Each zein gene class can produce polypeptides of different sizes

The family of zein proteins in maize consists of several prototypes whose amino sequence is characterized by a repetitive block structure. We have now isolated and characterized a new series of zein cDNA clones from a cDNA library made from protein-body polysomal mRNA. The nucleotide sequences and the hybrid-selected translation results indicate that anomalous zein genes with termination codons within the coding region are transcribed, translated and possibly accumulated into zein protein bodies. The sequence analysis of these new clones and their comparison with already characterized zein sequences, all from the same maize line, indicate a large variability in the amino acid sequence of the blocks both among the prototypes and within members of each prototype. Northern blot analysis of total RNA from endosperms at different maturation stages shows a differential expression of each prototype sequence with the production, within each family, of zein transcripts coding for discrete size classes of polypeptides. The results allow a better definition of the zein system, clarify the relationship between sequence types and polypeptide size classes, and suggest the possibility of inserting lysine codons to raise the nutritional value of the seed storage protein.