The composition of seed storage proteins is regulated by sulfur and nitrogen supplies. Under conditions of a low sulfur-to-nitrogen ratio, accumulation of the beta-subunit of beta-conglycinin, a sulfur-poor seed storage protein of soybean (Glycine max [L.] Merr.), is elevated, whereas that of glycinin, a sulfur-rich storage protein, is reduced. Using transgenic Arabidopsis thaliana [L.] Heynh., it was found that the promoter from the gene encoding the beta-subunit of beta-conglycinin up-regulates gene expression under sulfur deficiency and down-regulates gene expression under nitrogen deficiency. To obtain an insight into the metabolic control of this regulation, the concentrations of metabolites related to the sulfur assimilation pathway were determined. Among the metabolites, O-acetyl-l-serine (OAS), one of the precursors of cysteine biosynthesis, accumulated to higher levels under low-sulfur and high-nitrogen conditions in siliques of transgenic A. thaliana. The pattern of OAS accumulation in response to various levels of sulfur and nitrogen was similar to that of gene expression driven by the beta-subunit promoter. Elevated levels of OAS accumulation were also observed in soybean cotyledons cultured under sulfur deficiency. Moreover, OAS applied to in-vitro cultures of immature soybean cotyledons under normal sulfate conditions resulted in a high accumulation of the beta-subunit mRNA and protein, whereas the accumulation of glycinin was reduced. These changes were very similar to the responses observed under conditions of sulfur deficiency. Our results suggest that the level of free OAS mediates sulfur- and nitrogen-regulation of soybean seed storage-protein composition.