Luciferase activity is inducible by Tax. (A) Schematic representation of the transgenic construct used. Tax activates viral transcription by recruiting CRE binding transcription factor (CREB) to the U3 region of the 5′LTR. Luciferase activity in lystates from (B) 293T cells and (C) NIH3T3 cells 2 days after cotransfection with 100 ng of CMV-Tax and indicated amounts of LTR-Luc plasmid. (D) Luciferase activity from lysates of B5Luc cells, (B5 cells containing stably integrated LTR-Luc), cocultured with indicated numbers of irradiated HTLV-1 infected (Tax-expressing) cells or with (E) 10⁵ irradiated HTLV-1 infected cells for 3 weeks. Data represent the average of 2 experiments in duplicate.