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In previous studies (GL Creason et al. 1983 Biochem Biophys Res Commun 117: 658-662; LP Holowach et al. 1984 Plant Physiol 74: 576-583), we have shown that when soybean (Glycine max L. Merrill cv Provar) cotyledons are cultured in medium supplemented with l-methionine, the beta-subunit of 7S protein and beta-mRNA are absent. We have carried out further studies on the mechanism of the methionine action. In one experiment, cotyledons were cultured for 16 days with or without methionine. After 4 days, some cotyledons were transferred from methionine-supplemented to basal (no methionine) medium and vice versa. In basal medium, beta-subunit was detected at 4 days whereas in methionine-supplemented medium, no beta-subunit was present. When cotyledons were transferred from basal to methionine-supplemented medium, the beta-subunit increased within a 4 day period and then remained constant (on a per cotyledon basis). This result indicated that methionine was not acting by accelerating the degradation of the beta-subunit. Four days after transfer from supplemented to basal medium cotyledons contained beta-subunit, thus demonstrating that the inhibition was reversible. During this time, the uncombined methionine declined from 7 to 1.5 mumoles methionine per gram fresh weight. When beta-mRNA was measured by in vitro translation, functional beta-mRNA was absent in tissue that was not accumulating beta-subunit. The messenger RNA for the beta-subunit had a half-life of about 1 day in the presence of methionine. Hybridization of cotyledon mRNA with cDNA complementary to beta-mRNA revealed that the 1700 nucleotide beta-mRNA was not present in supplemented cotyledons. Thus, expression of the beta-subunit gene is controlled at the level of transcription, RNA processing, or RNA turnover, rather than at the level of translation.

Studies on the Mechanism of Regulation of the mRNA Level for a Soybean Storage Protein Subunit by Exogenous l-Methionine