Mechanism of differential utilization of the his3 TR and TC TATA elements

The yeast his3 promoter region contains two TATA elements, TC and TR, that are differentially utilized in constitutive his3 transcription and Gcn4-activated his3 transcription. TR contains the canonical TATAAA sequence, whereas TC is an extended region that lacks a conventional TATA sequence and does not support transcription in vitro. Surprisingly, differential his3 TATA-element utilization does not depend on specific properties of activator proteins but, rather, is determined by the overall level of his3 transcription. At low levels of transcription, the upstream TC is preferentially utilized, even though it is inherently a much weaker TATA element than TR. The TATA elements are utilized equally at intermediate levels, whereas TR is strongly preferred at high levels of transcription. This characteristic behavior can be recreated by replacing TC with moderately functional derivatives of a conventional TATA element, suggesting that TC is a collection of weak TATA elements. Analysis of promoters containing two biochemically defined TATA elements indicates that differential utilization occurs when the upstream TATA element is weaker than the downstream element. In other situations, the upstream TATA element is preferentially utilized in a manner that is independent of the overall level of transcription. Thus, in promoters containing multiple TATA elements, relative utilization not only depends on the quality and arrangement of the TATA elements but can vary with the overall level of transcriptional stimulation. We suggest that differential TATA utilization results from the combination of an intrinsic preference for the upstream element and functional saturation of weak TATA elements at low levels of transcriptional stimulation.