The interferon-inducible Stat2 : Stat1 heterodimer preferentially binds invitro to a consensus element found in the promoters of a subset of interferon-stimulated genes

Regulated expression of type I interferon (IFN)-stimulated genes (ISG) requ ires the binding of the signal transducer and activator of transcription (S tat) complexes to enhancer elements located in the ISG promoters. These enh ancer elements include the IFN-stimulated response element (ISRE) and the p alindromic IFN-gamma activation site (GAS) element. Regulated expression of ISRE containing ISG depends on IFN-stimulated gene factor 3 (ISGF3), a het erodimer involving Stat1 and Stat2 in association with a DNA-binding adapte r protein, p48/IFN regulatory factor-9 (IRF-9), Several GAS binding Stat co mplexes involving Stat1, Stat3, Stat4, and Stat5 have been described, but t heir contribution to GAS-dependent ISG expression remains to be established . We and others previously identified an IFN-alpha -inducible Stat2:1 heter odimer that exhibits binding to the GAS element of the IRF-1 gene. These pr evious studies raise the possibility that Stat2:1 may participate in the tr anscriptional activation of the subset of ISG containing GAS elements. To a ddress this question, we performed a PCR-assisted binding site selection pr ocedure to define the Stat2:1 recognition sequence. The data reveal that St at2:1 preferentially binds to a palindromic sequence similar to the consens us GAS element found in the promoter of several ISG, Our results establish that in addition to the classic complex formation involving Stat2, Stat1, a nd p48 associations, Stat2:1 heterodimers are formed in response to IFN tre atment that may play an important role in the transcriptional regulation of certain ISG.