Transcriptional activation of the HIV-1 LTR observed after HP1γ knock-down requires Sp1, P-TEFb and PCAF. HeLa LTR-Luc cells were transfected with siRNAs specific for HP1γ, Sp1, CDK9, PCAF, p300 or control (Sc), as indicated. Luciferase activity was measured 48 h after transfection. Expression levels of HP1γ, Sp1, CDK9, PCAF, p300 and HP1α were determined by immunoblotting. Results are presented as fold activation relative to transfection of control siRNA. The mean relative luciferase activities were obtained from three independent experiments.