Nucleotide Sequence of a cDNA Encoding a Low Molecular Weight Sulfur-Rich Protein in Soybean Seeds

A cDNA library constructed with poly(A+) RNAs purified from immature seeds of soybean (Glycine mux L.) was screened by differential hybridization with cDNA probes prepared from immature seeds and roots. It resulted in isolation of severa1 different cDNA clones. One of them, SE60, contains a cDNA of 420 bp. The cDNA insert was subcloned into pUC19 and was subjected to nucleotide sequencing. The cDNA contained one open reading frame encoding a protein of 74 amino acids, having a molecular mass of 8.4 kD (Table I). The sequence around the putative translation initiation codon contained A at -3 and G at +4 positions, obeying the Kozak rule (Kozak, 1981). Hydropathy analysis revealed that the SE60 protein contains a highly hydrophobic region at the N terminus, which seems to function as a signal sequence (Kyte and Doolittle, 1982). The cleavage site between the signal sequence and the mature protein was predicted to be between Gly (residue 28) and Arg (residue 29), which conformed to the (-3, -1) rule (von Heijne, 1986). The cleavage of the signal sequence of 28 amino acids may lead to a mature protein of 46 amino acids. The mature protein is rich in Cys, which makes up 17.4% of total amino acid residues. The amino acid sequence was compared with those of other plant proteins. The SE60 protein was homologous to a potato tuber-specific protein (Stiekema et al., 1988; Hendriks et al., 1991). They are identical in 37 amino acids of the 74 amino acids compared. The SE60 protein also showed homology to the cowpea seed protein that is known to be synthesized from stored mRNA in cotyledons (Ishibashi and Minamikawa, 1990; Ishibashi et al., 1990). It is interesting that eight Cys residues were located at the same positions in the three proteins. Although the potato tuber-specific protein