p53 Activity in MEF cells. (A) Mouse embryo fibroblasts (WT, □; p65^–/–, ▦ ; p50^–/–, ▨ ; and NIK^–/–, ▪) were transfected with use of Effectene reagent (Qiagen) to measure p53 activity on the reporter construct PG13-Luc (1.5 μg) in the absence (–; lanes 1, 3, 5, and 7) or presence (+; lanes 2, 4, 6, and 8) of Tax (0.3 μg). The RSV-βgalactosidase (RSV-βgal, 0.2 μg) construct was included to control for transfection efficiency. The amount of p53 activity in the absence of Tax was set at 100%. The relative amounts represent averages of at least 2 independent experiments. Error bars indicate SD. (B) Transfections were performed as described in panel A. Values for the RSV-βgal are given as percentage of activity. Error bars indicate SD. (C) Western blot analyses of nuclear extracts indicate the expression level of endogenous p53 (top panel, Ab7; Oncogene Research Product), transfected Tax (middle panel, Tab172), or p65 (lower panel; Santa Cruz Biotechnology) as visualized by chemiluminescence.