Transactivation of the Brassica napus napin promoter by ABI3 requires interaction of the conserved B2 and B3 domains of ABI3 with different cis-elements: B2 mediates activation through an ABRE, whereas B3 interacts with an RY/G-box

The transcriptional activator ABI3 is a key regulator of gene expression du ring embryo maturation in crucifers. In monocots, the related VP1 protein r egulates the Em promoter synergistically with abscisic acid (ABA). We ident ified cis-elements in the Brassica napus napin napA promoter mediating regu lation by ABI3 and ABA, by analyzing substitution mutation constructs of na pA in transgenic tobacco plantlets ectopically expressing ABI3. In transien t analysis using particle bombardment of tobacco leaf sections, a tetramer of the distB ABRE (abscisic acid-responsive element) mediated transactivati on by ABI3 and ABI3-dependent response to ABA, whereas a tetramer of the co mposite RY/G complex, containing RY repeats and a G-box, mediated only ABA- independent transactivation by ABI3. Deletion of the conserved B2 and B3 do mains of ABI3 abolished transactivation of napA by ABI3. The two domains of ABI3 interact with different cis-elements: B2 is necessary for ABA-indepen dent and ABA-dependent activations through the distB ABRE, whereas B3 inter acts with the RY/G complex. Thus B2 mediates the interaction of ABI3 with t he protein complex at the ABRE. The regulation of napA by ABI3 differs from Em regulation by VP1, in that the B3 domain of ABI3 is essential for the A BA-dependent regulation of napA.