Molecular characterization of HMW-GS 1Dx3(t) and 1Dx4(t) genes from Aegilops tauschii and their potential value for wheat quality improvement

Two x-type high molecular weight glutenin subunits (HMW-GS) in Aegilops tauschii, 1Dx3(t) and 1Dx4(t) were identified by SDS-PAGE and MALDI-TOF-MS. Their complete coding sequences were isolated by AS-PCR. 1Dx3(t) and 1Dx4(t) genes consist of 2535 bp and 2508 bp and encode 845 and 836 amino acid residues, respectively. The deduced molecular masses of 1Dx3(t) and 1Dx4(t) gene products are 87655.26 Da and 86664.24 Da, respectively, well corresponding to the molecular masses measured by MALDI-TOF-MS. A total of 18 SNPs were identified between 1Dx3(t) and 1Dx4(t). Comparing with 1Dx5 subunit, 1Dx3(t) had a six amino acid insertion at 146-151 while the 1Dx4(t) had a nine amino acid deletion when compared with 1Dx3(t) subunit. The authenticity of the cloned 1Dx3(t) and 1Dx4(t) genes were confirmed by successful expression of their ORFs in E. coli. Comparison and phylogenetic tree based on the amino acid and nucleotide sequences confirmed that 1Dx3(t) was most closely related to 1Dx5 subunit that is widely accepted as a superior subunit for bread-making property. The secondary structure prediction demonstrated that 1Dx3(t) subunit has significantly high α-helix and β-strand contents, suggesting it might have positive effects on dough quality.     

Two Novel Y-Type High Molecular Weight Glutenin Genes in Chinese Wheat Landraces of the Yangtze-River Region

High molecular weight glutenin subunits (HMW-GSs) are key determinants for the end-use quality of wheat. Chinese wheat landraces are an important resource for exploring novel HMW-GS genes to improve the wheat baking quality. Two novel Glu-1Dy HMW-GSs (designated as 1Dy12.6 and 1Dy12.7) were identified and cloned from two Chinese wheat landraces Huazhong830 and Luosimai. The 1Dy12.6 and 1Dy12.7 subunits were deposited as the NCBInr Acc. No KR262518, and KR262519, respectively. The full open reading frames (ORFs) of 1Dy12.6 and 1Dy12.7 were 2022 bp and 1977 bp, encoding for proteins of 673 and 658 amino acid residues, respectively. Each contains four typical primary regions of HMW-GSs (a signal peptide, N- and C-terminal regions, and a central repetitive region). Their deduced molecular masses (70,165 Da and 68,400 Da) were strikingly consistent with those identified by MALDI-TOF-MS (69,985Da and 68,407 Da). The 1Dy12.6 is the largest 1Dy glutenin subunits cloned in common wheat up to date, containing longer repetitive central domains than other 1Dy encoded proteins. In comparison with the most similar active 1Dy alleles previously reported, the newly discovered alleles contained a total of 20 SNPs and 3 indels. The secondary structure prediction indicated that 1Dy12.6 and 1Dy12.7 have similar proportion of α-helix, β-turn, and β-bend to those of 1Dy10 (X12929). The phylogenetic analysis illustrated that the x- and y-type subunits of glutenins were well separated, but both 1Dy12.6 and 1Dy12.7 were clustered with the other Glu-1Dy alleles. Our results revealed that the 1Dy12.6 and 1Dy12.7 subunit have potential to strengthen gluten polymer interactions, and are valuable genetic resources for wheat quality improvement.     

一种检测转1Dx5基因小麦植株的新方法

目的:建立一种有效区分Glu-1D等位基因的Multiplex-PCR体系,快速检测转1Dx5小麦植株。方法:根据1Dx5和1Dx2亚基基因的区别设计特异的PCR引物,通过多重PCR技术检测花粉管通道法转化1Dx5核心片段的转基因T1代植株。结果:Multiplex-PCR能够在转基因T1代材料中扩增Glu-1D等位基因的特征条带,分别为343bp、320bp的1Dx5基因特异片段以及361bp的1Dx2基因特异片段,与预期结果一致。结论:该技术能够检测多个靶基因,有效地区分转基因Glu-1D上的等位基因,证实外源1Dx5基因已整合到受体基因组中,对检测基因组庞大、外源基因序列GC含量高且与内源基因同源性高的转基因小麦十分有效。     

Molecular characterization and phylogenetic analysis of a novel glutenin gene (Dy10.1t) from Aegilops tauschii.

