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

Applications of Prolamin-Based Edible Coatings in Food Preservation: A Review

Foods are susceptible to deterioration and sour due to external environmental influences during production and storage. Coating can form a layer of physical barrier on the surface of foods to achieve the purpose of food preservation. Because of its good barrier properties and biocompatibility, prolamin-based film has been valued as a new green and environment-friendly material in the application of food preservation. Single prolamin-based film has weaknesses of poor toughness and stability, and it is necessary to select appropriate modification methods to improve the performance of film according to the application requirements. The practical application effect of film is not only affected by the raw materials and the properties of the film itself, but also affected by the selection of preparation methods and processing techniques of film-forming liquid. In this review, the properties and selection of prolamins, the forming mechanisms and processes of prolamin-based coatings, the coating techniques, and the modifications of prolamin-based coatings were systematically introduced from the perspective of food coating applications. Moreover, the defects and deficiencies in the research and development of prolamin-based coatings were also reviewed in order to provide a reference for the follow-up research on the application of prolamin-based coatings in food preservation.

Structure, Organization, and Expression of the Alpha Prolamin Multigenic Family Bring New Insights into the Evolutionary Relationships among Grasses

Prolamins are the major seed storage proteins of grasses. In maize and related species, prolamins are classified into α-, β-, γ-, and δ-subclasses by their solubility properties. α-prolamins are encoded by multigene families and have a secondary structure that consists of tandem α-helix repeats. Maize has two α-prolamin subclasses, namely the 19 and 22 kDa subclasses that contain nine and 10 α-helix repeats, respectively. Here, we present an evolutionary study based on the structure, organization, and expression of α-prolamins in maize, sugarcane, sorghum, and coix. True 22 kDa subclasses containing 10 repeats are conserved in all four species, but true 19 kDa subclasses containing nine repeats are found only in maize and sugarcane. We discovered a 19 kDa-like α-coixin that, as in sorghum, is encoded by few genes. These data suggest that a 19 kDa progenitor present in the ancestor common to maize, coix, sorghum, and sugarcane was preserved at low copy number in coix and sorghum, while amplified into multigene family architecture in maize and sugarcane. The expression profiling of α-prolamins, verified by two-dimensional gels, showed highly conserved multispot composition for the 19 kDa α-prolamins in maize and sugarcane. Coix and sorghum did not present true 19 kDa α-prolamin spots. Our data show remarkable similarity between maize and sugarcane 19 kDa α-prolamins regarding both gene structure and expression. Since the multigene architecture of 19 kDa α-canein appeared after sugarcane diverged from sorghum, our data suggest that maize and sugarcane might have acquired the multigene family encoding these storage proteins from a common ancestor.

Structure, morphology, and assembly behavior of kafirin

Prolamins from grains have attracted intensive attention in recent years due to their potential in satisfying the demand for environmentally friendly (biodegradable), abundantly available (sustainable), and cost-effective biomaterials. However, for kafirin, the prolamin from sorghum, its composition, structure, morphology, and self-assembly behaviors have not been fully characterized. In this paper, kafirin was extracted from the whole sorghum grain and found to contain 68, 14, 6, and 12% of α-, β-, and γ-fractions and cross-linked kafirin, respectively. Freeze-dried kafirin contained ∼49% α-helix in the solid state. When dissolved in 65% (v/v) isopropanol, 60% (v/v) tert-butanol, and 85% (v/v) ethanol aqueous solvents, the relative α-helix content in kafirin increased with the decrease of solvent polarity. Structural analysis using small-angle X-ray scattering (SAXS) indicated that kafirin (2 mg/mL) took stretched and extended conformations with dimensions of 118 × 15 × 15 and 100 × 11 × 11 Å in 60% tert-butanol and 65% isopropanol, respectively. More elongated conformation of individual kafirin with high-order assembly was observed in 85% ethanol. Protein aggregation occurred as protein concentration increased in its good solvent. The morphology of kafirin assemblies captured by atomic force microscopy (AFM) revealed that kafirin protein took uniform particle morphology at low concentration, and disk-like or rod-like structures resulting from solvent evaporation induced particle interactions emerged at high concentrations. These results suggest that both protein concentration and solvent polarity can effectively regulate kafirin assemblies from thick rod-like to slim rod-like structures, a convenient way to tune the fibrillation of prolamin-based biomaterials.

