Isolation of polyribosomes and messenger RNA active in in vitro synthesis of soybean seed proteins.

Polyribosome preparations containing low proportions of monosomes to polyribosomes have been isolated from developing seeds of Glycine max L. Merrill using a high pH-high KCl buffer. The polyribosomes were functional in in vitro protein synthesis reactions using wheat germ 23,000g supernatant preparations. Results of experiments using aurintricarboxylic acid indicated that most or all of the amino acid incorporation in vitro resulted from the completion of nascent polypeptides associated with the isolated polyribosmes. RNA purified from polyribosome preparations by affinity chromatography on oligo(dT)-cellulose was also active in vitro, and had different Mg and K requirements for translation than did the polyribosomes. Translation of oligo(dT)-cellulose-purified mRNA was inhibited by the addition of 7-methylguanosine 5'-phosphate, suggesting that soybean mRNAs are "capped" at their 5' ends. Some, but not all, of the products of these reactions were identical in electrophoretic mobility to radioactive polypeptides of storage proteins produced in soybean cotyledons grown in culture.