Interferon regulatory factor-1 (IRF-1), a putative tumor suppressor protein, is a transcriptional mediator of interferon-responsive signaling pathways that are involved in antiviral defense, apoptosis, immune response, and cell growth regulation. To delineate the IRF-1 domain responsible for transactivation, we performed a detailed deletion analysis of IRF-1. We found that the amino acid segment 217-260 was necessary and sufficient for transactivation. The structure of this region was predicted to be loop-helix-loop-sheet using the program PHD. Further studies indicated that casein kinase II and protein kinase C sites on each of the two loops are not important for transactivation, and a region containing amino acids 233-255 comprises the core activation domain. To verify the physiological role of segment 233-255, we constructed an IRF-1 deletion mutant lacking these amino acids and examined it using IRF-1 transactivation assay, fluorescence-activated cell sorting, and in situ beta-galactosidase staining techniques. From the results of these studies, we conclude that the amino acid segment 233-255 of IRF-1 comprises the core activation domain required for the physiological functions of IRF-1. (C) 2003 Elsevier Science (USA). All rights reserved.