The NF-KB transcription factor complex is composed of a 50-kDa (p50) and a 65-kDa (p65) subunit. Both subunits bind to similar DNA motifs and elicit transcriptional activation as either homo-or heterodimers. By using chimeric proteins that contain the DNA binding domain of the yeast transcriptional activator GALA and subdomains of p65, three distinct transcriptional activation domains were identified. One domain was localized to a region of 42 amino acids containing a potential leucine zipper structure, consistent with earlier reports. Two other domains, both acidic and rich in prolines, were also identified. Of perhaps more significance, the same minimal activation domains that were functional in mammalian cells were also functional in the yeast Saccharomyces cerevisiae. Coexpression of the NF-KcB inhibitory molecule, IKB, reduced the transcriptional activity of p65 significantly, suggesting the ability of IKB to function in a similar manner in S. cerevisiae. Surprisingly, while the conserved rel homology domain of p65 demonstrated no transcriptional activity in either mammalian cells or S. cerevisiae, the corresponding domain in p50 was a strong transcriptional activator in S. cerevisiae. The observation that similar domains elicit transcriptional activation in mammalian cells and S. cerevisiae demonstrates strong conservation of the transcriptional machinery required for NF-cB function and provides a powerful genetic system to study the transcriptional mechanisms of these proteins.Nuclear factor KB (NF-KB), originally described as a factor involved in tissue-specific control of the mouse K light-chain enhancer (65,66), is now known to be involved in the inducible expression of a wide range of genes (2, 6, 44). The KB DNA binding site has been identified in the regulatory elements of cytokine, cytokine receptor, major histocompatibility complex class I and II antigens, inflammatory and acute-phase response genes, and several viral enhancer elements (2, 6, 44). DNA binding activity appears to be constitutive in mature B cells (66) and in activated macrophages (24). However, in most other cells NF-KB is complexed with an inhibitory molecule (IKB) in the cytoplasm (3,4,21). Sequestration in the cytoplasm is thought to involve masking of the nuclear localization signal (8). Upon activation by a variety of stimuli, including mitogens, cytokines, and various viral proteins (2, 6, 44), NF-KB shuttles to the nucleus, a process thought to involve phosphorylation of IKB (21,68). Initial studies demonstrated that IKB inhibits binding of the p65 homodimer and the p5O/p65 heterodimer through interaction with p65 (5, 21, 54, 78). However, recent studies suggest that both subunits can interact with IKB (8).Biochemical studies have shown that the major form of NF-KB is composed of a heterodimer of a 50-kDa (p50) and a 65-kDa (p65) subunit (5). Both subunits of NF-KB have been cloned, and sequence analyses demonstrate strong similarity in the amino-terminal 300 amino acids to the rel proto-oncogene product (22,38,54,58), origi...