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.