A novel y-type high molecular mass glutenin subunit (HMM-GS) possessing a mobility that is slightly slower than that of the subunit Dy10 obtained by SDS-PAGE, named Dy10.1t, in the wild wheat Aegilops tauschii was identified by 1- and 2-dimensional gel electrophoresis, capillary electrophoresis, and matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF-MS). The gene encoding the HMM subunit Dy10.1t was amplified with allele-specific PCR primers, and the amplified products were cloned and sequenced. The coding domain of the Dy10.1t subunit gene consisted of 1980 bp encoding a protein of 658 residues with an M rs of 68 611 Da, which was similar to the M rs determined by MALDI-TOF-MS. The deduced amino acid sequence indicated that Dy10.1t subunit displayed a greater similarity to the Dy12 subunit, differing by only 8 amino acid substitutions. Six coding region single-nucleotide polymorphisms were discovered in the Dy10.1t gene by multiple alignments (1 per 330 bp), 1 in the N-terminal domain and the others in the central repeats. Five of them resulted in residue substitutions, whereas 3 created enzyme site changes. The homology and neighbour-joining trees constructed from code domain sequences of 20 x- and y-type glutenin genes from different Triticum species separated into 2 halves, which corresponded to the x-type and y-type HMM glutenin alleles. Phylogenetic analysis revealed that the Glu-1 gene duplication event probably occurred at about 16.83 million years ago, whereas the divergence times of A, B, and D genomes within x-type and y-type halves were before 7.047 and 10.54 million years ago, respectively.     

Hdf1, a yeast Ku-protein homologue, is involved in illegitimate recombination, but not in homologous recombination.

Hdf1 is the yeast homologue of the mammalian 70 kDa subunit of Ku-protein, which has DNA end-binding activity and is involved in DNA double-strand break repair and V(D)J recombination. To examine whether Hdf1 is involved in illegitimate recombination, we have measured the rate of deletion mutation caused by illegitimate recombination on a plasmid in an hdf1 disruptant. The hdf1 mutation reduced the rate of deletion formation by 20-fold, while it did not affect mitotic and meiotic homologous recombinations between two heteroalleles or homologous recombination between direct repeats. Hence Hdf1 participates in illegitimate recombination, but not in homologous recombination, in contrast to Rad52, Rad50, Mre11 and Xrs2, which are involved in both homologous and illegitimate recombination. The illegitimate recombination in the hdf1 disruptant took place between recombination sites that shared short regions of homology (1-4 bp), as was observed in the wild-type. Based on the DNA end-binding activity of Hdf1, we discuss models in which Hdf1 plays an important role in the late step of illegitimate recombination.     

Cloning and molecular characterization of a novel x-type glutenin gene Glu-D(t)1 from Aegilops tauschii

A novel gene encoding an x-type high molecular weight glutenin subunit (HMW-GS), designated 1Dx1.1 ^t , was isolated from Aegilops tauschii. It is the largest HMW-GS gene reported so far in this species and its product has a slower mobility than that of subunit 1Ax1 in SDS-PAGE. The open reading frame (ORF) of the gene was 2,628 bp, encoding a protein of 874 amino acid residues. Comparisons of amino acid sequences showed that subunit 1Dx1.1^t had high similarity with other 1Dx subunits but also had two unique characteristics. Firstly, a tripeptide of consensus LQE present in the N-terminal domains of other 1Dx subunits was absent from subunit Dx1.1^t. Secondly, three copies of tandem duplications of the tripeptide motif GQQ and a novel tripeptide sequence (GQL) were present in its central repetitive domain. Phylogenetic analysis showed that subunit 1Dx1.1^t clustered with other known 1Dx subunits.     

Cloning and sequencing of a cluster of genes encoding branched-chain alpha-keto acid dehydrogenase from Streptomyces avermitilis and the production of a functional E1 [alpha beta] component in Escherichia coli.

