Previous studies showed that the binding of p50/NFkappaB1 to particular kappaB DNA sites altered its conformation in a way that correlated with transcriptional activation. Here, we investigated the effects of protein-protein interactions on the transcriptional activity of p50. We show that the association of p50 with a mutant Rel-homology domain (RHD) defective for DNA binding led to synergistic activation of kappaB site-dependent transcription, whereas neither protein alone had any effect. Partial proteolytic analysis showed that the conformation of p50 in these complexes differed from that in wild-type c-Rel-RHD/p50 complexes, and correlated with activated transcription. These results suggest that the Rel-homology domain can act as an allosteric effector to promote transcription by p50/NFkappaB1 and that the configuration of p50 is important for its activity. This also suggests that Rel proteins can promote transcription by other Rel-family members without binding to their DNA recognition site. These studies emphasize the important role of protein-protein interactions in Rel and NFkappaB-mediated transcription.