A maize α-zein promoter drives an endosperm-specific expression of transgene in rice

An alpha-zein promoter isolated from maize containing P-box, E motif sequence TGTAAAGT, opaque-2 box and TATA box was studied for its tissue-specific expression in rice. A 1,098 bp promoter region of alpha-zein gene, fused to the upstream of gusA reporter gene was used for transforming rice immature embryos (ASD 16 or IR 64) via the particle bombardment-mediated method. PCR analysis of putative transformants demonstrated the presence of transgenes (the zein promoter, gusA and hpt). Nineteen out of 37 and two out of five events generated from ASD 16 and IR 64 were found to be GUS-positive. A histological staining analysis performed on sections of mature T1 seeds revealed that the GUS expression was limited to the endosperm and not to the pericarp or the endothelial region. GUS expression was observed only in the following seed development stages : milky (14-15 DAF), soft dough (17-18 DAF), hard dough (20-23 DAF), and mature stages (28-30 DAF) of zein-gusA transformed (T0) plants. On the contrary a constitutive expression of GUS was evident in CaMV35S-gusA plants. PCR and Southern blotting analyses on T1 plants demonstrated a stable integration and inheritance of transgene in the subsequent T1 generation. GUS assay on T2 seeds revealed that the expression of gusA gene driven by alpha-zein promoter was stable and tissue-specific over two generations. Results suggest that this alpha-zein promoter could serve as an alternative promoter to drive endosperm-specific expression of transgenes in rice and other cereal transformation experiments.

Modulation of kernel storage proteins in grain sorghum (Sorghum bicolor (L.) Moench)

Sorghum prolamins, termed kafirins, are categorized into subgroups α, β, and γ. The kafirins are co-translationally translocated to the endoplasmic reticulum (ER) where they are assembled into discrete protein bodies that tend to be poorly digestible with low functionality in food and feed applications. As a means to address the issues surrounding functionality and digestibility in sorghum, we employed a biotechnology approach that is designed to alter protein body structure, with the concomitant synthesis of a co-protein in the endosperm fraction of the grain. Wherein perturbation of protein body architecture may provide a route to impact digestibility by reducing disulphide bonds about the periphery of the body, while synthesis of a co-protein, with known functionality attributes, theoretically could impact structure of the protein body through direct association and/or augment end-use applications of sorghum flour by stabilizing ß-sheet formation of the kafirins in sorghum dough preparations. This in turn may improve viscoelasticity of sorghum dough. To this end, we report here on the molecular and phenotypic characterizations of transgenic sorghum events that are down-regulated in γ- and the 29-kDa α-kafirins and the expression of a wheat Dy10/Dx 5 hybrid high-molecular weight glutenin protein. The results demonstrate that down-regulation of γ-kafirin alone does not alter protein body formation or impacts protein digestibility of cooked flour samples. However, reduction in accumulation of a predicted 29-kDa α-kafirin alters the morphology of protein body and enhances protein digestibility in both raw and cooked samples.

Expression pattern of the alpha-kafirin promoter coupled with a signal peptide from Sorghum bicolor L. Moench

Regulatory sequences with endosperm specificity are essential for foreign gene expression in the desired tissue for both grain quality improvement and molecular pharming. In this study, promoters of seed storage α-kafirin genes coupled with signal sequence (ss) were isolated from Sorghum bicolor L. Moench genomic DNA by PCR. The α-kafirin promoter (α-kaf) contains endosperm specificity-determining motifs, prolamin-box, the O2-box 1, CATC, and TATA boxes required for α-kafirin gene expression in sorghum seeds. The constructs pMB-Ubi-gfp and pMB-kaf-gfp were microprojectile bombarded into various sorghum and sweet corn explants. GFP expression was detected on all explants using the Ubi promoter but only in seeds for the α-kaf promoter. This shows that the α-kaf promoter isolated was functional and demonstrated seed-specific GFP expression. The constructs pMB-Ubi-ss-gfp and pMB-kaf-ss-gfp were also bombarded into the same explants. Detection of GFP expression showed that the signal peptide (SP)::GFP fusion can assemble and fold properly, preserving the fluorescent properties of GFP.

