Transcriptional activation of the virus inducible enhancer of the human 
interferon-beta (IFN-beta) gene in response to virus infection requires the 
assembly of an enhanceosome, consisting of the transcriptional activators 
NF-kappaB, ATF-2/c-Jun, IRFs and the architectural protein of the mammalian high 
mobility group I(Y) [HMG I(Y)]. Here, we demonstrate that the first step in 
enhanceosome assembly, i.e. HMG I(Y)-dependent recruitment of NF-kappaB and 
ATF-2/c-Jun to the enhancer, is facilitated by discrete regions of HMG I and is 
mediated by allosteric changes induced in the DNA by HMG I(Y) and not by 
protein-protein interactions between HMG I(Y) and these proteins. However, we 
show that completion of the enhanceosome assembly process requires 
protein-protein interactions between HMG I(Y) and the activators. Finally, we 
demonstrate that once assembled, the IFN-beta enhanceosome is an unusually 
stable nucleoprotein structure that can activate transcription at high levels by 
promoting multiple rounds of reinitiation of transcription.