ABSTRAC1Supplemental methionine in a complete culture medium increased the methionine content of the protein fraction of cultured soybean (Glycine max L. Merrill) cotyledons (Thompson, Madison, Muenster 1981 Phytochemistry 20: 941-945). To explain the observed increase in protein methionine, we have measured the amounts and subunit compositions of 7S and 11S storage proteins and determined the amino acid compositions of the three major protein fractions (2-5S, 7S, 11S) of seeds developed on plants and of cultured cotyledons grown in the presence or absence of supplemental L-methionine. Development of cultured cotyledons was representative of development of seeds on plants. The ratios of 11S to 7S proteins, the subunit contents, and amino acid compositions of their storage protein fractions were similar, but not identical. Supplemental methionine increased the mole percent methionine in each of the three protein fractions of cultured cotyledons and changed the amounts of several other amino acids. Supplemental methionine inhibited expression of the 7S a-subunit gene. Concomitant with the absence of the a-subunit, which contains no methionine, was an increase in the ratio of 11S to 7S proteins, and an increase in the methionine content of the subunits composing these fractions. Inhibition of 6-subunit gene expression by methionine in cultured cotyledons provides a reproducible, easily controlled system for the study of eucaryotic gene expression.Soybean seed proteins lack sufficient quantities ofsulfur amino acids to satisfactorily meet the dietary requirements of some monogastric animals. In this respect, soybeans are typical legume seeds. Soybeans, however, accumulate about twice as much protein in their seeds as other legumes, and up to four times as much protein as grain seeds. The nutritional quality of soybean and other legume seeds could be improved by raising the sulfurcontaining amino acid content of their proteins (37). Since the 7S and Il S storage protein fractions constitute about 70% of total seed protein, they have been the focus of efforts to raise protein methionine content. The soybean 7S fraction consists of three major subunits, the a'-, a-, and a-subunits. (29,31) In vitro experiments were designed so that sets of 5 to 46 cotyledon pairs were subjected to control and experimental treatments. Individual seeds were divided so that one-half was placed in complete medium lacking methionine, and the other half in complete medium containing 8.4 mm L-methionine.Rates of growth and protein accumulation were variable in seeds grown on plants. Seeds matured more slowly in the winter than in the summer. Maturity was therefore measured by increments of seed fresh weight, which was found to be a more consistent measure of the time course of development than days after flowering (13,18