RT Journal Article SR Electronic T1 AUX/IAA Proteins Are Active Repressors, and Their Stability and Activity Are Modulated by Auxin JF The Plant Cell JO Plant Cell FD American Society of Plant Biologists SP 2809 OP 2822 DO 10.1105/tpc.010289 VO 13 IS 12 A1 Tiwari, Shiv B. A1 Wang, Xiao-Jun A1 Hagen, Gretchen A1 Guilfoyle, Tom J. YR 2001 UL http://www.plantcell.org/content/13/12/2809.abstract AB Aux/IAA genes are early auxin response genes that encode short-lived nuclear proteins with four conserved domains, referred to as I, II, III, and IV. Arabidopsis Aux/IAA proteins repressed transcription on auxin-responsive reporter genes in protoplast transfection assays. Mutations in domain II resulted in increased repression, whereas mutations in domains I and III partially relieved repression. Aux/IAA proteins fused to a heterologous DNA binding domain were targeted to promoters of constitutively expressed reporter genes and actively repressed transcription in an auxin-responsive and dose-dependent manner. In comparison with an unfused luciferase protein, luciferase fused to Aux/IAA proteins displayed less luciferase activity, which further decreased in the presence of auxin in transfected protoplasts. Domain II mutations increased and domain I mutations decreased luciferase activity with the fusion proteins. These results suggested that Aux/IAA proteins function as active repressors by dimerizing with auxin response factors bound to auxin response elements and that early auxin response genes are regulated by auxin-modulated stabilities of Aux/IAA proteins.