NUCLEOTIDE-SEQUENCE OF A GAMMA-TYPE GLUTENIN GENE FROM A DURUM-WHEAT - CORRELATION WITH A GAMMA-TYPE GLUTENIN SUBUNIT FROM THE SAME BIOTYPE

A gene coding for an apparent gamma-type glutenin subunit has been clo ned from the biotype Lira 45 of the durum wheat cultivar Lira and sequ enced. The nucleotide sequence of the gene from clone pTD9 showed almo st perfect homology with a complete gamma-type glutenin gene of Tritic um aestivum L. and with an incomplete, presumably gamma-type glutenin gene isolated from a different cultivar of Triticum durum Desf. All th ree genes code for proteins that have an additional cysteine residue i n the N-terminal region at residue 26; there are nine cysteines in the molecule rather than the eight that are typical of gamma-gliadins. Th is odd cysteine makes it reasonably certain that the equivalent protei n will be incorporated into glutenin polymers, because all other cyste ines were homologous to those in classical gamma-gliadins, making it l ikely that they form the usual four intramolecular disulfide bonds cha racteristic of gamma-gliadins. Accordingly, the ninth (odd) cysteine s hould be available for intermolecular disulfide bond formation. The ab ility 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 ch aracterized by N-terminal amino acid sequencing to show perfect homolo gy at the N-terminus with the protein sequence derived from sequencing of the pTD9 clone, including the cysteine at residue 26. A computer m olecular model of the protein equivalent to pTD9, based on the sequenc e and on the disulfide bond arrangements found for gamma-gliadins (Koh ler et al 1993), provides a hypothetical three-dimensional structure f or the gamma-type glutenin subunit.