Homeodomain leucine zipper proteins bind to the phosphate response domain of the soybean VspB tripartite promoter

The soybean (Glycine max L. Merr. cv Williams 82) genes VspA and VspB encod e vacuolar glycoprotein acid phosphatases that serve as vegetative storage proteins during seed fill and early stages of seedling growth. VspB express ion is activated by jasmonates (JAs) and sugars and down-regulated by phosp hate and auxin. Previous promoter studies demonstrated that VspB promoter s equences between -585 and -535 mediated responses to JA, and sequences betw een -535 and -401 mediated responses to sugars, phosphate, and auxin. Ln th is study, the response domains were further delineated using transient expr ession of VspB promoter-beta -glucuronidase constructs in tobacco protoplas ts. Sequences between -536 and -484 were identified as important for phosph ate responses, whereas the region from -486 to -427 mediated sugar response s. Gel-shift and deoxyribonuclease-I footprinting assays revealed four DNA- binding sites between -611 and -451 of the soybean VspB promoter: one in th e IA response domain, two in the phosphate response domain, and one binding site in the sugar response domain. The sequence CATTAATTAG present in the phosphate response domain binds soybean homeodomain leucine zipper proteins , suggesting a role for these transcription factors in phosphate-modulated gene expression.