Cereal Chem 72:443-449 |
Nucleotide Sequence of a gamma-Type Glutenin Gene from a Durum Wheat: Correlation with a gamma-Type Glutenin Subunit from the Same Biotype.
R. D'Ovidio, M. Simeone, S. Masci, E. Porceddu, and D. D. Kasarda. Copyright 1995 by the American Association of Cereal Chemists, Inc.
A gene coding for an apparent gamma-type glutenin subunit has been cloned from the biotype Lira 45 of the durum wheat cultivar Lira and sequenced. The nucleotide sequence of the gene from clone pTD9 showed almost perfect homology with a complete gamma-type glutenin gene of Triticum aestivum L. and with an incomplete, presumably gamma-type glutenin gene isolated from a different cultivar of Triticum durum Desf. All three genes code for proteins that have an additional cysteine residue in the N-terminal region at residue 26; there are nine cysteines in the molecule rather than the eight that are typical of gamma-gliadins. This odd cysteine makes it reasonably certain that the equivalent protein will be incorporated into glutenin polymers, because all other cysteines were homologous to those in classical gamma-gliadins, making it likely that they form the usual four intramolecular disulfide bonds characteristic of gamma-gliadins. Accordingly, the ninth (odd) cysteine should be available for intermolecular disulfide bond formation. The ability to form only a single intermolecular disulfide bond would cause the molecule to act as a chain terminator during formation of glutenin polymers. In support of this, a gamma-type glutenin was purified from a glutenin preparation derived from the same biotype (Lira 45) and characterized by N-terminal amino acid sequencing to show perfect homology at the N-terminus with the protein sequence derived from sequencing of the pTD9 clone, including the cysteine at residue 26. A computer molecular model of the protein equivalent to pTD9, based on the sequence and on the disulfide bond arrangements found for gamma-gliadins (K"hler et al 1993), provides a hypothetical three-dimensional structure for the gamma- type glutenin subunit.