Interferon-gamma (IFN-gamma) is a key cytokine of T lymphocytes with major regulatory functions in the immune system. To determine and compare protein/DNA interactions at the native IFN-gamma locus in T cells, we analyzed the human IFN-gamma promoter by ligation-mediated polymerase chain reaction (LM-PCR) techniques. Accordingly, Jurkat T cells and primary CD45RA and CD45R0 CD4+ T cell subsets isolated from peripheral blood using immunomagnetic beads were cultured and analyzed by LM-PCR. Constitutive and inducible protein/DNA interactions of the IFN-gamma promoter in vivo were detected in all T cells tested. Interestingly, an inducible footprint between -183 and -196 was consistently observed in Jurkat T cells and CD45RA and CD45R0 T helper subsets upon stimulation with phorbol 12-myristate 13-acetate+phytohemagglutinin (PMA+PHA) that was highly sensitive to treatment with corticosteroids. This novel target site, denoted the C-site, was shown by several criteria, including cell distribution studies, stimulation experiments, supershift assays, and cross-competition electrophoretic mobility shift assays to bind the transcription factor AP-1. Mutation of the C-site that prevented AP-1 binding to this site was sufficient strikingly to reduce inducible promoter activity in primary CD45R0 T cells. In summary, the data demonstrate that IFN-gamma gene transcription in primary T cells is regulated in vivo at the level of constitutive and inducible protein/DNA interactions. We propose a model where basal transcription is maintained by binding of various transcription factors to the IFN-gamma promoter, whereas PMA+PHA-inducible IFN-gamma transcription in CD45R0 T cells is associated with binding of AP-1 to the C-site.