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.