Genetic regulation of storaage protein content in maize endosperm

Efficient recovery of recombinant proteins from cereal endosperm is affected by interaction with endogenous storage proteins

Cereal seeds are versatile platforms for the production of recombinant proteins because they provide a stable environment for protein accumulation. Endogenous seed storage proteins, however, include several prolamin-type polypeptides that aggregate and crosslink via intermolecular disulfide bridges, which could potentially interact with multimeric recombinant proteins such as antibodies, which assemble in the same manner. We investigated this possibility by sequentially extracting a human antibody expressed in maize endosperm, followed by precipitation in vitro with zein. We provide evidence that a significant proportion of the antibody pool interacts with zein and therefore cannot be extracted using non-reducing buffers. Immunolocalization experiments demonstrated that antibodies targeted for secretion were instead retained within zein bodies because of such covalent interactions. Our findings suggest that the production of soluble recombinant antibodies in maize could be enhanced by eliminating or minimizing interactions with endogenous storage proteins.

Kernel lysine content does not increase in some maize opaque2 mutants

The recessive mutant allele of the opaque2 gene (o2) alters the endosperm protein pattern and increases the kernel lysine content of maize (Zea mays L.). In this study, sequencing results showed that the o2 mutant was successfully introgressed into 12 elite normal maize inbred lines by marker assisted selection (MAS). The average genetic similarity between these normal inbred lines and their o2 near-isogenic lines (NILs) was more than 95%. Kernel lysine content increased significantly in most of o2 NILs lines relative to normal elite inbreds, but remained unchanged in the genetic backgrounds Dan598o2 and Liao2345o2. Moreover, the kernel characteristics of these two o2 NILs did not differ from the other inbred lines. The results of lysine content analysis in the F1 hybrids between Liao2345o2 and Dan598o2 and other o2 NILs demonstrated that gene(s) other than opaque2 may control kernel lysine content in these two o2 NILs. The results of zein analysis showed that 22-kD α-zein synthesis was reduced or absent, and the 19-kD α-zein synthesis was greatly reduced compared with the recurrent parents in most o2 NILs except for Dan598o2 and Liao2345o2. Our results indicate that gene(s) other than opaque2 may play more important roles in zein synthesis and kernel lysine content in some maize genetic backgrounds.

Opaque7 encodes an acyl-activating enzyme-like protein that affects storage protein synthesis in maize endosperm

In maize, a series of seed mutants with starchy endosperm could increase the lysine content by decreased amount of zeins, the main storage proteins in endosperm. Cloning and characterization of these mutants could reveal regulatory mechanisms for zeins accumulation in maize endosperm. Opaque7 (o7) is a classic maize starchy endosperm mutant with large effects on zeins accumulation and high lysine content. In this study, the O7 gene was cloned by map-based cloning and confirmed by transgenic functional complementation and RNAi. The o7-ref allele has a 12-bp in-frame deletion. The four-amino-acid deletion caused low accumulation of o7 protein in vivo. The O7 gene encodes an acyl-activating enzyme with high similarity to AAE3. The opaque phenotype of the o7 mutant was produced by the reduction of protein body size and number caused by a decrease in the α-zeins concentrations. Analysis of amino acids and metabolites suggested that the O7 gene might affect amino acid biosynthesis by affecting α-ketoglutaric acid and oxaloacetic acid. Transgenic rice seeds containing RNAi constructs targeting the rice ortholog of maize O7 also produced lower amounts of seed proteins and displayed an opaque endosperm phenotype, indicating a conserved biological function of O7 in cereal crops. The cloning of O7 revealed a novel regulatory mechanism for storage protein synthesis and highlighted an effective target for the genetic manipulation of storage protein contents in cereal seeds.

Reducing the brittleness of zein films through chemical modification

Zein protein is a major coproduct of biofuel from corn. To reduce the brittleness of zein films, a new type of zein-based biomaterial, was synthesized by chemical modification of zein with lauryl chloride through an acylation reaction. The final products were confirmed by (1)H NMR, FT-IR analysis, and SDS-PAGE. Thermal analysis detected no microphase separation in the synthesized polymer matrix. As the content of lauryl moiety increased, the glass transition temperatures of modified zein decreased by as large as 25.8 °C due to the plasticization effect of the lauryl moiety. In addition, mechanical and surface properties of cast films from acylated zein were also investigated. The elongation at break of modified zein sheet was increased by about 7-fold at the high modification level with some loss of mechanical strength. The surfaces of modified zein films were as uniform as unmodified zein film but more hydrophobic, further suggesting that no microphase separation happened during the film formation process. This work indicated the potential of these new biomaterials in the development of biodegradable food packaging materials and delivery systems.

