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

Transcriptional activation of the virus inducible enhancer of the human int erferon-beta (IFN-beta) gene in response to virus infection requires the as sembly of an enhanceosome, consisting of the transcriptional activators NF- kappa B, ATF-2/c-Jun, IRFs and the architectural protein of the mammalian h igh mobility group I(Y) [HMG I(Y)]. Here, we demonstrate that the first ste p in enhanceosome assembly, i.e. HMG I(Y)-dependent recruitment of NF-kappa B and ATF-2/c-Jun to the enhancer, is facilitated by discrete regions of H MG I and is mediated by allosteric changes induced in the DNA by HMG I(Y) a nd not by protein-protein interactions between HMG I(Y) and these proteins, However, we show that completion of the enhanceosome assembly process requ ires protein-protein interactions between HMG I(Y) and the activators. Fina lly, we demonstrate that once assembled, the IFN-beta enhanceosome is an un usually stable nucleoprotein structure that can activate transcription at h igh levels by promoting multiple rounds of reinitiation of transcription.