PU.1 represses transcription driven by the c-myb promoter. (A) Schematic representation of c-myb promoter-CAT constructs used for transient transfection CAT assays. Numbering is relative to nucleotide +1.19 Location of the mutated PU.1 binding site is shown. (B) CAT activity in Jurkat cells transiently cotransfected with the c-myb-CAT constructs (wild-type and mutated) and a PU.1 expression vector (lanes 2, 4, 6, 8, and 10) or the empty vector (lanes 1, 3, 5, 7, and 9). (C) CAT activity in Tk⁻ts13 hamster fibroblasts transiently cotransfected with the c-myb–CAT constructs (wild-type and mutated) and a PU.1 expression vector (lanes 2, 4, 6, 8, and 10) or the empty vector (lanes 1, 3, 5, 7, and 9). Error bars indicate ±SD for three independent experiments. (D) CAT activity in 32Dcl3 cells transiently transfected with wild-type P1-CAT (lane 1) or mutant P1mut-CAT (lane 3) c-myb promoter in the presence of pSVPU.1 (lanes 2 and 5) or of pSV vector (lane 3).