Genetic Evidence for Promoter Competition in Saccharomyces cerevisiae

Abstract

The his4-912δ mutation is an insertion of the long terminal repeat (δ) of the yeast retrotransposon Ty into the HIS4 promoter region, such that the δ is 97 base pairs upstream of the HIS4 transcription initiation site. Strains carrying the his4-912δ allele are His^- at 23°C; this phenotype can be reversed either by growth at 37°C or by mutations in trans-acting SPT genes. Under conditions in which his4-912δ confers a His^- phenotype. HIS4 transcription initiates at the δ initiation site, rather than at the HIS4 initiation site, producing a longer, nonfunctional transcript. Under conditions in which the strain is His⁺, transcription initiates at the wild-type HIS4 initiation site. To understand how transcription is balanced between the δ and HIS4 promoters, we have selected for cis-acting suppressors of his4-912δ. Two classes defined by six independent mutations restore synthesis of a functional HIS4 ranscript. The first class is an A-to-G base change 1 base upstream of the proposed delta TATA sequence. These mutants do not synthesize the δ-initiated transcript; instead, they synthesize only the wild-type HIS4 transcript. The second class of mutations alters base pairs surrounding the functional HIS4 TATA sequence. The two strongest His⁺ mutants of this class synthesize the wild-type HIS4 transcript at levels consistent with their His⁺ phenotype. Surprisingly, these two mutants also have a reduced level of the δ-initiated transcript relative to the his4-912δ parent. Analysis of these mutants indicates that the level of transcription from one promoter can affect the level of transcription from the other promoter and suggests that δ and HIS4 transcription signals compete for initiation of transcription from each site.