Constitutive protein-DNA interactions on the abscisic acid-responsive element before and after developmental activation of the rab28 gene

Transcription of the rab28 gene from maize is induced in late embryo develo pment and in response to abscisic acid. We have studied the regulation of t he activity of the rab28 promoter in embryos. Two abscisic acid-responsive elements (ABREs) were necessary for expression in embryos of transgenic Ara bidopsis and in transient transformation in maize embryos. In vivo footprin ting showed that there was protein binding to the ABREs and to other cis el ements in the promoter in young embryos before expression of rab28. This sh ows that the rab28 promoter is in an open chromatin structure before develo pmental activation. The ABREs are important for the induction and have prot ein binding in young embryos. Nuclear proteins extracted from embryos befor e activation of rab28 bound to the ABREs in band shift assays. A complex wi th different mobility was formed between nuclear proteins and the ABREs aft er induction of rab28 suggesting a modification of the ABRE-binding factor or an exchange of proteins. The footprints on the ABREs were unaltered by i nduction with abscisic acid or during developmental activation of rab28. Th ese results indicate that constitutive binding of transcription factor(s) o n the ABRE is central in embryonic regulation of the rab28 gene.