Evolution and heterogeneity of the alpha-/beta-type and gamma-type gliadin DNA sequences

Near full length cDNA clones for both alpha-/beta- and gamma-type gliadins were isolated and studied for sequence diversity. Based on restriction site polymorphism and cross-hybridization studies, alpha-/beta- and gamma-type clones could be divided into five and three homology classes, respectively. Clones representing each of the different classes were sequenced and compared. Sequence divergence between the classes was due to single-base substitutions and to duplications or deletions within or near direct repeats. Thus, through numerous duplications and subsequent divergence, the gliadin multigene family encodes a polymorphic set of polypeptides differing in both isoelectric point and molecular size. Southern blot analysis of wheat DNA suggested that the number of genes encoding the alpha-/beta-type gliadins was extremely large (greater than 100 copies/haploid genome). Inasmuch as hybridization patterns were the same using DNA isolated from seeds or leaves, amplification or rearrangement of DNA does not occur during development. The complete coding sequence of a gamma-gliadin was similar to that observed for the alpha-/beta-gliadins, but with several notable differences. Comparison of gamma-type gliadin cDNA sequences showed that, unlike the conserved dodecamer repeat common to all the alpha-/beta-gliadins, the tandem repeat unit differed among gamma-gliadin clones.