Virus-mediated induction of interferon A gene requires cooperation between multiple binding factors in the interferon alpha promoter region.

Transcriptional activation of interferon A (IFNA) gene in virus-infected cells is controlled by a 35-nucleotide inducible element that is cell type specific. Within this region, two elements, alpha F1 and IRF-1 binding sites, were shown by mutation analysis to play a crucial role in the expression of inducible element. In this study, we have analyzed the binding of nuclear proteins to the alpha F1 sequence and have shown that the induction is associated with the formation of a novel complex alpha F1/B, which contains at least two DNA binding proteins of 68 and 96 kDa. In contrast, no binding of the purified interferon regulatory factor 1 (IRF-1) either to the alpha F1 or IRF-1 binding sites could be detected in vitro. However, the oligonucleotides corresponding to alpha F1 or IRF-1 binding sites competed efficiently for the induction of IFNA4 promoter region in a transient transfection assay. We suggest that the induction of IFNA promoter region requires cooperation between alpha F1 binding proteins and IRF-1. Interestingly, our data also show that the inability of IFNA6 promoter to be expressed in infected L-cells may be a result of a viral-induced repressor, which could act by binding and inactivating alpha F1 or by competing for the IRF-1 binding site. These results suggest that cell-specific expression of IFNA genes results from core-cruitment of trans-acting factors that bind to alpha F1 and the IRF-1 binding site with the cell-specific virus-induced activator or repressor.