Cell-free Synthesis of the Major Storage Protein of the Bean, Phaseolus vulgaris L.

As seeds of the French bean (Phaseolus vulgaris, L. cv. Tendergreen) mature, a single protein, G1 globulin (analogous to legumin), represents the majority of protein synthesized. Washed polysomes extracted from developing cotyledons had little endogenous activity in amino acid incorporation, but on addition of cell-free extracts from wheat germ, active incorporation was obtained, the level being similar to that with viral RNA as messenger. The Mg(2+) optimum for protein synthesis in the presence of bean polysomes was 6 mm compared with 4 mm for synthesis of viral polypeptides in the wheat germ system. Using T-2 toxin as an inhibitor, it was shown that 29% of the incorporation depended on initiation events. Electrophoretic analysis of the total polypeptide products of cell-free synthesis gave a disperse profile. Centrifugation to remove polysome-bound peptides after 60 minutes incubation gave a supernatant having a product with the same electrophoretic mobility as G1 globulin and containing 26% of the radioactivity present in the gel. Protein eluted from this peak was subjected to re-electrophoresis and shown to consist of the three polypeptide subunits characteristic of G1 globulin.