TATA-binding protein activates transcription when upstream of a GCN4-binding site in a novel yeast promoter.
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In the gal-his3 hybrid promoter, his3-GG1, GCN4 stimulates transcription at the position normally occupied by a TATA element. This expression requires two elements within gal1-10 sequences, a REB1-binding site and a second element, Z, which resides 20 base pairs upstream of the GCN4-binding site. No obvious TATA element is present in this promoter. To characterize the function of Z, we replaced it with short random oligonucleotides and selected for expression in vivo. Fourteen elements were identified and classified into groups based upon sequence and phenotypic similarities. Group 1 elements contained functional TATA sequences that were essential for activity. TATA elements can thus function when positioned upstream of a GCN4-binding site. The Group 2 elements activated transcription poorly when used as conventional TATA elements; however, mutational analyses demonstrated that their activity required TATA-like sequences. These TATA-like sequences bound the yeast TATA-binding protein (TBP) poorly in vitro but function in vivo as TBP interaction sites based upon two criteria. First mutations that improved their TATA character correspondingly improved function and second their activity could be enhanced in the presence of an altered binding specificity mutant of TBP. Furthermore, the Group 2 elements enabled the identification of mutations outside of the TATA-like core that contribute to transcriptional activation without adversely affecting TBP binding. The finding that low affinity TBP-binding sites can be used at unconventional positions suggests that many "TATA-less" promoters contain a cryptic interaction site for TBP.
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