2 AUXIN-RESPONSIVE DOMAINS INTERACT POSITIVELY TO INDUCE EXPRESSION OF THE EARLY INDOLEACETIC ACID-INDUCIBLE GENE PS-IAA4 5/

The plant growth hormone indole-3-acetic acid (IAA) transcriptionally activates expression of several genes in plants. We have previously id entified a 164-bp promoter region (-318 to -154) in the PS-IAA4/5 gene that confers IAA inducibility. Linker-scanning mutagenesis across the region has identified two positive domains: domain A (48 bp; -203 to -156) and domain B (44 bp; -299 to -256), responsible for transcriptio nal activation of PS-IAA4/5 by IAA. Domain A contains the highly conse rved sequence 5'-TGTCCCAT-3' found among various IAA-inducible genes a nd behaves as the major auxin-responsive element. Domain B functions a s an enhancer element which may also contain a less efficient auxin-re sponsive element. The two domains act cooperatively to stimulate trans cription; however, tetramerization of domain A or B compensates for th e loss of A or B function. The two domains can also mediate IAA-induce d transcription from the heterologous cauliflower mosaic virus 35S pro moter (-73 to +1). In vivo competition experiments with icosamers of d omain A or B show that the domains interact specifically and with diff erent affinities to low abundance, positive transcription factor(s). A model for transcriptional activation of PS-IAA4/5 by IAA is discussed .