A conserved 21-base pair element, designated as hex-1, located between -180 and -160 of the wheat histone H3 promoter, is known to interact with two tobacco nuclear factors, activating sequence factor 1 and hex-1-specific binding factor. We have shown previously that a mutant sequence (hex-3), which differs from hex-1 by three base pairs, can no longer bind these two factors significantly. In the present work, we examined the functional characteristics of these two sequences in transgenic tobacco. Surprisingly, we found that a tetramer of hex-3, but not of hex-1, confers high level expression in mature seeds. Expression of this synthetic promoter rapidly diminishes upon germination but can be reactivated in young seedlings and mature leaves by desiccation, NaCl, or the phytohormone abscisic acid (ABA). Treatment with auxin or cytokinin has no apparent effect on the expression. Since the endogenous ABA level of plant cells is known to increase upon water stress, our data suggest that hex-3, the mutated hex-1 sequence, is an abscisic acid-responsive element (abre). We propose that a tobacco nuclear factor, distinct from activating sequence factor 1 and hex-1-specific binding factor, interacts with this sequence and is involved in mediating the effects of ABA and water stress on gene expression.