Sequence analysis of a genomic clone encoding a Zc2 protein from Zea mays W64 A

Production of α-L-iduronidase in maize for the potential treatment of a human lysosomal storage disease

Lysosomal storage diseases are a class of over 70 rare genetic diseases that are amenable to enzyme replacement therapy. Towards developing a plant-based enzyme replacement therapeutic for the lysosomal storage disease mucopolysaccharidosis I, here we expressed α-L-iduronidase in the endosperm of maize seeds by a previously uncharacterized mRNA-targeting-based mechanism. Immunolocalization, cellular fractionation and in situ RT-PCR demonstrate that the α-L-iduronidase protein and mRNA are targeted to endoplasmic reticulum (ER)-derived protein bodies and to protein body-ER regions, respectively, using regulatory (5'- and 3'-UTR) and signal-peptide coding sequences from the γ-zein gene. The maize α-L-iduronidase exhibits high activity, contains high-mannose N-glycans and is amenable to in vitro phosphorylation. This mRNA-based strategy is of widespread importance as plant N-glycan maturation is controlled and the therapeutic protein is generated in a native form. For our target enzyme, the N-glycan structures are appropriate for downstream processing, a prerequisite for its potential as a therapeutic protein.

Expression of the gamma-zein protein of maize in seeds of transgenic barley: effects on grain composition and properties

A cDNA clone encoding the gamma-zein protein of maize was expressed in developing grain of barley using the starchy endosperm cell-specific promoter from the wheat Glu-1D-1 (HMW subunit 1Dx5) gene. Seven transgenic lines were recovered from 226 bombarded immature embryos, of which two were sterile and four tetraploid, while five were shown to express the gamma-zein protein based on western blotting. Southern blot analysis showed the presence of between about three and twelve transgene insertions. Detailed comparative studies of five null and five homozygous transformed sub-lines from transgenic line A showed that gamma-zein accounted for over 4% of the total prolamin fraction, corresponding to about 1.9% of the total grain N. Comparison of the proteins present in the gel protein fraction demonstrated that the gamma-zein was incorporated into polymers, as in maize. However, there was no effect on grain hardness measured using the Perten Single Kernel Characterisation System or on the vitreousness measured by visual inspection. This contrasts with the situation in maize where a clear association with vitreousness has been reported.

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).

Lysine-rich modified gamma-zeins accumulate in protein bodies of transiently transformed maize endosperms

During maize seed development, endosperm cells synthesize large amounts of storage proteins, alpha-, beta-, and gamma-zeins, which accumulate within endoplasmic reticulum (ER)-derived protein bodies. The absence of lysine in all zein polypeptides results in an imbalance in the amino acid composition of maize seeds. We modified the maize gamma-zein gene through the introduction of lysine-rich (Pro-Lys)n coding sequences at different sites of the gamma-zein coding sequence. Maize endosperms were transiently transformed by biolistic bombardment with Lys-rich gamma-zein constructs under the control of the 1.7 kb gamma-zein seed-specific promoter and the cauliflower mosaic virus (CaMV) 35S promoter. When (Pro-Lys)n sequences were inserted contiguous to or in substitution of the Pro-Xaa region of the gamma-zein, high levels of protein were observed. In contrast, when (Pro-Lys)n sequences were inserted five residues from the C-terminal, the transcript was present but modified protein was not detected. These results suggest that only an appropriate positioning of Lys-rich inserts leads to the modified molecule displaying correct folding and stability. Subcellular localization analyses and immunoelectron microscopy studies on isolated protein bodies demonstrated that modified gamma-zeins accumulate within these organelles and co-localized with endogenous alpha- and gamma-zeins. The studies reported here show the feasibility of manipulating the gamma-zein gene in order to obtain stable and correctly targeted Lys-rich zeins in maize seeds.

Tissue-specific expression in transgenic maize of four endosperm promoters from maize and rice

The tissue-specific, developmental, and genetic control of four endosperm-active genes was studied via expression of GUS reporter genes in transgenic maize plants. The transgenes included promoters from the maize granule-bound starch synthase (Waxy) gene (zmGBS), a maize 27 kDa zein gene (zmZ27), a rice small subunit ADP-glucose pyrophosphorylase gene (osAGP) and the rice glutelin 1 gene (osGT1). Most plants had a transgene expression profile similar to that of the endogenous gene: expression in the pollen and endosperm for the zmGBS transgene, and endosperm only for the others. Histological analysis indicated expression initiated at the periphery of the endosperm for zmGBS, zmZ27 and osGT1, while osAGP transgene activity tended to start in the lower portion of the seed. Transgene expression at the RNA level was proportional to GUS activity, and did not influence endogenous gene expression. Genetic analysis showed that there was a positive dosage response with most lines. Activity of the zmGBS transgene was threefold higher in a low starch (shrunken 2) genetic background. This effect was not seen with zmZ27 or osGTI transgenes. The expression of the transgenes is discussed relative to the known behaviour of the endogenous genes, and the developmental programme of the maize endosperm.

