A family of interferon (IFN) regulatory factors (IRFs) The virus-induced expression of interferon (IFN) genes in infected cells involves interplay of several constitutively expressed and virus-activated transcriptional factors (1). Two of these factors binding to the virus-inducible element of the IFN-3 gene have been proposed to play a crucial role in the regulation of expression of IFN-a and IFN-,3 genes. Interferon regulatory factor 1 (IRF-1) was shown to act as an activator, and the closely related IRF-2 was a repressor (2). It was proposed that induction of the IFN-,3 gene was the result of the removal of repressor IRF-2 and the subsequent binding of the activator IRF-1 (3). Several observations supported this model. Expression of IRF-1 was found to be upregulated in virus-infected cells. The IRF-1 binding sites were identified in the IFN-3 gene promoter region and reporter plasmids with multiple repeats of the AAGTGA hexanucleotides (which are the strong IRF-1 binding site) were inducible by overexpression of IRF-1. This transactivation could be repressed by (2). In embryonal carcinoma cells, overexpression of IRF-1 induced both the transfected and the endogenous IFN-a and -,B genes (4). Moreover, a decrease in IFN-,B induction was observed in cells expressing the IRF-1 antisense mRNA (5).

The family of interferon (IFN) regulatory factors (IRFs) encodes DNA-binding transcription factors, some of which function as modulators of virus-induced signaling. The IRF-3 gene is constitutively expressed in many tissues and cell types, and neither virus infection nor IFN treatment enhances its transcription. In infected cells, however, IRF-3 protein is phosphorylated at the carboxyl terminus, which facilitates its binding to the CBP͞p300 coactivator.

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