HTLV-I Tax induces binding of NF-IL6 to the IL1B promoter. DNA binding reactions were performed with a radiolabeled 6/IL-1 (lanes 1 to 5) and −131 to −59 HD probe (lanes 6 to 8). The oligonucleotide 6/IL-1 sequence containing the −91 to −83 NF-IL6 site of the IL1B promoter was described in the Materials and Methods. Nuclear extracts prepared as described in the legend to Fig 4 were used. In lanes 6 to 8, recombinant truncated NF-IL6 with a deletion of the activation domain (Δspl) and/or recombinant Tax protein were incubated with a radiolabeled HD probe. An oligonucleotide 6/6 contains a strong NF-IL6 binding site from the human IL-6 gene. The 6/6 competitor was used in a 50-fold molar excess over radiolabeled probe. The arrow locates the mobility of DNA-protein complex containing NF-IL6.