Beyond the enhanceosome: cluster of novel κB sites downstream of the human IFN-ß gene is essential for lipopolysaccharide-induced gene activation.

The expression of interferon-ß (IFN-ß) in virus-infected HeLa cells established a paradigm of multifactorial gene regulation, in which cooperative assembly of transcription factors (TFs) at the composite DNA element (enhanceosome), is central for amplification of weak activating signals provided by individual TFs. However, whether the same TFs and the same DNA element are essential for IFN-ß induction in response to bacterial stimuli are less well understood. Here we report that rapid and transient transcription of IFN-ß in response to TLR4 stimulation with bacterial lipopolysaccharide (LPS) follows nuclear factor-κB (NF-κB) RelA activation and recruitment to the IFN-ß genomic locus at multiple spatially separated regulatory regions. We demonstrate that the IFN-ß enhanceosome region is not sufficient for maximal gene induction in response to LPS and identify an essential cluster of homotypic κB sites in the 3' downstream of the gene. The cluster is characterized by elevated levels of histone 3 lysine 4 mono-methylation, a chromatin signature of enhancers, and efficiently binds RelA-containing NF-κB complexes in vitro and in vivo. These findings demonstrate that IFN-ß gene activation via multifactorial enhanceosome assembly is potentiated in LPS-stimulated cells by NF-κB interactions with all functional κB sites in the locus.