ACGT-containing abscisic acid response element (ABRE) and coupling element3 (CE3) are functionally equivalent

ACGT-containing ABA response elements (ABREs) have been functionally identi fied 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 in duction. A coupling element 3 (CE3) sequence, originally identified as such in the barley HVA1 promoter, is found approximately 30 bp downstream of mo tif A (ACGT-containing ABBE) in the promoter of the Osem gene. The relation ship 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-resp onsiveness 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 simila r, motif-exchange experiments were carried out. The experiments demonstrate d that motif A and CE3 were interchangeable by each other with respect to b oth ABA and VP1 regulation. In addition, both sequences were shown to be re cognized by a VP1-interacting, ABA-responsive bZIP factor TRAB1. These resu lts indicate that ACGT-containing ABREs and CE3 are functionally equivalent cis-acting elements. Furthermore, TRAB1 was shown to bind two other non-AC GT ABREs. Based on these results, all these ABREs including CE3 are propose d to be categorized into a single class of cis-acting elements.