Effect of kafirin protein coating on sensory quality and shelf-life of 'Packham's Triumph' pears during ripening

BACKGROUND

Pears are exported in large quantities from South Africa, resulting in large revenues. Minimisation of quality losses once the fruit has reached the export destination is as important as following strict export and distribution protocols. Kafirin can form edible films. In this study an edible coating made from 20 g kg(-1) kafirin coating solution was applied as a postharvest treatment to retard quality deterioration of 'Packham's Triumph' pears during storage at the typical ripening temperature (20 °C). Changes in physicochemical and sensory quality were monitored over a period of 24 days.

RESULTS

The kafirin coating was unable to retard the onset of ripening but decreased the respiration rate and retarded the progression of senescence. However, moisture loss was exacerbated in the kafirin-coated fruit during ripening at 20 °C, especially towards the end of the shelf-life.

CONCLUSION

The kafirin coating extended the eat-ripe quality of the pears by 1-2 weeks. However, the appearance of the fruit was unacceptable after 14 days of storage in terms of wrinkled skin. Further work is needed to improve the water barrier properties of the kafirin coating by incorporating a wax or triglyceride into the coating formulation or more simply by applying a kafirin coating to waxed fruit.

The amplification and evolution of orthologous 22-kDa α-prolamin tandemly arrayed genes in coix, sorghum and maize genomes

Tandemly arrayed genes (TAGs) account for about one-third of the duplicated genes in eukaryotic genomes. They provide raw genetic material for biological evolution, and play important roles in genome evolution. The 22-kDa prolamin genes in cereal genomes represent typical TAG organization, and provide the good material to investigate gene amplification of TAGs in closely related grass genomes. Here, we isolated and sequenced the Coix 22-kDa prolamin (coixin) gene cluster (283 kb), and carried out a comparative analysis with orthologous 22-kDa prolamin gene clusters from maize and sorghum. The 22-kDa prolamin gene clusters descended from orthologous ancestor genes, but underwent independent gene amplification paths after the separation of these species, therefore varied dramatically in sequence and organization. Our analysis indicated that the gene amplification model of 22-kDa prolamin gene clusters can be divided into three major stages. In the first stage, rare gene duplications occurred from the ancestor gene copy accidentally. In the second stage, rounds of gene amplification occurred by unequal crossing over to form tandem gene array(s). In the third stage, gene array was further diverged by other genomic activities, such as transposon insertions, segmental rearrangements, etc. Unlike their highly conserved sequences, the amplified 22-kDa prolamin genes diverged rapidly at their expression capacities and expression levels. Such processes had no apparent correlation to age or order of amplified genes within TAG cluster, suggesting a fast evolving nature of TAGs after gene amplification. These results provided insights into the amplification and evolution of TAG families in grasses.

Seed storage protein gene promoters contain conserved DNA motifs in Brassicaceae, Fabaceae and Poaceae

BACKGROUND

Accurate computational identification of cis-regulatory motifs is difficult, particularly in eukaryotic promoters, which typically contain multiple short and degenerate DNA sequences bound by several interacting factors. Enrichment in combinations of rare motifs in the promoter sequence of functionally or evolutionarily related genes among several species is an indicator of conserved transcriptional regulatory mechanisms. This provides a basis for the computational identification of cis-regulatory motifs.

