Abstract
To elucidate the molecular mechanisms responsible for seed-specific gene expression in plants, the promoter of the pea lectin (psl) gene, encoding an abundant seed protein, was used as a model. Leaf and seed nuclear proteins bound to a region in the psl promoter containing three overlapping TGAC-like motifs, which have been shown to be a binding site for basic/leucine zipper proteins, including TGA1a. A trimer of a 22-bp region of the psl promoter, containing the TGAC-like motifs, coupled to a heterologous minimal promoter conferred low reporter gene expression in root, stem, and leaf and high expression in seed of transgenic tobacco. Expression increased during the midmaturation stage of seed development and was observed in the endosperm as well as in the embryo, where it strongly decreased within a few days after germination. This expression pattern is qualitatively identical to the expression pattern conferred by a 2000-bp fragment of the psl promoter. Nucleotides within the TGAC-like motifs important for in vitro binding are also essential for in vivo transcription activation in vegetative tissue as well as in seed. The electrophoretic mobility of a DNA-protein complex containing seed nuclear protein was different from that formed with leaf nuclear protein. Furthermore, the TGA1a steady state mRNA level in immature seed was relatively low. These results suggest that a seed-specific factor different from TGA1a, but with similar binding specificity, is responsible for gene activation in seed. We conclude that the 22-bp region contains all the information, including an essential TGAGTCATCA sequence, necessary for seed-specific expression and very likely plays an essential role in the seed-specific expression pattern of the psl gene.