The 22 bp W1 element in the pea lectin promoter is necessary and, as a multimer, sufficient for high gene expression in tobacco seeds.

The pea lectin (Psl) gene encodes an abundant seed protein. Its seed-specific expression pattern is conserved in transgenic tobacco plants. Progressive 5' promoter deletions resulted in a gradual decrease of transcriptional activity in tobacco seed. A fragment of 115 bp still conferred seed-specific expression albeit at a low level. This fragment contains a 22 bp element (W1), which has been demonstrated to be important for seed-specific expression when coupled as a trimer to a heterologous TATA box (de Pater et al., Plant Cell 5:877-886, 1993). Here we show that deletion of W1 in the natural promoter context resulted in a strongly decreased level of gene expression. A 4 bp mutation of W1 reduced the expression of truncated derivatives of the Psl promoter. A single copy of W1 coupled to the TATA box of the CaMV 35S promoter directed low gene expression in seeds and leaves. Multimerization enhanced the expression in seeds up to 100-fold, to levels found with the Psl promoter, whereas the expression level in leaves remained low. These results demonstrate that the W1 element is an essential control element in the Psl promoter. When taken out of its natural context and multimerized, it is sufficient for high expression in seeds.