In cotyledon cells of developing field beans the RNA content per cell does not change in the second half of developmental period 2, whereas globulin biosynthesis continues. The constant RNA content per cell results from an equilibrium between RNA synthesis and degradation. All types of RNA are synthesized until the end of globulin biosynthesis, but poly(A)-containing RNA was preferentially labelled during maximum globulin formation.During stage 2 of seed development the poly(A)-containing RNA fraction represents a discrete peak in the 12 -18-S region on agarose gels and corresponds to the peak of poly(A)-containing RNA isolated from polysomes. %-Amanitin inhibits selectively the labelling of poly(A)-containing RNA and concomitantly globulin formation. Translation of total poly(A)-containing RNA, free and membrane-bound polysomes in a cell-free wheat germ system demonstrates that the globulins are preferentially produced on membrane-bound polysomes and that poly(A)-containing RNA includes the mRNA for both vicilin and legumin.

Initially membrane bound polysomes, microsomes and total cellular polyadenylated RNA were isolated from developing cotyledons of Vicia faba. The templates were translated in vitro employing the wheat germ system (polysomes and microsomes) or the rabbit reticulocyte system (poly-A RNA). The translation products were isolated with antibodies raised against vicilin and legumin and analyzed by SDS-polyacrylamide gel electrophoresis. Vicilin was synthesized as one major N-terminally extended precursor molecule, with a molecular weight of about 52,000. The precursor was vectorially discharged through the membrane of the endoplasmic reticulum in conjunction with removal of the N-terminal extension. Legumin isolated from mature Vicia faba seeds consists of two subunits which are linked by cystin bridges and have molecular weights of about 36,000 and 20,000, respectively. These two subunits are known to be processed from a precursor molecule of 60,000 molecular weight. Applying antibodies raised against a purified legumin fraction for immunoaffinity isolation of in vitro translation products from polysomal, microsomal and mRNA templates yielded several polypeptides. The largest four of the polypeptides occurred in pairs with molecular weights around 51,000 and 60,000, respectively. These and one lower molecular weight polypeptide were co-translationaUy transported into the lumen of the endoplasmic reticulum. Hybrid microsomes were reconstituted from Vicia faba stripped microsomes and initially membrane bound polysomes isolated from barley endosperm. These microsomes served as an active template for the in vitro synthesis of hordein-the storage protein of barley endosperm. Furthermore, hordein was vectorially discharged through the Vicia faba microsomal membranes. Abbreviations: DOC = sodium deoxycholate; DTT = dithiothreitol; HEPES = N-2-hydroxyethyl-piperazine-N'-2-ethane sulfonic acid; HKM = HEPES-K-Mg acetate buffer (see 2. I); IgG = immunoglobulin G; mol.wt. = molecular weight; SDS-PAGE=Sodium dodecylsulfate polyacrylamide gel electrophoresis; TCA =trichloroacetic acid.

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