Involvement of P-TEFb in GATA-1 cooperation with RUNX1/CBFβ. (A,B) K562 cells underwent transient transfection with the αIIb-598–luciferase reporter plus expression constructs for GATA-1 (G1), RUNX1 (R1), and CBFβ (Cb) as indicated. Results are mean of 3 independent experiments plus or minus SEM and are expressed as fold activation relative to GATA-1 alone or to vector. All transfections were normalized with cotransfected pCMVβGAL. (A) Transfections also included expression constructs for the dominant-negative mutants Cdk7 KK41/42NQ (dnCdk7) and Cdk9 D167N (dnCdk9). (B) Transfectants were treated the final 24 hours (of 48-hour incubation) with 100, 200, and 300 nM of flavopiridol (FP) or with 25, 50, and 100 μM of DRB. (C) Luciferase reporter assays were conducted as in panel A. K562 transfections included an expression construct for HEXIM1 (HEX) as indicated.