Developmental Biochemistry of Cottonseed Embryogenesis and Germination : XIII. REGULATION OF BIOSYNTHESIS OF PRINCIPAL STORAGE PROTEINS.

The synthesis of the principal cottonseed storage proteins during embryogenesis has been followed by analyses of stained protein gels and of fluorographs of protein synthesized in vivo and from purified RNA in vitro in the wheat germ system. The kinetics of in vivo labeling as well as immunochemical cross-reactivity indicate that the 52- and 48-kilodalton mature storage protein sets are derived from 70- and 67-kilodalton precursor protein sets that are abundant proteins in embryonic cotyledons and disappear in late embryogenesis. Identification of the initial translation products of the storage protein mRNA has not been clearly established although products of apparent molecular weights of 69,000 and 60,000 are the likely storage protein precursors.Storage protein synthesis falls off markedly in late embryogenesis simultaneously with the loss of a superabundant class of mRNAs (shown by cDNA:RNA reassociation) that are presumed to be those for the storage proteins. The synthesis of these proteins ceases abruptly when immature embryos are removed from the boll and allowed to germinate precociously or when this precocious germination is prevented by incubation in abscisic acid. Thus, abscisic acid is not implicated in the expression of the storage protein genes.A scheme involving co-translational processing into vesicles, glycosylation, and slow in situ cleavage to produce the mature storage proteins is proposed.