Fig. 7. Requirement of Sp1 and its functional cooperation with Smads in the transcription of p15Ink4B in Drosophila S2 cells. (A) Sp1 strongly transactivates the p15 promoter. S2 cells were transfected with the p15P113luc reporter plasmid and increasing amounts of the Sp1 expression plasmid Pac-Sp1, in the presence or absence of an activated TβRI. Luciferase assays were done as in HaCaT cells. (B) Inability of Smads to transactivate the p15 promoter in the absence of Sp1. S2 cells were transfected with the p15P113luc reporter and expression plasmids for Sp1 or for Smads, in the presence or absence of an activated TβRI, as shown. (C) Sp1 and Smads cooperate to activate the p15 promoter in response to TGF-β receptor activation. S2 cells were transfected with the p15P113luc reporter, the Sp1 expression plasmid and expression plasmids for Smads, as marked.