with that in endosperm tissue from developing kernels, it remains specific to the origin of explant, since no transcripts are detected in leaf tissue-derived suspension cultures. Transcript sizes are identical to those in developing seed endosperm tissue. Furthermore, accurate transcription initiation of the 10- and 27-kDa zein genes is observed by S1 nuclease mapping. Protoplasts isolated from endosperm cultures are capable of expressing foreign genes when transfected by electroporation. We demonstrate that the 5' flanking sequences of the 10- and 27-kDa zein genes are capable of promoting chloramphenicol acetyltransferase (CAT) gene expression in these transfected protoplasts. Our observations show that these maize endosperm cultures can be used as an efficient homologous system to study transcriptional regulation of zein genes.

Post-transcriptional regulation of methionine content in maize kernels

Message levels for a methionine-rich 10 kDa zein were determined in three inbred lines of maize and their reciprocal crosses at various stages during endosperm development. Inbred line BSSS-53, which overexpresses the 10 kDa protein in mature kernels, was shown to have higher mRNA levels in developing endosperm, as compared to inbred lines W23 and W64A. Differences in mRNA levels could not be explained by differences in transcription rate of the 10 kDa zein gene, indicating differential post-transcriptional regulation of this storage protein in the different inbred lines analyzed. Among progeny segregating for the BSSS-53 allele of the 10 kDa zein structural gene Zps10/(22), mRNA levels are independent of Zps10/(22) segregation, indicating that post-transcriptional regulation of mRNA levels takes place via a trans-acting mechanism. In the same progeny, mRNA levels are also independent of allelic segregation of the regulatory locus Zpr10/(22). Thus, the trans-acting factor encoded by Zpr10/(22) determines accumulation of 10 kDa zein at a translational or post-translational step. Multiple trans-acting factors are therefore involved in post-transcriptional regulation of the methionine-rich 10 kDa zein.

Characterization of the kafirin gene family from sorghum reveals extensive homology with zein from maize

Electrophoretic analysis of translation products of polyadenylated RNA isolated from mid-maturation sorghum seed in the presence of [(35)S]met, [(3)H]leu, or [(3)H]val revealed two major proteins of kDa and 21 kDa. These products were not detected when [(3)H]lys was supplied as the radioactive substrate. Under similar electrophoretic conditions, kafirin (a major seed storage prolamin of sorghum), migrated as two bands of 22 kDa and 19 kDa. Sequence analysis of two cDNA clones (pSK8 and pSKR2) from sorghum seed mRNA revealed them to be highly homologous with each other and to the 22 kDa zeins from maize, suggesting that they represented kafirin cDNAs. Compared with pSKR2, pSK8 had an insertion of 24 nucleotides and a deletion of 24 nucleotides, so that the coding regions were nearly identical in length. The deduced amino acid sequence for these cDNA clones reveals that kafirin, like zein, is rich in glutamine and nonpolar amino acids, but contains no lysine. Both kafirin and zein have a 21 amino acid signal peptide exhibiting 80% homology and eight copies of a repetitive amino acid block in the C-terminal domain with the consensus: infI (supP) LL finP (supA) LN infQ (supP) LALANPAAYLQQQQ.The kafirin cDNAs were used as probes to screen a sorghum genomic library; one genomic clone (λGK.1) was sequenced and found to be very similar (97.8%) to the pSK8 cDNA clone. Clone λGK.1 contains features typical for a functional gene in that the intronless open reading frame encoding 268 amino acids is flanked at the 5' end by sequences corresponding to the CAAT and TATA promoter boxes (positioned at about -60 and -30 bp, respectively, from the transcriptional initiation site), and at the 3' end by a consensus polyadenylation signal. In common with zein genomic clones, kafirin clones contain a 15 basepair consensus sequence centered at postion -320 relative to the transcriptional initiation site. Under similar hybridization conditions, genomic reconstruction analysis using an oligonucleotide probe indicated the presence of less than 20 copies of kafirin per haploid sorghum genome compared with approximatley 140 copies of zein per haploid maize genome.

Aggregation of lysine-containing zeins into protein bodies in Xenopus oocytes

Zeins, the storage proteins of maize, are totally lacking in the essential amino acids lysine and tryptophan. Lysine codons and lysine- and tryptophan-encoding oligonucleotides were introduced at several positions into a 19-kilodalton zein complementary DNA by oligonucleotide-mediated mutagenesis. A 450-base pair open reading frame from a simian virus 40 (SV40) coat protein was also engineered into the zein coding region. Messenger RNAs for the modified zeins were synthesized in vitro with an SP6 RNA polymerase system and injected into Xenopus laevis oocytes. The modifications did not affect the translation, signal peptide cleavage, or stability of the zeins. The ability of the modified zeins to assemble into structures similar to maize protein bodies was assayed by two criteria: assembly into membrane-bound vesicles resistant to exogenously added protease, and ability to self-aggregate into dense structures. All of the modified zeins were membrane-bound; only the one containing a 17-kilodalton SV40 protein fragment was unable to aggregate. These findings suggest that it may be possible to create high-lysine corn by genetic engineering.

