Independent modes of transcriptional activation by the p50 and p65 subunits of NF-κB

Abstract

Recombinant subunits of the transcription factor NF-κB, p50 and p65, were analyzed both for binding to various κB motifs and in vitro activation. The subunits preferentially form a heterodimer that activates transcription. Although p50 and p65 bind DNA individually as homodimers and are structurally related, their activation mechanisms are distinct. p65 activates transcription by its unique carboxy-terminal activation domain. (p50)₂ displays higher affinity DNA binding than (p65)₂ for many distinct κB motifs and provides strong transcriptional activation only when adopting a chymotrypsin-resistant conformation induced by certain κB motifs but not others. Thus, (p50)₂ acts as a positive regulator in vitro, consistent with its isolation as a putative constitutive regulator of MHC class I genes. Both subunits of NF-κB, therefore, contribute independently to provide regulation at given κB motifs.