Core promoter specificities of the Sp1 and VP16 transcriptional activation domains

The core promoter compositions of mammalian protein-coding genes are highly variable; some contain TATA boxes, some contain initiator (Inr) elements, and others contain both or neither of these basal elements. The underlying reason for this heterogeneity remains a mystery, as recent studies have suggested that TATA-containing and Inr-containing core promoters direct transcription initiation by similar mechanisms and respond similarly to a wide variety of upstream activators. To analyze in greater detail the influence of core promoter structure on transcriptional activation, we compared activation by GAL4-VP16 and Sp1 through synthetic core promoters containing a TATA box, an Inr, or both TATA and Inr. Striking differences were found between the two activators, most notably in the relative strengths of the TATA/Inr and Inr core promoters: the TATA/Inr promoter was much stronger than the Inr promoter when transcription was activated by GAL4-VP16, but the strengths of the two promoters were more comparable when transcription was activated by Sp1. To define the domains of Sp1 responsible for efficient activation through an Inr, several Sp1 deletion mutants were tested as GAL4 fusion proteins. The results reveal that the glutamine-rich activation domains, which previously were found to interact with Drosophila TAF110, preferentially stimulate Inr-containing core promoters. In contrast, efficient activation through TATA appears to require additional domains of Sp1. These results demonstrate that activation domains differ in their abilities to function with specific core promoters, suggesting that the core promoter structure found in a given gene may reflect a preference of the regulators of that gene. Furthermore, the core promoter preference of an activation domain may be related to a specific mechanism of action, which may provide a functional criterion for grouping activation domains into distinct classes.