HTLV-I Tax induces binding of Spi-1 to the IL1B promoter. The −59 to +12 DT fragment was used as a radiolabeled probe. Nuclear extracts were prepared from THP-1 cells carrying either pcTax (4 μg; lanes 3 and 4) or control pcDNA1 vector (4 μg; lanes 1 and 2). THP-1 cells were transfected with either pcTax or pcDNA1. At 24 hours after transfection, cells were treated with 100 ng/mL of LPS for 45 minutes (lanes 2 and 4) or left untreated (lanes 1 and 3). In vitro expressed recombinant Spi-1 was incubated with the radiolabeled DT probe in the presence (lane 6) or absence (lane 5) of in vitro expressed recombinant Tax. The total amount of added reticulocyte lysate in each reaction mixture was kept constant by the addition of incubated control lysate. The arrow locates the mobility of Spi-1.