A subgroup of genes induced by IFN-γ requires both STAT1 and IRF1 for transcriptional activation. Using WT,stat1−/−, orirf1−/−cells, we analyzed the changes induced by IFN-γ ingbp2promoter chromatin. STAT1 associated with the promoter independently of IRF1 and played an essential role in the ordered recruitment of the coactivator/histone acetyl transferase CREB-binding protein (CBP) and the histone deacetylase HDAC1. Hyperacetylation of histone 4 also required STAT1. Phosphorylation at S727 in the transactivating domain increased transcriptional activity of STAT1. In cells expressing a STAT1S727A-mutant CBP recruitment, histone 4 hyperacetylation and RNA polymerase II association with thegbp2promoter were strongly reduced. IRF1 association with thegbp2promoter followed that of STAT1, but STAT1 association with DNA or histone hyperacetylation were not necessary for IRF1 binding. RNA polymerase II association with thegbp2promoter required both STAT1 and IRF1, suggesting that both proteins mediate essential steps in transcriptional activation. IRF1, but not STAT1, was found to coimmunoprecipitate with RNA polymerase II. Together, the data support the assumption that the main role of STAT1 in activatinggbp2transcription is to provide transcriptionally competent chromatin, whereas the function of IRF1 may lie in directly contacting RNA polymerase II-containing transcriptional complexes.