RESULTS

We have used a discriminative seeding DNA motif discovery algorithm for an in-depth analysis of 54 seed storage protein (SSP) gene promoters from three plant families, namely Brassicaceae (mustards), Fabaceae (legumes) and Poaceae (grasses) using backgrounds based on complete sets of promoters from a representative species in each family, namely Arabidopsis (Arabidopsis thaliana (L.) Heynh.), soybean (Glycine max (L.) Merr.) and rice (Oryza sativa L.) respectively. We have identified three conserved motifs (two RY-like and one ACGT-like) in Brassicaceae and Fabaceae SSP gene promoters that are similar to experimentally characterized seed-specific cis-regulatory elements. Fabaceae SSP gene promoter sequences are also enriched in a novel, seed-specific E2Fb-like motif. Conserved motifs identified in Poaceae SSP gene promoters include a GCN4-like motif, two prolamin-box-like motifs and an Skn-1-like motif. Evidence of the presence of a variant of the TATA-box is found in the SSP gene promoters from the three plant families. Motifs discovered in SSP gene promoters were used to score whole-genome sets of promoters from Arabidopsis, soybean and rice. The highest-scoring promoters are associated with genes coding for different subunits or precursors of seed storage proteins.

CONCLUSION

Seed storage protein gene promoter motifs are conserved in diverse species, and different plant families are characterized by a distinct combination of conserved motifs. The majority of discovered motifs match experimentally characterized cis-regulatory elements. These results provide a good starting point for further experimental analysis of plant seed-specific promoters and our methodology can be used to unravel more transcriptional regulatory mechanisms in plants and other eukaryotes.

Kafirin microparticle encapsulation of catechin and sorghum condensed tannins

To exploit the porous nature of previously developed kafirin microparticles, encapsulation of the bioactive polyphenols, catechin and sorghum condensed tannins, was investigated. The antioxidant release profiles of the encapsulated substances were studied under simulated gastric conditions. Kafirin microparticles encapsulating catechin or sorghum condensed tannins were similar in size to control kafirin microparticles (5-6 mum). TEM showed that kafirin microparticles encapsulating catechin had a rough porous surface. Microparticles encapsulating sorghum condensed tannins were irregular in shape, some apparently joined together, with a mixture of rough and smooth surfaces. Over a period of 4 h, catechin and sorghum condensed tannin encapsulated kafirin microparticles showed virtually no protein digestion but released approximately 70 and 50%, respectively, of total antioxidant activity. Thus, the use of kafirin microparticles to encapsulate catechin and sorghum condensed tannins has potential as an effective method of controlled release of dietary antioxidants.

Organization of the prolamin gene family provides insight into the evolution of the maize genome and gene duplications in grass species

Zea mays, commonly known as corn, is perhaps the most greatly produced crop in terms of tonnage and a major food, feed, and biofuel resource. Here we analyzed its prolamin gene family, encoding the major seed storage proteins, as a model for gene evolution by syntenic alignments with sorghum and rice, two genomes that have been sequenced recently. Because a high-density gene map has been constructed for maize inbred B73, all prolamin gene copies can be identified in their chromosomal context. Alignment of respective chromosomal regions of these species via conserved genes allow us to identify the pedigree of prolamin gene copies in space and time. Its youngest and largest gene family, the alpha prolamins, arose about 22-26 million years ago (Mya) after the split of the Panicoideae (including maize, sorghum, and millet) from the Pooideae (including wheat, barley, and oats) and Oryzoideae (rice). The first dispersal of alpha prolamin gene copies occurred before the split of the progenitors of maize and sorghum about 11.9 Mya. One of the two progenitors of maize gained a new alpha zein locus, absent in the other lineage, to form a nonduplicated locus in maize after allotetraplodization about 4.8 Mya. But dispersed copies gave rise to tandem duplications through uneven expansion and gene silencing of this gene family in maize and sorghum, possibly because of maize's greater recombination and mutation rates resulting from its diploidization process. Interestingly, new gene loci in maize represent junctions of ancestral chromosome fragments and sites of new centromeres in sorghum and rice.