A cluster of genes encoding the E1 alpha, E1 beta, and E2 subunits of branched-chain alpha-keto acid dehydrogenase (BCDH) of Streptomyces avermitilis has been cloned and sequenced. Open reading frame 1 (ORF1) (E1 alpha), 1,146 nucleotides long, would encode a polypeptide of 40,969 Da (381 amino acids). ORF2 (E1 beta), 1,005 nucleotides long, would encode a polypeptide of 35,577 Da (334 amino acids). The intergenic distance between ORF1 and ORF2 is 73 bp. The putative ATG start codon of the incomplete ORF3 (E2) overlaps the stop codon of ORF2. Computer-aided searches showed that the deduced products of ORF1 and ORF2 resembled the corresponding E1 subunit (alpha or beta) of several prokaryotic and eukaryotic BCDH complexes. When these ORFs were overexpressed in Escherichia coli, proteins of about 41 and 34 kDa, which are the approximate masses of the predicted S. avermitilis ORF1 and ORF2 products, respectively, were detected. In addition, specific E1 [alpha beta] BCDH activity was detected in E. coli cells carrying the S. avermitilis ORF1 (E1 alpha) and ORF2 (E1 beta) coexpressed under the control of the T7 promoter.     

Silencing of HMW glutenins in transgenic wheat expressing extra HMW subunits

Wheat HMW glutenin subunit genes 1Ax1 and 1Dx5 were introduced, and either expressed or overexpressed, into a commercial wheat cultivar that already expresses five subunits. Six independent transgenic events were obtained and characterized by SDS-PAGE and Southern analysis. The 1Dx5 gene was overexpressed in two events without changes in the other endosperm proteins. Overexpression of 1Dx5 increased the contribution of HMW glutenin subunits to total protein up to 22%. Two events express the 1Ax1 subunit transgene with associated silencing of the 1Ax2* endogenous subunit. In the SDS-PAGE one of them shows a new HMW glutenin band of an apparent M_r lower than that of the 1Dx5 subunit. Southern analysis of the four events confirmed transformation and suggest that the transgenes are present in a low copy number. Silencing of all the HMW glutenin subunits was observed in two different events of transgenic wheat expressing the 1Ax1 subunit transgene and overexpressing the Dx5 gene. Transgenes and expression patterns were stably transmitted to the progenies in all the events except one where in some of the segregating T₂ seeds the silencing of all HMW glutenin subunits was reverted associated with a drastic lost of transgenes from a high to a low copy number. The revertant T₂ seeds expressed the five endogenous subunits plus the 1Ax1 transgene. Received: 16 June 1999 / Accepted: 29 July 1999     

Functional properties of flours from field grown transgenic wheat lines expressing the HMW glutenin subunit 1Ax1 and 1Dx5 genes

The high molecular weight glutenin subunits (HMW-GS) of wheat are major determinants of the viscoelastic properties of gluten and dough. The bread making quality of field grown transgenic lines of bread wheat expressing the HMW-GS 1Ax1 or 1Dx5 genes were evaluated over a two year period. Subunit 1Ax1 represented about 29% and 48% of the total HMW-GS in lines 1-2 and 2-2, respectively, while subunit 1Dx5 represented 65.4% and 62% of the total HMW-GS in transgenic lines 6-2 and 9, respectively. The expression of subunits 1Ax1 or 1Dx5 in transgenic wheat led to corresponding decreases in the proportions of endogenous HMW-GS. HMW-GS 1Ax1 and 1Dx5 had contrasting effects on dough quality determined by the Alveograph and sedimentation test. Subunit 1Ax1 increased the tenacity (P), extensibility (L), deformation work (W), and sedimentation value, with the increase being related to the level of expression. In contrast, subunit 1Dx5 led to a smaller increment in the tenacity (P), but to drastic decrease in both extensibility (L), deformation work (W), and the sedimentation value. Expression of subunit 1Ax1 in transgenic wheat resulted in lines with improved rheological properties whereas the lines expressing subunit 1Dx5 resulted in unsuitable breadmaking-related characteristics.     

