No abstract

We have identified the promoter region of the GALIO 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 GALIO leader DNA with.a GALlO 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 CYCI (iso-l-cytochrome c) promoter [Guarente, L. & Ptashne, M. (1981) Proc. NatL Acad. Sci USA 78,[2199][2200][2201][2202][2203]. The.hybrid promoter is composed of DNA encoding CYCI mRNA start sites and the GAL segment upstream of these sites. This promoter is regulated in a manner analogous to GALIO; 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 CYCI 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~pic-ture of the GALIO promoter that emerges is one of an upstream activation site that responds to the GAL4 product plus galactose, and a region of transcription initiation thatmay contain sequences that mediate glucose repression. Experiments employing strains inducible (GAL80) or constitutive (galSO) for GALIO expression indicate that an additional component of glucose repression is inducer exclusion.Expression ofprokaryotic genes or eukaryotic genes transcribed by RNA polymerase II is regulated by DNA sequences that lie upstream of coding sequences. In the cases of the simian virus 40 early region (1, 2), the sea urchin histone H2A gene (3), or the yeast CYC1 gene (refs. 4 and 5) (unpublished data), these regulatory sequences lie in two regions, one in close proximity to where transcription initiates, and the other upstream of the initiation region. The CYCJ.gene, in particular, contains an upstream activation site (UASC) about 250 base pairs upstream of the startpoint of transcription that enhances expression about 50-fold (4).To further study the role of UAS regions, we have probed whether yeast genes other than CYCI contain such sites. The focus of this report is the GALlO gene of Saccharomyces cerevisiae. The product of GALJO (UDP-galactose epimerase), along with the products of GALl (galactokinase) and GAL7 (galactose-l-phosphate uridylyltransferase), forms the pathway for utilization of galactose as carbon source in S. cerevisiae. These three genes are closely linked in a cluster on chromosome II (6, 7) and are coordinately induced about 1,000-fold at the level of transcription by growth on galactose (8-10). This coordinate control is exercised by the constitutively synthesized protein products of the GAL4 and GAL80 genes, which are linked neither to each other nor to the structural gene cluster (11)(12)(13). The GAL...