Expression of the sorghum 10-member kafirin gene cluster in maize endosperm

Functional analysis of chromosomal segments containing linked genes requires the insertion of contiguous genomic sequences from bacterial artificial chromosomes (BACs) into the genome. Therefore, we introduced a 90-kb large BAC clone carrying a 10-copy tandem array of kafirin storage protein genes from sorghum linkage group J, mixed with a selectable marker gene, directly into maize cells using the particle bombardment method. Transgenic plants were regenerated and seeds from eight different transgenic lines were produced. One such transgenic plant was selected that had the entire kafirin gene cluster on a single continuous DNA fragment spanning more than 45 kb integrated into its genome. When alcohol-soluble proteins from individual T2 and T3 seeds of this event were analyzed, significant levels of kafirin were found in addition to the endogenous zein storage proteins, demonstrating that the large exogenous DNA segment is stably integrated into the maize genome and expressed at high levels in subsequent generations. Therefore, we could provide a new utility of plant transformation by the particle bombardment method for functional genomics of multigene families and the modification of the nutritive quality of cereal grains. Despite a tandem array of highly homologous sequences at the transgenic locus, no gene silencing was observed, probably owing to the effects of co-transformed flanking sequences. The expression studies of the transgenic locus also revealed new features of storage protein gene promoters that differed from previous transient gene expression studies, thereby illustrating the significance of the concentration and configuration of DNA-protein interactions in the regulation of gene expression.

Mosaic organization of orthologous sequences in grass genomes

Although comparative genetic mapping studies show extensive genome conservation among grasses, recent data provide many exceptions to gene collinearity at the DNA sequence level. Rice, sorghum, and maize are closely related grass species, once sharing a common ancestor. Because they diverged at different times during evolution, they provide an excellent model to investigate sequence divergence. We isolated, sequenced, and compared orthologous regions from two rice subspecies, sorghum, and maize to investigate the nature of their sequence differences. This study represents the most extensive sequence comparison among grasses, including the largest contiguous genomic sequences from sorghum (425 kb) and maize (435 kb) to date. Our results reveal a mosaic organization of the orthologous regions, with conserved sequences interspersed with nonconserved sequences. Gene amplification, gene movement, and retrotransposition account for the majority of the nonconserved sequences. Our analysis also shows that gene amplification is frequently linked with gene movement. Analyzing an additional 2.9 Mb of genomic sequence from rice not only corroborates our observations, but also suggests that a significant portion of grass genomes may consist of paralogous sequences derived from gene amplification. We propose that sequence divergence started from hotspots along chromosomes and expanded by accumulating small-scale genomic changes during evolution.

Sequence, regulation, and evolution of the maize 22-kD alpha zein gene family

We have isolated and sequenced all 23 members of the 22-kD alpha zein (z1C) gene family of maize. This is one of the largest plant gene families that has been sequenced from a single genetic background and includes the largest contiguous genomic DNA from maize with 346,292 bp to date. Twenty-two of the z1C members are found in a roughly tandem array on chromosome 4S forming a dense gene cluster 168,489-bp long. The twenty-third copy of the gene family is also located on chromosome 4S at a site approximately 20 cM closer to the centromere and appears to be the wild-type allele of the floury-2 (fl2) mutation. On the basis of an analysis of maize cDNA databases, only seven of these genes appear to be expressed including the fl2 allele. The expressed genes in the cluster are interspersed with nonexpressed genes. Interestingly, some of the expressed genes differ in their transcriptional regulation. Gene amplification appears to be in blocks of genes explaining the rapid and compact expansion of the cluster during the evolution of maize.

Characterization of the maize prolamin box-binding factor-1 (PBF-1) and its role in the developmental regulation of the zein multigene family

A maize prolamin box (P-box)-binding factor (PBF-1) has been purified and characterized from immature endosperm tissue. PBF-1 has a molecular weight of 38kDa. It is detected only in endosperm, but not in root or leaf tissues, consistent with its tissue-specific function. Site-directed mutagenesis experiments reveal that both the P-box and its flanking sequences are important for PBF-1 DNA binding. Developmental studies show that PBF-1 accumulates in the endosperm from 8 to at least 30days after pollination (DAP). From 16 to 24DAP, however, multiple shifted bands of protein(s)-DNA complexes can be observed, which correlate with an increase in zein gene expression. PBF-1 can also bind to the P-box from '22-kDa' and '19-kDa' zein promoters, but at a lower affinity than to the '27-kDa' zein promoter. The effects of protein dephosphorylation and zinc ion chelators on PBF-1 DNA binding activity are also shown. A model is proposed where PBF-1 serves as a 'recruiter' of class-specific transcription factors like Opaque2 (O2).