A seed high-lysine trait is negatively associated with the TCA cycle and slows down Arabidopsis seed germination

• Lysine is a nutritionally important essential amino acid, but significant elevation of its levels in Arabidopsis seeds, by enhancing its synthesis and blocking its catabolism, causes a retardation of germination. Here, we hypothesized that this negative effect is associated with changes in primary metabolism and gene expression programs that are essential for early germination. • Seeds at different stages of germination sensu stricto of the seed-high-lysine genotype were subjected to detailed analysis of primary metabolism, using GC-MS, as well as microarray analysis and two-dimensional, isoelectric focusing, sodium dodecylsulfate polyacrylamide gel electrophoresis, to detect storage protein mobilization. • Our results exposed a major negative effect of the seed-specific increased lysine synthesis and knockout of its catabolism on the levels of a number of TCA cycle metabolites. This metabolic alteration also influences significantly the transcriptome, primarily attenuating the boost of specific transcriptional programs that are essential for seedling establishment, such as the onset of photosynthesis, as well as the turnover of specific transcriptional programs associated with seed embryonic traits. • Our results indicate that catabolism of the aspartic acid family of amino acids is an important contributor to the energy status of plants, and hence to the onset of autotrophic growth-associated processes during germination.

Maize authentication: quality control methods and multivariate analysis (chemometrics)

Maize is one of the most important cereals because of its numerous applications in processed foods where it is the major or minor component. Apart from maize authenticity issues related to cultivar and geographical origin (national and/or international level), there is another important issue related to genetically modified maize. Various objective parameters such as fatty acids, phenolic compounds, pigments, heavy metals were determined in conjunction with subjective (sensory analysis) in order to identify the maize authenticity. However, the implementation of multivariate analysis (principal component analysis, cluster analysis, discriminant analysis, canonical analysis) is of great importance toward reaching valid conclusions on authenticity issues. This review summarized the most important finding of both objective and subjective evaluations of maize in five comprehensive tables in conjunction with the discussion.

Extraction of protein from distiller's grain

We have investigated the feasibility of extracting the oil and protein from distiller's grain (DG) to obtain a higher-valued protein-rich product and a carbohydrate-rich residue better suited for conversion to fermentable sugars. Protein extractions based on aqueous ethanol, alkaline-ethanol, and aqueous enzyme treatments were compared. Three of the methods extracted a significant amount of the protein from dried, defatted DG (DDDG). The enzymatic extraction decreased the crude protein content in the solid phase for both milled and unmilled DDDG from 41% (dry weight) to approximately 10% (dry weight) protein in the residual solid; this corresponded to extraction of 90% of the protein in the original DDDG. The alkaline-ethanol extraction was similarly effective for milled but not unmilled DG. Simple extraction with alcohol was not as effective. Amino acid analysis of each protein extract was consistent with it consisting mainly of zeins. For the protease-assisted extractions, 95% the proteins were in the form of peptides smaller than 10kDa.

Organization and regulation of zein genes in maize endosperm

Zein, the major storage protein of maize endosperm, is constituted by a group of similar polypeptides encoded by a multigene family. The structural genes are located into three main clusters on chromosomes 4, 7 and 10. The rate of accumulation of zein polypeptides is under the control of several positive regulatory loci. The mutant alleles at these loci (O2, O6, O7, F12, De-B30, Mc) reduce more or less drastically the rate of zein deposition. By analysing the interactions among the mutants, epistatic, additive and synergistic effects were observed indicating the existence of multiple pathways controlling zein deposition. Proteins, other than zeins, associated with theO2, O6andF12loci have been identified and characterized.

A defective signal peptide in the maize high-lysine mutant floury 2

The maize floury 2 (fl2) mutation enhances the lysine content of the grain, but the soft texture of the endosperm makes it unsuitable for commercial production. The mutant phenotype is linked with the appearance of a 24-kDa alpha-zein protein and increased synthesis of binding protein, both of which are associated with irregularly shaped protein bodies. We have cloned the gene encoding the 24-kDa protein and show that it is expressed as a 22-kDa alpha-zein with an uncleaved signal peptide. Comparison of the deduced N-terminal amino acid sequence of the 24-kDa alpha-zein protein with other alpha-zeins revealed an alanine to valine substitution at the C-terminal position of the signal peptide, a histidine insertion within the seventh alpha-helical repeat, and an alanine to threonine substitution with the same alpha-helical repeat of the protein. Structural defects associated with this alpha-zein explain many of the phenotypic effects of the fl2 mutation.