The maize GapC4 promoter confers anaerobic reporter gene expression and shows homology to the maize anthocyanin regulatory locus C1

The cytosolic glyceraldehyde-3-phosphate dehydrogenase (GapC) gene family of maize is differentially expressed in response to anaerobic stress. While GapCl and GapC2 are downregulated, GapC3 and GapC4 are anaerobically induced. We have sequenced and analyzed a 3073 bp promoter fragment of GapC4. The promoter confers anaerobic induction of a reporter gene construct in a transient gene expression system in maize. Deletion analysis of the GapC4 promoter revealed a 270 bp long DNA region required for anaerobic induction. This region contains sequence motifs resembling the cis-acting sequences of the anaerobically induced maize Adh1 and Adh2 genes. Furthermore, the 3073 bp GapC4 promoter fragment displays homology to long terminal repeats of maize retrotransposons and to the 3' region of the maize anthocyanin regulatory locus C1.

Zeon-1, a member of a new maize retrotransposon family

We have previously shown that the tandemly duplicated 27 kDa maize storage protein locus underwent mitotic rearrangement to yield a single-copy allele in isolates of the inbred line A188. This rearrangement contains a new LTR retrotransposon, designated Zeon-1. This middle repetitive element of 7313 bp had two long terminal repeats, a primer binding site, a polypurine tract and a gag-related open reading frame of 375 amino acids. Transcripts of the gag-related region were detected by the polymerase chain reaction (PCR) in certain maize tissues, and Western blots detected the gag-related protein in the same tissues. Moreover, the product of this mitotic rearrangement was shown to contain the same insertion site and 3' LTR as Zeon-1, suggesting that this rearrangement occurs with unusual precision. Zeon elements were found to be present in teosinte and not present in the Gramineae wheat, barley, sorghum and rye.

Identification of two opaque2 modifier loci in quality protein maize

Genetic modifiers of opaque2 convert the soft, starchy endosperm of opaque2 maize mutants to a hard, vitreous phenotype, while maintaining the enhanced lysine content of the grain. Genetic analysis of F2 segregating seeds from crosses of opaque2 by modified opaque2 genotypes indicated that the modifiers are complex traits that act codominantly. We developed two different segregating F2 populations and mapped the modifier loci by restriction fragment length polymorphism (RFLP) analysis. A relationship was found between formation of vitreous endosperm and the locus encoding the gamma-zein storage protein, which maps near the centromere of chromosome 7. Endosperm modification was consistently associated with the presence of two rather than one gamma-zein gene at this locus. A second modifier locus was mapped near the telomere of chromosome 7L.

Narrow A/T-rich zones present at the distal 5'-flanking sequences of the zein genes Zc1 and Zc2 bind a unique 30 kDa HMG-like protein

Nuclear extracts from maize endosperm were used to investigate protein-DNA interactions in the 5'-upstream region of the Zc1 and Zc2 genes. These genes encode for zeins of apparent molecular mass (MWapp) 16 and 28 kDa, respectively, which accumulate in the endosperm during seed maturation. Binding assays revealed specific binding of a nuclear protein to three A/T-rich elements, 0.9-1.0 kbp upstream from the initiation codon. One of these elements (41 bp, 88% A/T), present in Zc1, contained a 13 nucleotide duplication. The other two (28 bp, 86% A/T; 42 bp alternating A-T) are consecutive elements in Zc2. Competition experiments strongly suggest that the three elements bind to the same protein. Protein-DNA interaction was detected in endosperm nuclear extracts of 8 to 21 days after pollination (DAP), as well as in 25 DAP embryos and in different tissues from plantlets. The protein factor has an MWapp of ca. 30 kDa. This factor has properties suggesting it is an HMG-like protein. These results are consistent with a growing accumulation of data for a number of genes indicating that A/T-rich elements, located at distal and proximal zones of the 5'-flanking sequences, interact with HMG-like proteins.

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.

Tourist: a large family of small inverted repeat elements frequently associated with maize genes

The wx-B2 mutation results from a 128-bp transposable element-like insertion in exon 11 of the maize Waxy gene. Surprisingly, 11 maize genes and one barley gene in the GenBank and EMBL data bases were found to contain similar elements in flanking or intron sequences. Members of this previously undescribed family of elements, designated Tourist, are short (133 bp on average), have conserved terminal inverted repeats, are flanked by a 3-bp direct repeat, and display target site specificity. Based on estimates of repetitiveness of three Tourist elements in maize genomic DNA, the copy number of the Tourist element family may exceed that of all previously reported eukaryotic inverted repeat elements. Taken together, our data suggest that Tourist may be the maize equivalent of the human Alu family of elements with respect to copy number, genomic dispersion, and the high frequency of association with genes.

Sequence variation between alleles reveals two types of copy correction at the 27-kDa zein locus of maize

In many inbred lines of maize, two 27-kDa storage protein (zein) genes are found within tandem duplications of 12 kb. Both genes of the duplicated allele from the maize inbred line A188 were sequenced and compared to a similar duplicated allele in another inbred line, W22, and to a single-copy allele in the inbred line W64A. The comparisons reveal interesting patterns in the distribution of sequence changes between these alleles. Differences between the two duplicated alleles that are conserved between the two genes of each allele are found exclusively in the 5' region. In contrast, differences between the individual genes of each allele in the 3' region are conserved between the two alleles. The first case is indicative of an intraallelic copy correction mechanism, whereas the second may result from interallelic copy correction. These may be mediated by gene conversion processes, as previously described for other multigene families.