A New Class of Wheat Gliadin Genes and Proteins

The utility of mining DNA sequence data to understand the structure and expression of cereal prolamin genes is demonstrated by the identification of a new class of wheat prolamins. This previously unrecognized wheat prolamin class, given the name delta-gliadins, is the most direct ortholog of barley gamma 3-hordeins. Phylogenetic analysis shows that the orthologous delta-gliadins and gamma 3-hordeins form a distinct prolamin branch that existed separate from the gamma-gliadins and gamma-hordeins in an ancestral Triticeae prior to the branching of wheat and barley. The expressed delta-gliadins are encoded by a single gene in each of the hexaploid wheat genomes. This single delta-gliadin/gamma 3-hordein ortholog may be a general feature of the Triticeae tribe since examination of ESTs from three barley cultivars also confirms a single gamma 3-hordein gene. Analysis of ESTs and cDNAs shows that the genes are expressed in at least five hexaploid wheat cultivars in addition to diploids Triticum monococcum and Aegilops tauschii. The latter two sequences also allow assignment of the delta-gliadin genes to the A and D genomes, respectively, with the third sequence type assumed to be from the B genome. Two wheat cultivars for which there are sufficient ESTs show different patterns of expression, i.e., with cv Chinese Spring expressing the genes from the A and B genomes, while cv Recital has ESTs from the A and D genomes. Genomic sequences of Chinese Spring show that the D genome gene is inactivated by tandem premature stop codons. A fourth delta-gliadin sequence occurs in the D genome of both Chinese Spring and Ae. tauschii, but no ESTs match this sequence and limited genomic sequences indicates a pseudogene containing frame shifts and premature stop codons. Sequencing of BACs covering a 3 Mb region from Ae. tauschii locates the delta-gliadin gene to the complex Gli-1 plus Glu-3 region on chromosome 1.