A 22-BP FRAGMENT OF THE PEA LECTIN PROMOTER CONTAINING ESSENTIAL TGAC-LIKE MOTIFS CONFERS SEED-SPECIFIC GENE-EXPRESSION

To elucidate the molecular mechanisms responsible for seed-specific ge ne expression in plants, the promoter of the pea lectin (psl) gene, en coding an abundant seed protein, was used as a model. Leaf and seed nu clear proteins bound to a region in the psl promoter containing three overlapping TGAC-like motifs, which have been shown to be a binding si te for basic/leucine zipper proteins, including TGA1a. A trimer of a 2 2-bp region of the psl promoter, containing the TGAC-like motifs, coup led to a heterologous minimal promoter conferred low reporter gene exp ression in root, stem, and leaf and high expression in seed of transge nic tobacco. Expression increased during the midmaturation stage of se ed development and was observed in the endosperm as well as in the emb ryo, where it strongly decreased within a few days after germination. This expression pattern is qualitatively identical to the expression p attern conferred by a 2000-bp fragment of the psl promoter. Nucleotide s 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 comple x containing seed nuclear protein was different from that formed with leaf nuclear protein. Furthermore, the TGA1a steady state mRNA level i n immature seed was relatively low. These results suggest that a seed- specific factor different from TFA1a, but with similar binding specifi city, is responsible for gene activation in seed. We conclude that the 22-bp region contains all the information, including an essential TGA GTCATCA sequence, necessary for seed-specific expression and very like ly plays an essential role in the seed-specific expression pattern of the psl gene.