Synthesis of an unusual alpha-zein protein is correlated with the phenotypic effects of the floury2 mutation in maize

The soft, starchy endosperm of the maize (Zea mays L.) floury 2 mutant is associated with a reduction in zein mRNA and protein synthesis, unique protein body morphology, and enhanced levels of a 70 kDa protein, that has been shown to be the maize homolog of a chaperonin found in the endoplasmic reticulum. We found an unusual alpha-zein protein of 24 kDa to be consistently associated with the zein fraction from floury 2 mutants. Three additional alpha-zein proteins with molecular weights ranging from ca. 25 to 27 kDa are detected in the storage protein fraction of a high percentage of floury 2 kernels and a low percentage of normal kernels in a genetically segregating population. The four proteins in a genetically segregating population. The four proteins can be distinguished from one another by immunostaining on Western blots. Synthesis of the 24 kDa protein is regulated by Opaque2, since the 24 kDa protein is lacking in the storage protein fraction of opaque2/floury2 double mutants. The synthesis of an abnormal alpha-zein protein in floury2 could explain many features of the mutant, such as the abnormal protein body morphology, induction of the 70 kDa chaperonin, and hypostasis to opaque2 (o2). Although we cannot prove that the accumulation of this protein is responsible for the floury2 phenotype, we were able to detect a restriction fragment length polymorphism (RFLP) linked to the floury2 locus with a 22 kDa alpha-zein probe. We hypothesize that the unique characteristics of the floury2 mutant could be a response to the accumulation of a defective alpha-zein protein which impairs secretory protein synthesis.

The origin of lysine-containing proteins in opaque-2 maize endosperm

The reduction of zein synthesis in the maize (Zea mays L.) opaque-2 mutant is associated with an increased percentage of lysine in the endosperm protein. When expressed on an endosperm basis, we found that W64A opaque-2 contains 490 micrograms of lysine compared with 350 micrograms in W64A normal. SDS-PAGE analysis of endosperm proteins indicated that several non-zein proteins are more abundant in the mutant than in normal genotype. To determine the subcellular origin of these proteins, we separated an endosperm homogenate from developing kernels by sucrose density gradient centrifugation and used marker enzyme assays and immunoblot analyses to identify cellular components. Amino acid analysis of proteins in the gradient fractions showed that the majority of the lysine occurs in soluble proteins at the top of the gradient. To identify these proteins, we prepared a complex antiserum against the entire soluble protein fraction and used it to immunoscreen an endosperm cDNA expression library. Sequence analysis of clones identified mRNAs involved in carbohydrate metabolism, amino acid biosynthesis, and protein synthesis. RNA dot blot hybridization analysis with these clones revealed significant variation in the levels of transcripts between normal and opaque-2 endosperm, but we identified several mRNAs that are elevated in opaque-2 and that may encode proteins responsible for the enhanced lysine content.

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.

Deletion of DNA sequences flanking an Mr 19,000 zein gene reduces its transcriptional activity in heterologous plant tissues

Analysis of a series of clones containing deletions in the 5' noncoding sequence of a gene encoding an Mr 19,000 zein allowed identification of a region required for maximal transcription. Transcriptional activity was assayed in two heterologous plant systems. In one system, the Ti plasmid was used to introduce the modified zein genes into the sunflower genome. In the other system, electroporation was used to transform carrot protoplasts with plasmids containing the zein genes. For the electroporation experiments, the 5' noncoding sequences from the zein clones were linked to the protein coding sequence of chloramphenicol acetyl transferase. The results showed that an upstream sequence, delimited by nucleotides -337 and -125 with respect to the mRNA cap site, is required for maximal transcription of the gene. In contrast, very low levels of transcription were directed by constructs that contained 125 bp of 5' noncoding sequence that included the CAAT and TATA boxes, suggesting that the additional sequences (-337 to -125) further 5' exert a quantitative effect on transcription. Examination of the additional 5' sequences showed five regions that share homology with the SV40 enhancer core sequence.

