FORMATION OF STAT1-STAT2 HETERODIMERS AND THEIR ROLE IN THE ACTIVATION OF IRF-1 GENE-TRANSCRIPTION BY INTERFERON-ALPHA

An upstream inverted repeat (IR) element mediates transcriptional acti vation of the interferon response factor-1 gene (IRF-1) by interferon (IFN)-alpha and IFN-gamma. IFN-alpha and IFN-gamma fail to induce IRF- 1 in cells that lack signal transducer and activator of transcription 1 (STAT1), and STAT1 homodimers bind to IR elements in extracts of IFN -alpha-treated cells. We now report that STATE also plays an important role in the IFN-alpha-mediated transcriptional activation of the IRF- 1 gene. A new factor, most likely a STAT1-STAT2 heterodimer, was de te cted with an IR probe in extracts of IFN-alpha-treated cells. STAT1 an d STAT2 are already known to combine with p48, a DNA-binding protein, to form IFN-stimulated gene factor 3 (ISGF3), which binds to IFN-stimu lated response elements (ISREs) distinct from the IR of the IRF-1 gene . In extracts of U2A cells, which lack p48, STAT1-STAT2 heterodimers w ere still formed, indicating that they do not contain p48. We manipula ted the intracellular levels of STAT1-STAT2 heterodimers and STAT1 hom odimers to examine their roles in the induction of IRF-1 by IFN-alpha. Although both dimers can induce IRF-1 transcription, the heterodimers are more potent and thus may be the major activators in vivo. Deletio n analysis reveals that the C-terminal domain of STAT2 is important fo r transcriptional activation mediated by both STAT1-STAT2 heterodimers and ISGF3.