Independent modes of transcriptional activation by the p50 and p65 subunits of NF-kappa B.

Recombinant subunits of the transcription factor NF-kappa B, p50 and p65, were analyzed both for binding to various kappa B 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 kappa B motifs and provides strong transcriptional activation only when adopting a chymotrypsin-resistant conformation induced by certain kappa B 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-kappa B, therefore, contribute independently to provide regulation at given kappa B motifs.