Modulation of gene expression by DNA-protein and protein-protein interactions in the promoter region of the zein multigene family

A common cis-acting element in the promoter region of many genes expressed during endosperm development of cereal seeds, the prolamine-box or P-box, is only 20bp upstream of the alpha-class 22-kDa zein gene-specific cis element, the O2-box, which is recognized by the b-ZIP transcription factor, Opaque-2 (O2). The proximity of these two boxes has prompted a study of how two DNA-binding proteins of a different hierarchy might be involved in the activation and modulation of the 22-kDa zein-encoding genes. This was accomplished by utilizing a highly purified P-box-binding-factor-1 (PBF-1) and a bacterially expressed truncated form of the O2 protein. After adding the recombinant O2 to the purified fraction of PBF-1, binding studies were performed with a series of DNA probes combining the P- and O2-boxes from zein promoters. These studies have revealed an interesting inhibitory effect of PBF-1 over O2 function dependent on their ratio, consistent with its in-vivo properties and the developmental expression profiles of zein genes. We also could show that the P-box is specifically recognized by topoisomerase II and single-strand DNA-binding proteins, indicating a possible additional linkage between P-box and the scaffold-attachment-region (SAR).

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.

Structural characterization and promoter activity analysis of the gamma-kafirin gene from sorghum

A genomic clone encoding the gamma-kafirin gene from sorghum was isolated and sequenced. A 2938 bp sequenced fragment includes an intronless open reading frame of 636 nucleotides encoding a putative polypeptide of 212 amino acids. Comparison of the deduced amino acid sequence of gamma-kafirin with the published sequences of gamma-prolamins of maize, and Coix revealed highly conserved domains. The N-terminal region of these proteins contains the conserved hexapeptide PPPVHL, which is repeated eight times in gamma-zein, four times in gamma-kafirin and three times in gamma-coixin. The number of PPPVHL repeats accounts predominantly for the differences in the molecular weights of gamma-prolamins. Several putative regulatory sequences common to the gamma-kafirin and gamma-zein genes were identified in both the 5' and the 3' flanking regions. Putative GCN4-like regulatory sequences were found at positions -192 and -476 in the 5' flanking region of gamma-kafirin. In the 3' noncoding region, three putative polyadenylation signals, two AATAAT and one AATGAA, were found at positions +658, +716, and +785, respectively. In order to investigate the role of the putative GCN4-like motifs and other possible cis-acting element(s) of the gamma-kafirin promoter, a series of deleted and chimeric promoter constructs were introduced into maize, Coix and sorghum tissues by particle bombardment. Histochemical analysis of beta-glucuronidase (GUS) activity in different tissues indicated that the element(s) responsible for tissue specificity is probably located in the 285-bp proximal region of the promoter, while the remaining promoter sequence seems to carry the element(s) responsible for the quantitative response.

Structural and functional analysis of an Opaque-2-related gene from sorghum

The Opaque-2 (O2) gene from maize encodes a transcriptional activator of the b-ZIP class. We have isolated and characterized a gene from sorghum, related in sequence to the O2 gene from maize. A single copy of the gene is present in sorghum. Both genomic and cDNA sequences of the O2-related sorghum gene were determined. The sequence is highly homologous to maize O2 both in the promoter and in the coding region. The most closely related sequences contain the b-ZIP domain with only 11 amino acid substitutions in a total of 122 residues. In transient expression assays, the sorghum O2-related coding sequence, expressed from a CaMV 35S promoter, activates expression from the maize b-32 promoter as effectively as that obtained with the maize O2 sequence.

