T_C, an Unusual Promoter Element Required for Constitutive Transcription of the Yeast HIS3 Gene

Abstract

T_C is the proximal promoter element required for constitutive his3 transcription that occurs in the absence of the canonical TATA element (T_R) and is initiated from the +1 site. The T_C element, unlike T_R, does not respond to transcriptional stimulation by the GCN4 or GAL4 activator protein. Analysis of deletion, substitution, and point mutations indicates that T_C 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 T_C function by base pair substitutions. Of 15 derivatives averaging four substitutions in the T_C region and representing 40% of all possible single changes, only 1 inactivated the T_C element. Moreover, the phenotypes of mutant and hybrid elements indicated that inactivation of T_C required multiple changes. The spacing between T_C 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 T_C 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.