Multiplex PCR identification of wheat HMW glutenin subunit genes by allele-specific markers

Wheat bread-making quality is closely correlated with composition and quantity of gluten proteins, in particular with high-molecular weight (HMW) glutenin subunits encoded by the Glu-1 genes. A multiplex polymerase chain reaction (PCR) method was developed to identify the allele composition of HMW glutenin complex Glu-1 loci (Glu-A1, Glu-B1 and Glu-D1) in common wheat genotypes. The study of multiplex PCR to obtain a well-balanced set of amplicons involved examination of various combinations of selected primer sets and/or thermal cycling conditions. One to three simultaneously amplified DNA fragments of HMW glutenin Glu-1 genes were separated by agarose slab-gel electrophoresis and differences between Ax1, Ax2* and Axnull genes of Glu-A1 loci, Bx6, Bx7 and Bx17 of Glu-B1, and Dx2, Dx5 and Dy10 genes of Glu-D1 loci were revealed. A complete agreement was found in identification of HMW glutenin subunits by both multiplex PCR analysis and SDS-PAGE for seventy-six Polish cultivars/strains of both spring and winter common wheat. Rapid identification of molecular markers of Glu-1 alleles by multiplex PCR can be an efficient alternative to the standard separation procedure for early selection of useful wheat genotypes with good bread-making quality.     

Introgression of 1Dx5+1Dy10 into Tritordeum

The uses of hexaploid tritordeum as a crop for human consumption require improvement of its bread-making quality. For this purpose chromosome 1D of bread wheat with the Glu-D1 allele encoding for high-molecular-weight glutenin subunits Dx5+Dy10 was introgressed into tritordeum. Different primary tritordeums were crossed with wheats carrying subunits Dx5+Dy10. The hybrids were backcrossed to tritordeum and seeds for the next backcross (or selfing) were selected for the presence of chromosome 1D using SDS-PAGE. Forty two chromosome plants carrying subunits Dx5+Dy10 were obtained after two backcrosses and selfing. Chromosome characterization of these plants using fluorescence in situ hybridisation (FISH) proved that either chromosome substitution 1H(ch)/1D or 1A/1D had been obtained. A homozygous plant with a translocation of the entire 1DL arm to 1H(ch)S was also obtained. The complete chromosome substitution lines have better agronomic characteristics than the lines with translocations.     

Conservation in wheat high-molecular-weight glutenin gene promoter sequences: comparisons among loci and among alleles of the GLU-B1-1 locus

 The high-molecular-weight glutenin (HMW) genes and encoded subunits are known to be critical for wheat quality characteristics and are among the best-studied cereal research subjects. Two lines of experiments were undertaken to further understand the structure and high expression levels of the HMW-glutenin gene promoters. Cross hybridizations of clones of the paralogous x-type and y-type HMW-glutenin genes to a complete set of six genes from a single cultivar showed that each type hybridizes best within that type. The extent of hybridization was relatively restricted to the coding and immediate flanking DNA sequences. Additional DNA sequences were determined for four published members of the HMW-glutenin gene family (encoding subunits Ax2^*, Bx7, Dx5, and Dy10) and showed that the flanking DNA of the examined genes diverge at approximately −1200 bp 5′ to the start codon and 200–400 bp 3′ to the stop codon. These divergence sites may indicate the boundaries of sequences important in gene expression. In addition, promoter sequences were determined for alleles of the Bx gene (Glu-B1-1), a gene reported to show higher levels of expression than other HMW-glutenin genes and with variation among cultivars. The sequences of Bx promoters from three cultivars and one wild tetraploid wheat indicated that all Bx alleles had few differences and contained a duplicated portion of the promoter sequence “cereal-box” previously suspected as a factor in higher levels of expression. Thus, the “cereal-box” duplication preceeded the origin of hexaploid wheat, and provides no evidence to explain the variations in Bx subunit synthesis levels. One active Bx allele contained a 185-bp insertion that evidently resulted from a transposition event. Received: 5 August 1997 / Accepted: 6 November 1997     

Mitotic illegitimate recombination is a mechanism for novel changes in high-molecular-weight glutenin subunits in wheat-rye hybrids

