Figure 3

The P-TEFb inhibitors DRB, seliciclib and flavopiridol release P-TEFb from the large form. Low-salt cytosolic extract (CE) containing the large form of P-TEFb and high-salt nuclear extracts (NE) containing the free form of P-TEFb were generated from (A) DRB-treated HeLa cells, (B) DRB treated Jurkat cells, (C) seliciclib-treated HeLa37 cells or (D) flavopiridol-treated Jurkat cells. Quantitative western blotting was performed on low salt cytosolic extracts (CE) and high-salt nuclear extracts (NE) to detect the percentage of Cdk9 and cyclin T1 present in the free and large form of the P-TEFb complex. The percent of P-TEFb in the large form of the complex (low-salt or CE) was calculated as a fraction of the total amount of P-TEFb (low-salt + high-salt P-TEFb) and plotted as a function of the concentration of P-TEFb inhibitor.