The p65 subunit of the inducible transcription factor NF-kappa B contains at least two strong transactivation domains (TADs) within its C terminus. The first domain, TA1, is contained within the last 30 amino acids of p65, whereas TA2 comprises the adjacent 90 amino acids. In this study, squelching experiments revealed that both TADs of p65, as well as the related subunit c-Rel, compete for the same cofactor(s) mediating transactivation. Both TADs of p65 share a common sequence motif, which is evolutionarily conserved and displays a remarkable degree of spatial organization when aligned on an alpha-helical surface. The functional importance of the common sequence motif was confirmed by deletion analysis of TA2. Within the conserved sequence motif, a 7-amino-acid repeat was noted. Idealized heptad repeats fused to the DNA binding domain of Gal4 were transcriptionally active, but only as multimers. Phosphorylation and transcriptional activity of a defined region within the TA2 domain was found to be stimulated by phorbol ester treatment of cells. In contrast, TA1 was constitutively phosphorylated, and its activity did not significantly respond to phorbol ester stimulation. The stimulatory effect of phorbol ester on transcription of the TA2 domain was completely blocked by the protein kinase C inhibitor. These data suggest that protein kinase C has a dual effect on NF-kappa B activity. It not only causes removal of I kappa B-alpha from cytoplasmic NF-kappa B but also augments the transactivation potential of activated nuclear NF-kappa B.