Wide hybrids can have novel traits or changed expression of a quantitative trait that their parents do not have. These phenomena have long been noticed, yet the mechanisms are poorly understood. High-molecular-weight glutenin subunits (HMW-GS) are seed storage proteins encoded by Glu-1 genes that only express in endosperm in wheat and its related species. Novel HMW-GS compositions have been observed in their hybrids. This research elucidated the molecular mechanisms by investigating the causative factors of novel HMW-GS changes in wheat-rye hybrids. HMW-GS compositions in the endosperm and their coding sequences in the leaves of F(1) and F(2) hybrids between wheat landrace Shinchunaga and rye landrace Qinling were investigated. Missing and/or additional novel HMW-GSs were observed in the endosperm of 0.5% of the 2078 F(1) and 22% of 36 F(2) hybrid seeds. The wildtype Glu-1Ax null allele was found to have 42 types of short repeat sequences of 3-60 bp long that appeared 2 to 100 times. It also has an in-frame stop codon in the central repetitive region. Analyzing cloned allele sequences of HMW-GS coding gene Glu-1 revealed that deletions involving the in-frame stop codon had happened, resulting in novel ～1.8-kb Glu-1Ax alleles in some F(1) and F(2) plants. The cloned mutant Glu-1Ax alleles were expressed in Escherichia coli, and the HMW-GSs produced matched the novel HMW-GSs found in the hybrids. The differential changes between the endosperm and the plant of the same hybrids and the data of E. coli expression of the cloned deletion alleles both suggested that mitotic illegitimate recombination between two copies of a short repeat sequence had resulted in the deletions and thus the changed HMW-GS compositions. Our experiments have provided the first direct evidence to show that mitotic illegitimate recombination is a mechanism that produces novel phenotypes in wide hybrids.     

Marker-assisted selection of high molecular weight glutenin alleles related to bread-making quality in Iranian common wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.)

Bread-making quality in hexaploid wheats is a complex trait. It has been shown that the amount and composition of protein can influence dough rheological properties. The high-molecular-weight (HMW) glutenins are encoded by a complex locus, Glu-1, on the long arm of group-1 homoeologus chromosome of the A, B and D genomes. In this work we used PCR-based DNA markers as a substitution tool to distinguish wheat bread-making quality. We detected PCR-based DNA markers for coding sequence of Glu-A1x, Glu-B1x and Glu-D1x to be 2300 bp, 2400 bp and 2500 bp respectively. DNA markers related to coding sequence of Glu-A1y, Glu-B1y and Glu-D1y were; 1800 bp, 2100 bp and 1950 bp, however, the repetitive region of their coding sequence were shown to be about 1300 bp, 1500 bp and 1600 bp. The results demonstrate that the size variation was due to different lengths of the central repetitive domain. Good or poor bread-making quality in wheat is associated with two allelic pairs of Glu-D1, designated 1Dx5-1Dy10 and 1Dx2-1Dy12. The 1Bx7 allele has moderate-to-good quality score. The specific DNA markers, of 450 bp, 576 bp, 612 bp and 2400 bp respectively were characterized for 1Dx5, 1Dy10, 1Dy12 and 1Bx7 alleles. These markers are very important in screening of wheat for bread-making quality.     

Molecular cloning of a novel chimeric HMW glutenin subunit gene <Emphasis Type="Italic">1Dx5′</Emphasis> from a common wheat line W958

A novel chimeric high-molecular-weight (HMW) glutenin subunit gene from a new common wheat line W958 (2n = 6x = 42) was isolated and characterized. SDS–PAGE analysis revealed that this glutenin subunit has similar electrophoretic mobility to 1Dx5, so it was designated 1Dx5′. Genomic DNA from W958 was amplified and a 2,505-bp fragment was obtained. The 1Dx5′ subunit showed a chimeric primary structure of 1Dx5 and 1Dx2, with the 1Dx5 sequence in the 5′ and middle repetitive regions and the 1Dx2 sequence in the repetitive domain and 3′ region. MALDI-TOF-MS analysis demonstrated that 1Dx5′ had a molecular weight of 86815.1 Da, close to that of an x-type glutenin subunit. Secondary structure analysis showed that this subunit had six helixes and one strand, including four helixes in the repetitive domain which could enhance the dough properties. Additionally, the promoter of 1Dx5′ was obtained and showed the same sequence as 1Dx5 or 1Dx2 except for a few base conversions. The promoter analysis indicated that the cis-acting regulatory elements of 1Dx5′ were the same as those of 1Dx5 and/or 1Dx2. Previously, we have demonstrated that this novel glutenin subunit is associated with good bread-making quality and comprises a very large proportion of the F2 segregation population. Consequently, we suggest that the amino acid residue composition and the secondary structure of the subunit may contribute to the bread-making quality. In summary, the novel 1Dx5′ gene could have greater potential in wheat quality improvement.     

