Submitted to: Theoretical and Applied Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/19/2004
Publication Date: 4/22/2004
Citation: Gu, Y.Q., Crossman, C.C., Kong, X., Luo, M., You, F.M., Coleman Derr, D., Dubcovsky, J., Anderson, O.D. 2004. Genomic organization of the complex a-gliadin gene loci in wheat. Theoretical and Applied Genetics. 109:648-657. Interpretive Summary: Wheat grain, of which 9-15 %(dry eight) is protein, is a major source of energy and nutrition in the human diet. The majority of the seed storage proteins are prolamins, the compositions of which determine wheat flour's unique biophysical and biochemical properties, which allow a wide range of food products; i.e. leavened and unleavened breads, biscuits, cookies, and pasta, etc. The a-gliadins are the major components of the prolamins. The high content of this protein in wheat grain is primarily due to the presence of large numbers of the a-gliadins genes in the wheat genome. However, little is known about the distribution of a-gliadins genes along the chromosomes. In this manuscript, we isolated bacterial artificial chromosome (BAC) clones that carry large fragments of wheat genomic DNAs containing a-gliadins genes. The insert DNAs were then digested with restriction endonuclease enzymes that cut the DNAs into fragments by recognizing specific sequences. Computer program was used to identify shared fragments among the BAC clones and used these fragments to joint the overlapping BAC clones to form large contiguous wheat chromosome regions. The results indicate that many of the a-gliadins genes are clustered in a small region, while others are distantly away from each other. The knowledge will enhance our understanding of the organization of a-gliadins genes in the wheat genome and help us develop a better strategy for the improvement of wheat storage protein content and composition.
Technical Abstract: To better understand the molecular evolution of the large a-gliadin gene family, a half-million BAC library clones from tetraploid durum wheat, Triticum turgidum ssp. durum (2n = 4x = 28, genome AB), were screened for large genomic segments carrying the a-gliadin genes of the Gli-2 loci on the group 6 homoeologous chromosomes. The resulting 220 positive BAC clones, each containing between one and four copies of a-gliadin sequences, were fingerprinted for contig assembly to produce contiguous chromosomal regions covering the Gli-2 loci. While contigs consisting of as many as 21 BAC clones and containing up to17 a-gliadin genes were formed, many BAC clones remained as singletons. The accuracy of the order of BAC clones in the contigs was verified by the Southern hybridization analysis of the BAC fingerprints using an a-gliadin probe. These results indicate that a-gliadin genes are not evenly dispersed in the Gli-2 locus regions. Hybridization of these BACs with probes for long terminal repeat (LTR) retrotransposons was used to determine the abundance and distribution of these repetitive DNAs in this region. Sequencing of BAC ends indicated that 70 % of the sequences were significantly similar to different classes of retrotransposons, suggesting that these elements are abundant in this region. Several mechanisms underlying the dynamic evolution in the Gli-2 loci are discussed.