The role of HMG I(Y) in the assembly and function of the IFN-β enhanceosome

Transcriptional activation of the virus inducible enhancer of the human interferon-β (IFN-β) gene in response to virus infection requires the assembly of an enhanceosome, consisting of the transcriptional activators NF-κB, 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-κB 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-β enhanceosome is an unusually stable nucleoprotein structure that can activate transcription at high levels by promoting multiple rounds of reinitiation of transcription.