In Vitro Synthesis of a Precursor to the Methionine-rich Polypeptide of the Zein Fraction of Corn.
The messenger ribonucleic acid fraction isolated from a protein bodyenriched fraction of developing corn (Zea mays L.) endosperm stimulated the incorporation of radioactive amino acids into at least five polypeptides when added to a wheat germ extract capable of protein synthesis. Of these, the two major polypeptides formed with messenger from freshly frozen corn were identified as precursors to zein A and B, the two major polypeptides of the prolamine fraction of corn meal (21,600 and 19,600 molecular weight). The identification was based on the relative incorporations of radioactive leucine, lysine, and methionine, and the susceptibility of the zein A precursor, but not the zein B precursor to cleavage with cyanogen bromide. Using extracts from stored frozen corn, a third polypeptide of 14,500 molecular weight was identified as a major in vitro product. It was preferentially labeled with methionine and slightly larger than a similar peptide in the zein fraction of corn meal. Two other polypeptides of still lower molecular weight could be detected above the background of probably incomplete polypeptides.
Melcher
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Department of Biochemistry, Oklahoma State University, Stillwater, Oklahoma 74074.