DNA element downstream of the κB site in the Lcn2 promoter is required for transcriptional activation by IκBζ and NF-κB p50.

The nuclear protein IκBζ activates transcription of a subset of NF-κB-dependent innate immune genes such as Lcn2 encoding the antibacterial protein lipocalin-2. IκBζ functions as a coactivator via its interaction with NF-κB p50, which contains a DNA-binding Rel-homology domain but lacks a transcriptional activation domain. However cis-regulatory elements involved in IκBζ function have remained unknown. Here, we show that, although IκBζ by itself is unable to associate with the Lcn2 promoter, IκBζ interacts with the promoter via p50 binding to the NF-κB-binding site (κB site) and the interaction also requires the pyrimidine-rich site (CCCCTC) that localizes seven bases downstream of the κB site. The pyrimidine-rich site is also essential for IκBζ-mediated activation of the Lcn2 gene. Introduction of both sites into an IκBζ-independent gene culminates in IκBζ-p50-DNA complex formation and transcriptional activation. Furthermore, spacing between the two sites is crucial for both IκBζ-DNA interaction and IκBζ-mediated gene activation. Thus, the pyrimidine-rich IκBζ-responsive site plays an essential role in productive interaction of IκBζ with the p50-DNA complex.