ACGT-containing abscisic acid response element (ABRE) and coupling element 3 (CE3) are functionally equivalent

ACGT-containing ABA response elements (ABREs) have been functionally identified in the promoters of various genes. In addition, single copies of ABRE have been found to require a cis-acting, coupling element to achieve ABA induction. A coupling element 3 (CE3) sequence, originally identified as such in the barley HVA1 promoter, is found approximately 30 bp downstream of motif A (ACGT-containing ABRE) in the promoter of the Osem gene. The relationship between these two elements was further defined by linker-scan analyses of a 55 bp fragment of the Osem promoter, which is sufficient for ABA-responsiveness and VP1 activation. The analyses revealed that both motif A and CE3 sequence were required not only for ABA-responsiveness but also for VP1 activation. Since the sequences of motif A and CE3 were found to be similar, motif-exchange experiments were carried out. The experiments demonstrated that motif A and CE3 were interchangeable by each other with respect to both ABA and VP1 regulation. In addition, both sequences were shown to be recognized by a VP1-interacting, ABA-responsive bZIP factor TRAB1. These results indicate that ACGT-containing ABREs and CE3 are functionally equivalent cis-acting elements. Furthermore, TRAB1 was shown to bind two other non-ACGT ABREs. Based on these results, all these ABREs including CE3 are proposed to be categorized into a single class of cis-acting elements.     

The conserved B3 domain of VIVIPAROUS1 has a cooperative DNA binding activity.

The biochemical activities that underlie the genetically defined activator and repressor functions of the VIVIPAROUS1 (VP1) protein have resisted in vitro analysis. Here, we show that a glutathione S-transferase (GST) fusion protein, including only the highly conserved B3 domain of VP1, has a highly cooperative, sequence-specific DNA binding activity. GST fusion proteins that include larger regions of the VP1 protein have very low activity, indicating that removal of the flanking protein sequences is necessary to elicit DNA binding in vitro. DNA competition and DNase I footprinting analyses show that B3 binds specifically to the Sph element involved in VP1 activation of the C1 gene, whereas binding to the G-box-type VP1-responsive element is of low affinity and is nonspecific. Footprint analysis of the C1 promoter revealed that sequences flanking the core TCCATGCAT motif of Sph also contribute to the recognition of the Sph element in its native context. The salient features of the in vitro GST-B3 DNA interaction are in good agreement with the protein and DNA sequence requirements defined by the functional analyses of VP1 and VP1-responsive elements in maize cells.