Specific binding of high-mobility-group I (HMGI) protein and histone H1 to the upstream AT-rich region of the murine beta interferon promoter: HMGI protein acts as a potential antirepressor of the promoter.

The high-mobility-group I (HMGI) protein is a nonhistone component of active chromatin. In this work, we demonstrate that HMGI protein specifically binds to the AT-rich region of the murine beta interferon (IFN-beta) promoter localized upstream of the murine virus-responsive element (VRE). Contrary to what has been described for the human promoter, HMGI protein did not specifically bind to the VRE of the murine IFN-beta promoter. Stably transfected promoters carrying mutations on this HMGI binding site displayed delayed virus-induced kinetics of transcription. When integrated into chromatin, the mutated promoter remained repressed and never reached normal transcriptional activity. Such a phenomenon was not observed with transiently transfected promoters upon which chromatin was only partially reconstituted. Using UV footprinting, we show that the upstream AT-rich sequences of the murine IFN-beta promoter constitute a preferential binding region for histone H1. Transfection with a plasmid carrying scaffold attachment regions as well as incubation with distamycin led to the derepression of the IFN-beta promoter stably integrated into chromatin. In vitro, HMGI protein was able to displace histone H1 from the upstream AT-rich region of the wild-type promoter but not from the promoter carrying mutations on the upstream high-affinity HMGI binding site. Our results suggest that the binding of histone H1 to the upstream AT-rich region of the promoter might be partly responsible for the constitutive repression of the promoter. The displacement by HMGI protein of histone H1 could help to convert the IFN-beta promoter from a repressed to an active state.