Interferon gamma (IFN-gamma) interacts synergistically with bacterial lipopolysaccharide (LPS) to induce transcription of iNOS, the isoform of nitric oxide synthase whose activity is independent of elevated Ca2+ and exogenous calmodulin. To define a cis-acting element mediating IFN-gamma-dependent synergy, we made deletions in iNOS promoter constructs fused to reporter genes, transfected RAW 264.7 macrophages, and treated the cells with IFN-gamma and/or LPS. This analysis implicated the region from positions -951 to -911, a cluster of four enhancer elements known to bind IFN-gamma-responsive transcription factors, including an interferon regulatory factor binding site (IRF-E) at nucleotides -913 to -923. Site-specific substitution of two conserved nucleotides within IRF-E in the context of the full-length iNOS promoter ablated IFN-gamma's contribution to synergistic enhancement of transcription. Electromobility shift assays performed with a probe containing IRF-E revealed the existence of a complex in nuclei of RAW 264.7 macrophages that was present only after treatment with IFN-gamma, which reacted specifically with anti-IRF-1 immunoglobulin G and which included a species migrating at 40-45 kD, consistent with the apparent molecular weight of murine IRF-1. Thus, the synergistic contribution of IFN-gamma to transcription of iNOS in RAW 264.7 macrophages requires that IRF-1 bind to IRF-E in the iNOS promoter. In conjunction with the work of Kamijo et al. (Kamijo, R., H. Harada, T. Matsuyama, M. Bosland, J. Gerecitano, D. Shapiro, J. Le, K. S. Im, T. Kimura, S. Green et al. 1994. Science [Wash. DC]. 263:1612), these findings identify iNOS as the first gene that requires IRF-1 for IFN-gamma-dependent transcriptional regulation.