The promoter element activation sequence-1 (as-1) confers tissue-specific and signal-responsive transcription in plants. Hormone and chemical stress cues are thought to activate as-1-dependent transcription through specific basic/leucine-zipper proteins, termed TGA factors, that bind this element. We report here that a highly conserved TGA factor of tobacco, TGA1a, can selectively activate transcription in response to micromolar concentrations of auxin hormones or their analogs. This induction is chemically specific, as a range of other compounds tested at similar concentrations had little or no effect. Auxin was found to augment the transactivation potential of TGA1a through carboxyl-terminal residues. The amino-terminal domain of TGA1a, by gain-of-function assays, was found to both constitutively activate transcription and maximize the response to auxin. Further evidence indicates that the trans-activation potential of this domain in TGA1a is repressed, under basal conditions, by carboxyl-terminal residues. Because TGA1a and endogenous TGA factors are stimulated by auxin only at concentrations that inhibited cell growth, this response is likely to involve chemical stress.The expression of nuclear genes in response to cellular and environmental cues is largely regulated by trans-acting factors and their cognate cis-elements. In plants, auxin hormones (e.g., indole-3-acetic acid) promote growth and affect a number of cellular processes. Auxins are believed to mediate at least some of their effect by enhancing the expression of specific genes through target cis-elements (1, 2). The prototype for one class of auxin-responsive element is activation sequence-1 (as-1), 1 which was first identified by Lam et al. (3) in the cauliflower mosaic virus (CaMV) 35 S promoter. Homologs of as-1 (e.g., ocs and nos), moreover, occur in the promoters of plant-transforming T-DNA genes of agrobacteria (4 -6). Apart from regulating these genes in plants, a broader role for as-1 elements has been suggested by their functional presence in plant glutathione S-transferase genes (7-9). For example, auxin-responsive expression of at least one member of the tobacco GST gene family, GNT35, is mediated by as-1 (8, 9). Unlike other auxin-responsive elements, as-1 requires physiologically high (i.e. micromolar) concentrations of auxins to activate transcription, suggesting that this response may involve chemical stress. This view is further supported by the observation that biologically inactive analogs of auxins are also strong inducers of as-1-dependent activity.Multiple TGA factors that bind selectively to as-1 and related motifs have been cloned from several plant species, and some or all of these factors are likely to mediate as-1-dependent transcription. Based on their amino acid homology, at least three subclasses of TGA factors occur in plants, with multiple members in each class. Heterodimerization between these factors may generate new combinations with potentially distinct functional properties (10). Posttranscriptional regulation ...