I kappa B proteins regulate Rel/NF-kappa B transcription complexes through a direct protein-protein interaction. In addition, we have previously shown that certain I kappa B proteins (I kappa B alpha and I kappa B gamma) can act as activators of transcription when fused to the DNA-binding domain of GAL4. We now show that a mutant chicken I kappa B alpha protein that cannot interact with Rel proteins in vitro did not activate transcription when fused to GAL4 in chicken embryo fibroblasts (CEF) and Saccharomyces cerevisiae, and did not inhibit growth in yeast; in contrast, an I kappa B alpha mutant that can still interact in vitro with Rel proteins activated transcription in both CEF and yeast and inhibited growth in yeast. In CEF, GAL4-I kappa B alpha mediated transcription activation was inhibited by co-transfection with an expression vector for a RelA (p65) protein that contained sequences needed for interaction with I kappa B alpha but that was deleted of its transcription activation domain. Therefore, it appears that GAL4-I kappa B alpha activates transcription by interacting with an endogenous Rel family protein in CEF. In contrast, the activation domain from I kappa B gamma behaved as a genuine acidic activator of transcription and did not inhibit growth when expressed in yeast. Since transcription activation and growth inhibition by GAL4-I kappa B alpha mutants in yeast correlated with their ability to interact with vertebrate Rel proteins, our results suggest that these activities of GAL4-I kappa B alpha are mediated through interaction with a Rel-like protein in yeast, which is important for cell growth.