The nuclear factor kappa B (NF-kappa B) transcription factor is responsive to specific cytokines and stress and is often activated in association with cell damage and growth arrest in eukaryotes. NF-kappa B is a heterodimeric protein, typically composed of 50- and 65-kilodalton subunits of the Rel family, of which RelA(p65) stimulates transcription of diverse genes. Specific cyclin-dependent kinases (CDKs) were found to regulate transcriptional activation by NF-kappa B through interactions with the coactivator p300. The transcriptional activation domain of RelA(p65) interacted with an amino-terminal region of p300 distinct from a carboxyl-terminal region of p300 required for binding to the cyclin E-Cdk2 complex. The CDK inhibitor p21 or a dominant negative Cdk2,which inhibited p300-associated cyclin E-Cdk2 activity, stimulated kappa B-dependent gene expression which was also enhanced by expression of p300 in the presence of p21. The interaction of NF-kappa B and CDKS through the p300 and CBP coactivators provides a mechanism for the coordination of transcriptional activation with cell cycle progression.