Localization of zein genes in maize

The band patterns of zein polypeptides were determined for many commercial inbred corn lines and maize stocks using isofocusing in agarose gels and sodium dodecyl sulfate (SDS)-urea gels. Each inbred line or homozygous maize strain genotype has a distinct zein profile which has been catalogued according to the distance of charge migration and molecular weight (kilodaltons). Several zein polypeptides were mapped to chromosomes 4 and 10 with the use of reciprocal translocations. The mapping of at least two polypeptides on distal 4L and 10L had not been previously reported. The general methods used in the present research will permit the mapping of all the zein polypeptides to chromosomal sites.

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.

Zein synthesis in the embryo and endosperm of maize mutants

Zein synthesis in the developing (22 and 50 days postpollination) endosperm and embryo of maize (Zea mays L.) double mutants, brittle-1; opaque-2 and brittle-2;opaque-2, were compared and correlated with sucrose concentration and RNase activity in order to test the hypothesis that high sucrose concentrations may prevent the interaction between zein polyribosomes and endoplasmic reticulum and make the zein mRNAs more susceptible to hydrolysis by high RNase activity, resulting in a severe reduction in zein synthesis. The double-mutant combinations of opaque-2 with each of the starch-deficient mutants, brittle-1 and brittle-2, maintained not only a high sucrose concentration in the endosperm but also a higher RNase activity than either one of the single mutants alone. Consequently, these double mutants severely suppressed the synthesis of two major zein components in their endosperms. In contrast to the endosperm system, embryos of the double mutants produced amounts of zein (and electrophoretic patterns) similar to that of the opaque-2 embryo, and their embryos contained levels of sucrose and RNase activity comparable to that of the o2 and normal control. These results are consistent with the notion that a posttranscriptional degradation of zein mRNAs by RNase, rather than a specific transcriptional block, is involved in the endosperm to suppress zein synthesis in these double mutants.

Zein gene organization in maize and related grasses

Zein cDNA clones were used to study the organization of zein genes within the genome of the inbred maize W64A. When individual clones for the two larger molecular-weight classes of zein proteins (Mr = 22,000; Mr = 19,000) were used as probes for Southern blot hybridizations of genomic DNA, multiple restriction fragments were found to hybridize. Reconstruction analyses using moderately stringent criteria were used to estimate a total of 70-80 zein sequences within the genome of this inbred maize. The hybridization patterns suggest that zein sequences are clustered within the same restriction fragment. When criteria permitting less cross-hybridization of homologous sequences (Tm - 10 degrees C) were used, the banding pattern changed, with some of the bands being reduced in intensity or eliminated entirely. Therefore, by control of hybridization criteria, particular zein genes may be more readily distinguished in a Southern blot analysis. The Southern blot hybridization pattern for the Mr = 15,000 zein was less complex. Only a single major band was found, with sufficient hybridization intensity for two or three genes. Genomic Southern analyses of other inbred maizes and related grasses showed similarly complex hybridization patterns with cDNA probes for the 19,000- and 22,000-molecular-weight zeins, suggesting that these sequences have been conserved over evolutionary time. The zein multigene family may therefore have arisen by gene duplication before divergence of the maize, teosinte, and Tripsacum species from a common ancestor.

Zein of maize grain: I--Isolation by gel filtration and characterization of monomeric and dimeric species

Unreduced zein chromatographed on Sephadex G 200 in 8 M urea, on G 100 in 1.5 or 2.5% sodium dodecyl sulfate (SDS) and on hydroxypropylated G 100 in 70% ethanol was resolved into two minor fractions A and B and two major ones D and M irrespective of the medium. The quantitative importance of the fraction M was dependent on the isolation conditions of zein. It decreased from 53% of the proteins contained in ethanolic extract and chromatographed as they were extracted, to 40% of the purified zein. The molecular weight values obtained from SDS-polyacrylamide gel electrophoresis and amino acid compositional data indicated that fractions D and M, as isolated from purified zein in the presence of ethanol, represented respectively dimeric and monomeric forms of a mixture of Mr 22 000 and 24 000 polypeptides with threonine or phenylalanine as NH2-terminal residue. Electrophoretic analysis of selectively carbamylated fraction M on starch gel at pH 3.5 revealed that zein subunits comprised several polypeptides differing in the number and the nature of basic amino acids. At least one of these polypeptides contained one lysyl residue.

