Messenger RNA for G1 protein of French bean seeds: Cell-free translation and product characterization

The fraction of poly(A)-containing RNA isolated from ripening bean (Phaseolus vulgaris) cotyledons that sedimented at 16 S in linear logarithmic sucrose gradients was at least as active a messenger as viral RNA when added to a cell-free protein-synthesizing system from wheat germ. The major products synthesized in vitro were polypeptides of about 47,000 and 43,000 daltons, corresponding to two of the three subunits of G1 protein, the most abundant bean seed storage protein. No trace of the largest (53,000 daltons) subunit was found among the polypeptides synthesized in vitro. Proof that the 47,000- and 43,000-dalton polypeptides coded by the 16S RNA were indeed subunits of G1 protein was obtained by immunoprecipitation with monovalent antibody to G1 protein and by electrophoretic mapping of peptides on acrylamide gels after digestion of mixtures of authentic protein and radioactive translation products with protease V8, chymotrypsin, and trypsin. The subunits synthesized in vitro were slightly smaller than the native subunits, probably because they lacked the sugar residues present on the holoprotein.