DNA methylation and tissue-specific transcription of the storage protein genes of maize

We investigated the methylation state of a set of storage protein genes of maize, coding for zeins and glutelins, in different somatic tissues and in developing endosperms. These genes, present as multigene families in the maize genome and organized in clusters on different chromosomes, are coordinately and specifically transcribed only in endosperm cells. Southern blot analysis of DNA digested with methylation-sensitive restriction enzymes shows a specific and extensive undermethylation of zein and glutelin sequences in the endosperm, while a common methylated pattern is detected in the different somatic tissues and in the embryo. However, a constant fraction of endosperm DNA (∼35%) is methylated at all zein sequences, which are found to be heavily modified in pollen DNA as well. Undermethylation is extended along a zein cluster and cannot be explained by reduced levels of 5-methylcytosine in endosperm DNA with respect to other tissues. The undermethylated state of storage protein genes is already established at an early stage of endosperm development, when transcripts levels for both genes are almost undetectable.

Differential expression of a gene for a methionine-rich storage protein in maize

A methionine-rich 10 kDa zein storage protein from maize was isolated and the sequence of the N-terminal 30 amino acids was determined. Based on the amino acid sequence, two mixed oligonucleotides were synthesized and used to probe a maize endosperm cDNA library. A full-length cDNA clone encoding the 10 kDa zein was isolated by this procedure. The nucleotide sequence of the cDNA clone predicts a polypeptide of 129 amino acids, preceded by a signal peptide of 21 amino acids. The predicted polypeptide is unique in its extremely high content of methionine (22.5%). The maize inbred line BSSS-53, which has increased seed methionine due to overproduction of this protein, was compared to W23, a standard inbred line. Northern blot analysis showed that the relative RNA levels for the 10 kDa zein were enhanced in developing seeds of BSSS-53, providing a molecular basis for the overproduction of the protein. Southern blot analysis indicated that there are one or two 10 kDa zein genes in the maize genome.

Transposon tagging and molecular analysis of the maize regulatory locus opaque-2

Genetic analyses suggested that the opaque-2 (o2) locus in maize acts as a positive, transacting, transcriptional activator of the zein seed storage-protein genes. Because isolation of the gene is requisite to understanding the molecular details of this regulation, transposon mutagenesis with the transposable element suppressor-mutator (Spm) was carried out, and three mutable o2 alleles were obtained. One of these alleles contained an 8.3-kilobase autonomous Spm, another a 6.8-kilobase nonautonomous Spm, and the third an unidentified transposon that is unrelated to Spm. A DNA sequence flanking the autonomous Spm insertion was verified to be o2-specific and provided a probe to clone a wild-type allele. Northern blots indicated that the gene is expressed in wild-type endosperm but not in leaf tissues or in endosperms homozygous for a mutant allele of the O2 gene. A transcript was detected in endosperms homozygous for mutations at opaque-7 and floury-2, an indication that O2 expression is independent of these two other putative regulators of zein synthesis.

Structural and transcriptional analysis of DNA sequences flanking genes that encode 19 kilodalton zeins

The nucleotide sequence of gz19ab11, a clone that corresponds to the coding and flanking sequences of an Mr 19,000 alpha zein, was determined. Comparison of the DNA sequences flanking this gene with those of other members of the gene subfamily showed that sequence conservation extends 820 nucleotides into the 5' flanking region and 130 nucleotides into the 3' flanking region. Southern blot analysis of maize DNA indicated that highly repetitive sequences are located within 950 bp 5' and 300 bp 3' to the protein coding region of these genes. The coding region of gz19ab11 is similar to but not identical with cDNA clones corresponding to Mr 19,000 zeins, and analysis of zein transcripts indicated that this gene is expressed exclusively in endosperm tissue. RNAs which correspond to transcripts originating 60 nucleotides, and more than 800 nucleotides, upstream of the initiation codon were detected for this and a related gene. However, the concentration of the large RNA species was several orders of magnitude less than that of the shorter RNAs. The functional significance of these large RNA transcripts in zein gene expression is unclear.

Specific transcription of a 15-kilodalton zene gene in HeLa cell extracts

A maize genomic clone containing a 15 kilodalton zein gene was used as a template in an in vitro transcription system for HeLa cells. A runoff assay indicated transcription was initiating 5' to the map position of the open reading frame for the protein. Fine-structure mapping of RNAs synthesized in vitro showed two transcription start sites separated by 24 bases. One start site is 27 bases downstream of a consensus TATA sequence; the other is 30 bases downstream of a TATG sequences. The initiation sites for RNA synthesized in vitro map to the same region of the genomic clone as zein RNA isolated from developing maize kernels.