Nucleotide sequence analysis of a zein genomic clone with a short open reading frame

The nucleotide sequence of the zein genomic clone (W22)Z7 and its flanking sequences from the W22 inbred line of maize is reported. The sequence is 1587 bp long and contains 444 bp of 5' noncoding sequence and 342 bp of 3' noncoding sequence. The Z7 sequence belongs to a large complex multigene family and is a member of the B49 subfamily. It is 86% homologous to other known sequences from the same subfamily, but contains four in-frame termination codons caused by single base changes. Its flanking regions contain the usual eukaryotic transcriptional signals.

Sequence analysis of zein cDNAs obtained by an efficient mRNA cloning method

A cDNA library was generated from mRNA isolated from the developing endosperm of W22 maize inbred. cDNA clones for zein, the maize storage protein family, were isolated and analyzed by DNA sequencing. The DNA sequences of four clones containing cDNA copies of mRNAs belonging to one zein subfamily were determined. The data support the following conclusions: a) genes encoding the larger of the two zein species contain eleven instead of nine repeat units within the coding sequence of the gene; b) transcription can be terminated at either of the two polyadenlation signals and c) transcription starts 31 basepairs downstream from the first T in the TATA box. To facilitate this analysis a new method for the construction of cDNA libraries was developed. The mRNA was annealed to linearized and oligo-dT tailed pUC9 plasmid DNA, which then primed synthesis of the first strand of the cDNA. Oligo-dG tails were added to the cDNA-plasmid molecules, which were then centrifuged through an alkaline sucrose gradient. The gradient step removed small molecules and separated the two cDNAs which were formerly attached to the same double stranded plasmid molecule. An excess of oligo-dC tailed denatured pUC9 DNA was added and the DNA was renatured under conditions that favor the circularization of monomers by the oligo-dC and oligo-dG tails. The oligo-dC tail served as primer for the synthesis of the second strand of the cDNA. The library was screened by colony hybridization using 32P-labelled cDNA and DNA from genomic zein clones as probes. We obtained 20,000 clones hybridizing total cDNA starting with 1 microgram of plasmid DNA and 1 microgram of mRNA.

Three mutations in Zea mays affecting zein accumulation: a comparison of zein polypeptides, in vitro synthesis and processing, mRNA levels, and genomic organization

We studied three mutations, opaque-2 (o2), opaque-7 (o7), and floury-2(fI2), each of which causes a depression in zein synthesis. We examined the processing efficiencies of the rough endoplasmic reticulum membranes in vitro, the levels of RNA transcription using cloned zein probes, and the genomic organization of the zein sequences as possible sites for the genetic defects. The results obtained indicate that the steps in prezein translation and processing occurring on the protein body membranes are not accountable for the lowered zein content in any of the mutations. The o2 mutation that typically shows a paucity of 22.5-kdalton zein polypeptides was found to have a concomitant reduction in a particular subgroup of mRNAs coding for this size class. Southern analyses suggest that the o2 mutation is not the result of a large deletion of tandem-linked zein genes.

Primary structure of a genomic zein sequence of maize

The nucleotide sequence of a genomic clone (termed Z4 ) of the zein multigene family was compared to the nucleotide sequence of related cDNA clones of zein mRNAs. A tandem duplication of a 96-bp sequence is found in the genomic clone that is not present in the related cDNA clones. When the duplication is disregarded, the nucleotide sequence homology between Z4 and its related cDNAs was approximately 97%. The nucleotide sequence is also compared to other isolated cDNAs. No introns in the coding region of the zein gene are detected. The first nucleotide of a putative TATA box, TATAAATA , was located 88 nucleotides upstream of the first nucleotide of the first ATG codon which initiated the open reading frame. The first nucleotide of a putative CCAAT box, CAAAAT , appeared 45 nucleotides upstream of the first nucleotide of the zein cDNA clones in the 3' non-coding region also appeared in the genomic sequence at the same locations. The amino acid composition of the polypeptide specified by the Z4 nucleotide sequence is similar to the known composition of zein proteins.

The primary structure of a plant storage protein: zein

The protein sequence of a representative of the zeins, the major storage proteins of maize, has been derived from the nucleotide sequence of a zein cDNA clone. This cDNA was sequence both by the Maxam and Gilbert and the M13-dideoxy techniques. The nucleotide sequence encompasses the non-translated 3' terminus of the mRNA, the entire coding sequence specifying both the mature zein protein and a small signal peptide, and a portion of the non-translated 5' region. The deduced amino acid composition and the amino-terminal amino acid sequence closely resemble those derived from chemical analysis of the zein protein fraction. The data presented represent the first complete amino acid sequence of a plant storage protein.

