PVALF, AN EMBRYO-SPECIFIC ACIDIC TRANSCRIPTIONAL ACTIVATOR ENHANCES GENE-EXPRESSION FROM PHASEOLIN AND PHYTOHEMAGGLUTININ PROMOTERS

Mutations in Vp1 and ABI3 genes of maize and Arabidopsis lead to drast ic reductions in the synthesis of a subset of maturation-specific prod ucts including seed storage proteins. Gene Phaseolus vulgaris ABI3-lik e factor (PvAlf), whose protein product is similar to the ABI3 and Vp1 proteins, has been cloned. Here, it is shown that PvAlf positively re gulates phaseolin and phytohemagglutinin (PHA-L) promoters in particle bombardment assays. PvAlf mRNA expression is embryo-specific and temp orally complex. PvAlf mRNA abundance is highest during two periods (9- 14 and 22-35 days after flowering) that precede the onsets of seed mat uration and seed abscission, respectively. Protein fusions with the DN A-binding domain of the yeast transcriptional activator GAL4 demonstra ted that the N-terminal 243 amino acids of PvAlf function as a strong transcriptional activation domain in yeast (Saccharomyces cerevisiae) and plant cells. This domain consists of a central cluster rich in ser ine, threonine and proline (STP duster) flanked by two negatively char ged regions containing bulky hydrophobic residues similar to acidic ac tivation domains of Vp1, the herpes simplex virus virion protein VP16 and transcription factors GCN4 and HAP4 from yeast. Together with the Vp1 proteins of maize and rice and ABI3, PvAlf constitutes a class (Vp 1/ABI3-like factors or VAlfs) of regulatory factors that are pivotal f or the promotion of seed maturation and dormancy in angiosperms.