Sequence analysis of 22 kDa-like alpha-coixin genes and their comparison with homologous zein and kafirin genes reveals highly conserved protein structure and regulatory elements

Several genomic and cDNA clones encoding the 22 kDa-like alpha-coixin, the alpha-prolamin of Coix seeds, were isolated and sequenced. Three contiguous 22 kDa-like alpha-coixin genes designated alpha-3A, alpha-3B and alpha-3C were found in the 15 kb alpha-3 genomic clone. The alpha-3A and alpha-3C genes presented in-frame stop codons at position +652. The two genes with truncated ORFs are flanking the alpha-3B gene, suggesting that the three alpha-coixin genes may have arisen by tandem duplication and that the stop codon was introduced before the duplication. Comparison of the deduced amino acid sequences of alpha-coixin clones with the published sequences of 22 kDa alpha-zein and 22 kDa-like alpha-kafirin revealed a highly conserved protein structure. The protein consists of an N-terminus, containing the signal peptide, followed by ten highly conserved tandem repeats of 15-20 amino acids flanked by polyglutamines, and a short C-terminus. The difference between the 22 kDa-like alpha-prolamins and the 19 kDa alpha-zein lies in the fact that the 19 kDa protein is exactly one repeat motif shorter than the 22 kDa proteins. Several putative regulatory sequences common to the zein and kafirin genes were identified within both the 5' and 3' flanking regions of alpha-3B. Nucleotide sequences that match the consensus TATA, CATC and the ca. -300 prolamin box are present at conserved positions in alpha-3B relative to zein and kafirin genes. Two putative Opaque-2 boxes are present in alpha-3B that occupies approximately the same positions as those identified for the 22 kDa alpha-zein and alpha-kafirin genes. Southern hybridization, using a fragment of a maize Opaque-2 cDNA clone as a probe, confirmed the presence of Opaque-2 homologous sequences in the Coix and sorghum genomes. The overall results suggest that the structural and regulatory genes involved in the expression of the 22 kDa-like alpha-prolamin genes of Coix, sorghum and maize, originated from a common ancestor, and that variations were introduced in the structural and regulatory sequences after species separation.

Studies of the zein-like alpha-prolamins based on an analysis of amino acid sequences: implications for their evolution and three-dimensional structure

alpha-Prolamins are the major seed storage proteins of species of the grass tribe Andropogonea. They are unusually rich in glutamine, proline, alanine, and leucine residues and their sequences show a series of tandem repeats presumed to be the result of multiple intragenic duplication. Two new sequences of alpha-prolamin clones from Coix (pBCX25.12 and pBCX25.10) are compared with similar clones from maize and Sorghum in order to investigate evolutionary relationships between the repeat motifs and to propose a schematic model for their three-dimensional structure based on hydrophobic membrane-helix propensities and helical "wheels." A scheme is proposed for the most recent events in the evolution of the central part of the molecule (repeats 3 to 8) which involves two partial intragenic duplications and in which contemporary odd-numbered and even-numbered repeats arise from common ancestors, respectively. Each pair of repeats is proposed to form an antiparallel alpha-helical hairpin and that the helices of the molecule as a whole are arranged on a hexagonal net. The majority of helices show six faces of alternating hydrophobic and polar residues, which give rise to intersticial holes around each helix which alternate in chemical character. The model is consistent with proteins which contain different numbers of repeats, with oligomerization and with the dense packaging of alpha-prolamins within the protein body of the seed endosperm.

Functional analysis of the -300 region of maize zein genes

A 43 bp fragment containing the -300 region upstream control element common to the endosperm expressed zein genes of Zea mays L. has been analyzed by in vivo and in vitro techniques. Transient transformation studies with protoplasts from a maize endosperm culture indicate that the element positively affects CaMV 35S promoter-driven gene expression, and that this effect is both orientation- and position-dependent. Band-shift and Southwestern blotting experiments demonstrate that the element is specifically bound by different sets of DNA-binding proteins from seedling and endosperm nuclei. A 2 bp substitution within the most conserved region of the element both reduces the stimulatory effect on transcription and alters the binding of nuclear proteins from both tissues.