Subcellular compartmentalization of maize storage proteins in Xenopus oocytes injected with zein messenger RNAs

Maize storage proteins synthesized in oocytes were compartmentalized in membrane vesicles because they were resistant to hydrolysis by protease, unless detergent was present. The site of storage protein deposition within the oocyte was determined by subcellular fractionation. Optimal separation of oocyte membranes and organelles was obtained when EDTA and high concentrations of NaCl were included in the homogenization and gradient buffers. Under these conditions, fractions in sucrose gradients containing a heterogeneous mixture of smooth membranes (presumably endoplasmic reticulum, Golgi apparatus, and plasma membrane, density = 1.10-1.12 g/cm3), mitochondria (densities = 1.14 and 1.16 g/cm3), yolk platelets (density = 1.21 g/cm3), and a dense matrix material (density = 1.22 g/cm3) could be separated. Some zein proteins were recovered in the mixed membrane fraction, but the majority occurred in vesicles sedimenting with yolk platelets and granular material at a density of approximately 1.22 g/cm3. When metrizamide was included in the gradient to increase the density, little of the dense matrix material was isolated, and vesicles containing zein proteins were separated from other oocyte components. These vesicles were similar to protein bodies in maize endosperm because they were of identical density and contained the same group of polypeptides.

Tissue-specific zein synthesis in maize kernel

Zein may account for as much as 10% of the total protein in the mature embryo of maize inbred W64A. This protein exhibited an electrophoretic pattern on SDS gels similar to that of the endosperm. Like the endosperm system, the synthesis of zein components in the embryo was controlled by the opaque-2 and floury-2-mutations. However, unlike zein synthesis in the endosperm, zein synthesis in the embryo could not be increased by nitrogen fertilizer. Variations in amino acid composition were observed between the zein components of the embryo and those of the endosperm.

Genes and mRNAs coding for zein polypeptides in Zea mays

Zein messenger RNAs from maize endosperm were purified by successive oligo(dT)-cellulose chromatography and sucrose gradient centrifugation. Polyacrylamide gel electrophoresis under denaturing conditions revealed the presence of two size classes of zein messenger RNAs of Mr 3.5 x 10(5) and 4.10 x 10(5). The mRNA was shown to synthesize the major zein polypeptides, to have a base composition characteristic of a poly(A)-containing RNA and to be transcribed by reverse transcriptase into complementary DNA. The r0t1/2 of the hybridization curve of cDNA hybridized to an excess of mRNA was shown to be 7 x 10(-2) M . s indicating that about 15 non-cross-hybridizing sequences are present in the zein mRNA preparations. The kinetics of cDNA annealing with an excess of maize DNA from 2 n cells suggest a ten-times reiteration of each mRNA sequence. This result is confirmed from saturation experiments, where in cDNA excess to DNA, the number of zein genes per haploid maize genome was estimated as about 120 copies. Similar experiments carried out on DNA from normal and mutant endosperms (3n cells) indicate the absence of large amplifications or deletions of zein genes in the tissue devoted to zein synthesis.

[Zein of maize grain. Preparation and characterization]

A laboratory procedure for isolation and purification of zein from grains of 4 varieties of Maize was described. The preparations were characterized by their physicochemical properties. Upon polyacrylamide gel electrophoresis in sodium dodecyl sulphate (SDS), native zein (from INRA 260 hybrid) was resolved into 2 major classes with average molecular weights of 45,000 and 22,000. After reduction with mercaptoethanol zein contained only two subunits of 22,000 and 24,000 daltons. Upon starch gel electrophoresis in 6 M urea at pH 3.5, native zein exhibited five major or medium intensity bands and several minor ones. The latter, under reducing conditions, disappeared to reinforce the major bands or to yield some new minor bands. Amino acid analysis revealed a very low content of lysine. The NH2-terminal amino acids were determined to be threonine and phenylalanine with a preponderance of the former. Zeins isolated from the varieties studied appeared tohave the same NH2-terminal residues and similar amino acid compositions with an arginine/histidine ratio ranging from 1.1 to 1.2. They differed in relative importance of components, detected by electrophoresis in the presence of SDS or urea. Changes in zein characteristics with the grain genotype allow one to conclude that the components of molecular weights of 22,000 and 24,000 consist of several subunits differing in charge and amino acid content.