Tc, an unusual promoter element required for constitutive transcription of the yeast HIS3 gene.

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Abstract
Tc is the proximal promoter element required for constitutive his3 transcription that occurs in the absence of the canonical TATA element (TR) and is initiated from the +1 site. The TC element, unlike TR, does not respond to transcriptional stimulation by the GCN4 or GAL4 activator protein. Analysis of deletion, substitution, and point mutations indicates that Tc mapped between nucleotides -54 and -83 and is a sequence-dependent element because it could not be functionally replaced by other DNA sequences. However, in contrast to the behavior of typical promoter elements, it was surprisingly difficult to eliminate Tc function by base pair substitutions. Of 15 derivatives averaging four substitutions in the Tc region and representing 40% of all possible single changes, only 1 inactivated the Tc element. Moreover, the phenotypes of mutant and hybrid elements indicated that inactivation of Tc required multiple changes. The spacing between Tc and the initiation region could be varied over a 30-base-pair range without significantly affecting the level of transcription from the +1 site. From these results, we consider it possible that Tc may not interact with TFIID or some other typical sequence-specific transcription factor, but instead might influence transcription, either directly or indirectly, by its DNA structure.