Aux/IAA proteins are short-lived nuclear proteins that repress expression of 
primary/early auxin response genes in protoplast transfection assays. Repression 
is thought to result from Aux/IAA proteins dimerizing with auxin response factor 
(ARF) transcriptional activators that reside on auxin-responsive promoter 
elements, referred to as AuxREs. Most Aux/IAA proteins contain four conserved 
domains, designated domains I, II, III, and IV. Domain II and domains III and IV 
play roles in protein stability and dimerization, respectively. A clear function 
for domain I had not been established. Results reported here indicate that 
domain I in Aux/IAA proteins is an active repression domain that is transferable 
and dominant over activation domains. An LxLxL motif within domain I is 
important for conferring repression. The dominance of Aux/IAA repression domains 
over activation domains in ARF transcriptional activators provides a plausible 
explanation for the repression of auxin response genes via ARF-Aux/IAA 
dimerization on auxin-responsive promoters.