Effects of Mutations of Rad50, Rad51, Rad52, and Related Genes on Illegitimate Recombination in Saccharomyces Cerevisiae

To examine the mechanism of illegitimate recombination in Saccharomyces cerevisiae, we have developed a plasmid system for quantitative analysis of deletion formation. A can1 cyh2 cell carrying two negative selection markers, the CAN1 and CYH2 genes, on a YCp plasmid is sensitive to canavanine and cycloheximide, but the cell becomes resistant to both drugs when the plasmid has a deletion over the CAN1 and CYH2 genes. Structural analysis of the recombinant plasmids obtained from the resistant cells showed that the plasmids had deletions at various sites of the CAN1-CYH2 region and there were only short regions of homology (1-5 bp) at the recombination junctions. The results indicated that the deletion detected in this system were formed by illegitimate recombination. Study on the effect of several rad mutations showed that the recombination rate was reduced by 30-, 10-, 10-, and 10-fold in the rad52, rad50, mre11, and xrs2 mutants, respectively, while in the rad51, 54, 55, and 57 mutants, the rate was comparable to that in the wild-type strain. The rad52 mutation did not affect length of homology at junction sites of illegitimate recombination.     

Role of DNA ligase in the illegitimate recombination that generates lambdabio-transducing phages in Escherichia coli

We studied the role of DNA ligase in illegitimate recombination in Escherichia coli. A temperature-sensitive mutation in the lig gene reduced the frequency with which lambdabio-transducing phages were generated to 10-14% of that of wild type under UV irradiation. Reintroduction of the lig gene into this mutant restored the frequency of recombinant phage generation to that of wild type. Furthermore, overexpression of DNA ligase enhanced illegitimate recombination by 10-fold with or without UV irradiation. In addition, when DNA ligase was present in only limited amounts, UV-induced or spontaneous illegitimate recombination occurred exclusively at hotspot sites that have relatively long sequences of homology (9 or 13 bp). However, when DNA ligase was overexpressed, most of the illegitimate recombination took place at non-hotspot sites having only short sequences of homology (<4 bp). Thus, the level of ligase activity affects the frequency of illegitimate recombination, the length of sequence homology at the recombination sites, and the preference for recombination at hotspots, at least after UV irradiation. These observations support our hypothesis that the illegitimate recombination that generates lambdabio-transducing phages is mediated by the DNA break-and-join mechanism.     

Identification and molecular characterisation of HMW glutenin subunit 1By16* in wild emmer

In this study, a novel y-type high molecular weight glutenin subunit (HMW-GS) in wild emmer wheat Triticum turgidum L. var. dicoccoides (K?rn.) accession KU1952 was identified by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), capillary electrophoresis (CE) and matrix-assisted laser desorption ionisation/time-of-flight/mass spectrometry (MALDI-TOF-MS). Its electrophoretic mobility and molecular weight were similar to those of 1By16 and was designated as 1By16*. The complete coding sequence of the 1By16* gene isolated by allelic-specific polymerase chain reaction (AS-PCR) consists of 2,157 bp, encoding 729 amino acid residues. The real presence and authenticity of the 1By16* gene in KU1952 were further confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), heterologous expression and Western blotting. The molecular structure as well as phylogenetic analysis revealed that 1By16* had 21 single-nucleotide polymorphism (SNP) variations and possessed greater similarity with superior quality subunits 1By15 and 1By16 of common wheat. Secondary structure prediction displayed higher α-helix and β-strand contents in the 1By16* subunit, which could form a superior gluten structure and, consequently, might have positive effects on dough quality. Our results suggest that 1By16* is expected to be a new potential gene for wheat quality improvement.