Dissociation of ribosomes and seed germination

Ribosomes from rice embryos (Oryza sativa) were dissociated into ribosomal subunits in vitro by systematic reduction of the Mg(2+) concentration. Ribosomes from imbibed (28 C) embryos were more easily dissociated than those from nonimbibed embryos. This was not observed with ribosomes from either imbibed, nonviable embryos, or from embryos imbibed at 0 C. Ribosomes from embryos which had been imbided and subsequently dehydrated resembled ribosomes from nonimbibed embryos in their resistance to dissociation. The change in the resistance to dissociation was essentially complete after the first 20 minutes of imbibition at 28 C, and accompanied activation in vivo of protein synthesis as determined by amino acid incorporation in vitro. Ribosomes from either imbibed or nonimbibed embryos could be dissociated into subunits by 0.5 m KCl. These subunits were separated by density gradient centrifugation, and, if recombined, were active for polyphenylalanine synthesis in vitro. The individual subunits prepared from nonimbibed embryos could be replaced by the corresponding subunit fraction from imbibed embryos without loss of capacity to support polyphenylalanine synthesis. The change in the ease of dissociation of ribosomes appears to be a physiological process, and its possible relationship to the initiation of protein synthesis during seed germination is discussed.