Upregulation of the c-fos promoter activity by Tat in (A) Jurkat cells activated by 15% FCS or 10⁻⁷ mol/L PMA plus 5 μg/mL PHA and (B) U937 cells activated by 15% FCS or 10⁻⁷ mol/L PMA 24 hours after serum-starvation (1% FCS). Cells were transiently cotransfected with FC3 plus tat/S or tat/AS or pRPneo-SL3 control vector. CAT activity is expressed as folds of activation with respect to cell lines transfected with FC3 alone. Data are reported as means ± SDs of four independent transfection experiments performed in duplicate.