Abstract
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 δ-gliadins, is the most direct ortholog of barley γ3-hordeins. Phylogenetic analysis shows that the orthologous δ-gliadins and γ3-hordeins form a distinct prolamin branch that existed separate from the γ-gliadins and γ-hordeins in an ancestral Triticeae prior to the branching of wheat and barley. The expressed δ-gliadins are encoded by a single gene in each of the hexaploid wheat genomes. This single δ-gliadin/γ3-hordein ortholog may be a general feature of the Triticeae tribe since examination of ESTs from three barley cultivars also confirms a single γ3-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 δ-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 δ-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 δ-gliadin gene to the complex Gli-1 plus Glu-3 region on chromosome 1.
Highlights
The c-type seed prolamins are widely distributed within the Triticeae, have been studied most extensively in wheat (c-gliadins), barley (c-hordeins), and rye (c-secalins), and have been proposed to be the most ancestral of the Triticeae prolamins [1]
The wheat prolamins have historically been divided into glutenin and gliadin (a, c, and v-gliadin) types dependent on whether they form polymers or exist mainly as monomers, respectively
The wheat a-gliadins are found on wheat chromosome 6 and related genes exist in many other Triticeae, but not barley, and are believed to have arisen as a translocation of one or more ancestral gliadin genes from chromosome 1 to chromosome 6 [1]
Summary
The c-type seed prolamins are widely distributed within the Triticeae, have been studied most extensively in wheat (c-gliadins), barley (c-hordeins), and rye (c-secalins), and have been proposed to be the most ancestral of the Triticeae prolamins [1]. There are relatively few gene sequences for barley chordeins in Genbank; e.g., only two Hordeum vulgare c3-hordein sequences – one covering a complete coding region (AK251750, [5]) and a partial sequence (X72628, [6]) along with 21 partial or complete more divergent H. chilense coding sequences [7]. Both c1 and c2 barley probes of Genbank return the same three matches Comparisons have indicated a orthologous relationship between the wheat cgliadins and barley c1- and c2-hordeins [3], but no closely related wheat sequence to c3-hordein has been reported
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