Sp1 and AP2 regulate but do not constitute TATA-less human TAF_II55 core promoter activity

Human TAF_II55 (hTAF_II55), a component of the general transcription factor TFIID, is the only general transcription factor encoded by an intronless gene identified thus far. Analysis of the TATA-less hTAF_II55 promoter-proximal sequence reveals putative binding sites for STAT-1, MEF2, Sp1, AP2, AREB6 and E47. Using chromatin immunoprecipitation, DNase I footprinting and electrophoretic mobility shift assays, we demonstrate that Sp1 and AP2 can bind simultaneously to juxtaposed Sp1- and AP2-binding sites in the hTAF_II55 promoter-proximal region and functionally modulate hTAF_II55 promoter activity, as evidenced by reporter gene assays performed in transiently transfected human C-33A and insect SL2 cell lines. Interestingly, removal of all the promoter-proximal Sp1-binding sites does not impair the function of the hTAF_II55 core promoter. Moreover, a 52-bp DNA fragment containing only the hTAF_II55 initiator (Inr) and downstream promoter element (DPE) is able to support Ga14-VP16-mediated activation in vivo and in vitro. Our data suggest that Sp1, although it plays an enhancing role in hTAF_II55 gene expression, is not essential for hTAF_II55 core promoter activity. Interestingly, mutations introduced at the Inr and DPE differentially affect the selection of transcription start sites, suggesting that these two core promoter elements play a non-redundant role in the function of TATA-less promoters.