Constitutive silencing of IFN-β promoter is mediated by NRF (NF-κB-repressing factor), a nuclear inhibitor of NF-κB

Transcriptional regulation of the interferon-β (IFN-β) gene is characterized by strict constitutive repression and virus-specific activation. Previous studies have shown that the IFN-β promoter is constitutively repressed by a negative regulatory element (NRE). Isolated NRE acts as a constitutive and positionindependent silencer on the NF-κB-binding sites. Here, we describe the identification and functional characterization of the NRE-binding protein, called NRF (NF-κB-repressing factor), which abolishes the transcriptional activity of the bordering NF-κBbinding sites by a distance-independent mechanism. Deletion studies show that a minimal repression domain of NRF is sufficient to exert its inhibitory effect. In vitro, NF-κB proteins bind to purified NRF by a direct protein–protein interaction. We demonstrate that NRF is a ubiquitous and constitutive nuclear protein. In fibroblasts, the expression of the NRF antisense RNA releases the endogenous IFN-β gene transcription. Our data strongly suggest that the NRF-mediated inhibition of NF-κB is a critical component of the IFN-β gene silencing prior to viral infection.