A GAL10-CYC1 hybrid yeast promoter identifies the GAL4 regulatory region as an upstream site.

We have identified the promoter region of the GAL10 gene (whose product is UDP-galactose epimerase) of Saccharomyces cerevisiae; this promoter mediates galactose induction of transcription in conjunction with the product of the GAL4 regulatory gene. This identification was achieved by excising a 365-base-pair fragment of GAL10 leader DNA with a GAL10 proximal endpoint greater than 100 base pairs upstream of the transcriptional start site and substituting it in place of the upstream activation site of the CYC1 (iso-1-cytochrome c) promoter [Guarente, L. & Ptashne, M. (1981) Proc. Natl. Acad. Sci. USA 78, 2199-2203]. The hybrid promoter is composed of DNA encoding CYC1 mRNA start sites and the GAL segment upstream of these sites. This promoter is regulated in a manner analogous to GAL10; i.e., it is induced by galactose and responds to mutations in the GAL4 and GAL80 regulatory loci. The activity of the hybrid promoter requires sequences in the region of the CYC1 mRNA start sites but does not require a precise spacing between these sequences and the GAL segment. The transposed GAL segment appears not to contain sequences that mediate glucose repression. Thus, the picture of the GAL10 promoter that emerges is one of an upstream activation site that responds to the GAL4 product plus galactose, and a region of transcription initiation that may contain sequences that mediate glucose repression. Experiments employing strains inducible (GAL80) or constitutive (gal80) for GAL10 expression indicate that an additional component of glucose repression is inducer exclusion.