IRF-7 plays an essential role in virus-activated transcription of IFNA genes. To analyze functional domains of IRF-7 we have constructed an amino-terminal deletion mutant of IRF-7 (237-514) which exerted a dominant negative (DN) effect on virus-induced expression of the endogenous Type I IFN genes. Focusing on the molecular mechanism underlying the dominant negative effect of IRF-7 DN, we found that virus-activated transcription of endogenous IFNA genes requires full-length IRF-7 and that Serine 483 and 484 play an essential role. While IRF-7 DN had no effect on virus-stimulated nuclear translocation of IRF-3 and IRF-7, the binding of IRF-7 DN to IRF-3 and IRF-7 was detected by GST pull-down assay as well as by immunoprecipitation in infected cells, indicating that IRF-7 DN targets both IRF-7 and IRF-3. The region by which IRF-7 interacts with IRF-3 was mapped between amino acid 418 and 473. Overexpression of IRF-7 DN in virus-infected 2FTGH cells resulted in an inhibition of IFN synthesis and in a significant reduction of binding of both IRF-3 and IRF-7 to the IFNA1 promoter. Interestingly, the IRF-7 DN-mediated suppression of IFNA gene expression can be negated by overexpression of IRF-3. Altogether these results suggest that the IRF-3/IRF-7 complexes are biologically active and are involved in virus-activated transcription of endogenous IFNA genes.