Short tandem repeats shared by B- and C-hordein cDNAs suggest a common evolutionary origin for two groups of cereal storage protein genes

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Abstract
We have identified cDNA clones coding for the major sulphur‐rich and sulphur‐poor groups of barley storage proteins (the B‐ and C‐hordeins, respectively). Hybridization studies have revealed unexpected homologies between B‐ and C‐hordein mRNAs. Using a deletion mutant (Risø 56), we have mapped some C‐hordein‐related sequences within, or closely associated with, B‐hordein genes at the Hor 2 locus. Nucleotide sequencing has shown that the primary structure of B‐hordein polypeptides can be divided into at least two domains: domain 1 (repetitive, proline‐rich, sulphur‐poor), which is homologous to C‐hordein sequences, and domain 2 (non‐repetitive, proline‐poor, sulphur‐rich), which makes up two‐thirds of the polypeptide and is partially homologous to a 2S globulin storage protein found in dicotyledons. The coding sequences that are homologous in B‐ and C‐hordein mRNAs have an asymmetric base composition (>80% C‐A) and are largely composed of a degenerate tandem repeat based on a 24 nucleotide consensus that encodes Pro‐Gln‐Gln‐Pro‐Phe‐Pro‐Gln‐Gln. We discuss the evolutionary implications of the domain structure of the B‐hordeins and the unusual relationship between the two groups of barley storage proteins.