Promoter activity assay. (A) Luciferase reporter activity was examined with various amounts (nanograms) of pcDNA6-HA-AML1–, pcDNA6-HA-AML1-ETO–, or pcDNA6-HA-AML1-ETO9a–transfected 293T cells. pBAAE35-TK-luc (TC, 50 ng, wild type), pCMV5-CBFb 21.5 (150 ng), and pNull-renilla luciferase (5 ng) were cotransfected. The value of y-axis is a fold change of luciferase activity compared with vector alone. The results are presented as the mean value of 3 independent experiments; error bars represent standard deviation. The almost equal level of protein expression induced from each amount of plasmid DNA was confirmed by immunoblotting with anti-HA antibody (data not shown). (B) Luciferase reporter assay was performed with 293T cells transfected with 50 ng wild-type (TC) or mutant (mC and Tm) of pBAAE35-TK-luc, 150 ng pCMV5-CBFb 21.5, and 5 ng pNull-renilla luciferase and increasing amount of pcDNA6-HA-AML1-ETO expression plasmid. (C) U937T-AML1-ETO cells were stably transfected with wild-type phSeparase-luc (TC) or the constructs including mutated AML1-binding sites (mC, Tm, mm). Two independent cell pools stably transfected with each construct were used in this study. The value of y-axis is a fold change of luciferase activity compared with day 0 of tetracycline (tet) withdrawal. The results are presented as the mean value of 3 independent experiments; error bars represent standard deviation. At day 1 of the tet withdrawal, statistically significant P values compared with mm control (*t test, P < .001 for TC, mC, and Tm) are indicated. At day 2 of the tet withdrawal, statistically significant P value compared with mm control (**t test, P = .005 for TC) is indicated. The expression of AML1-ETO was controlled by tet withdrawal, and protein expression was confirmed by Western blotting using anti-AML1 antibody (◀). Ponceau staining shows the relative amount of protein loading. As a negative control, all of these phSeparase-luc constructs were also stably transfected into parental U937T cells and no change of luciferase activity was detected after tet withdrawal (data not shown).