Fig. 5. Identification of a GAS element in the c–myc promoter that is necessary but not sufficient for suppression. (A) The full-length 1.7 kb c–myc promoter or two deletion constructs were linked to luciferase and transfected transiently into 2fTGH cells. Luciferase activity was determined with or without IFN–γ treatment (1500 IU/ml for 6 h). The data shown represent duplicate experiments in three independent trials (standard deviations). (B) Constructs containing eight copies of the consensus GAS (TTCTCGGAA) or seven copies of the c–myc GAS (TTCTGGGAA) linked to luciferase were transfected transiently into 2fTGH cells. Luciferase activity was determined with or without IFN–γ treatment (1500 IU/ml for 6 h). Data are presented from three independent experiments, with standard deviations. (C) NIH 3T3 cells stably transfected with the 1.7 kb c–myc promoter linked to the cell-surface protein cd2 were serum-starved and treated with PDGF alone (200 ng/ml), PDGF plus IFN–γ (1000 IU/ml) or were untreated. cd2 expression was determined by FACScan analysis.