The candidate proto-oncogene bcl-3 encodes a protein that shares structural features with IκB-α and other proteins that bind to members of the Rel protein family. Here, we show that in contrast to the inhibitory activity of IκB-α, the bcl-3 gene product superactivates NF-κB p50 homodimer-mediated gene expression both in vivo and in vitro. BCL-3 protein can, as well, selectively associate with p50 homodimers in the presence of DNA containing a κB motif. These results strongly suggest that BCL-3 can act as a transcriptional coactivator, acting through DNA-bound p50 homodimers
