Structure, expression, and heterogeneity of the rice seed prolamines.

By screening two rice (Oryza sativa L.) seed cDNA libraries, recombinant cDNA clones encoding the rice prolamine seed storage protein were isolated. Based on cross-hybridization and restriction enzyme map analyses, these clones can be divided into two homology classes. All clones contain a single open reading frame encoding a putative rice prolamine precursor (molecular weight = 17,200) possessing a typical 14-amino acid signal peptide. The deduced primary structures of both types of prolamine polypeptides are devoid of repetitive sequences, a feature prevalent in other cereal prolamines. Clones of these two homology classes diverge mainly by insertions/deletions of short nucleotide stretches and point mutations. An isolated genomic clone about 15.5 kilobases in length displays a highly conserved 2.5-kilobase EcoRI fragment, repeated in tandem four times, each containing the prolamine coding sequence. Close homology is exhibited by the coding segments of the genomic and cDNA sequences, although the 5' ends of the untranslated regions are widely divergent. The sequence heterogeneity displayed by these genomic and cDNA clones and large gene copy number ( approximately 80-100 copies/haploid genome) indicate that the rice prolamines are encoded by a complex multigene family.