Figure 3.

(a) Identification of nuclear proteins binding to the Sp1 sites. HeLa nuclear extract was mixed with radiolabeled oligonucleotide probe and analyzed by EMSA. The proteins bound to the probe were identified by preincubation of nuclear extracts with antibodies specific for Sp1, Sp3, or control antibodies specific for Ets2 and Ap2. Complexes specific to Sp1 and Sp3 are indicated to the left of the panel. The antibodies used are indicated at the top of the figure. (b) The Sp1 transcription factor upregulates and Sp3 transcription factor downregulates the hTERC promoter. The wild-type hTERC-luciferase plasmid was cotransfected with increasing amounts of either an Sp1 or Sp3 expression vector (0.25 µg to 2.0 µg) into the 5637 cell line. The mean luciferase activity normalized for protein and transfection efficiency is shown with the standard deviation of duplicate samples.