The general regulatory factor Reb1p controls basal, but not Gal4p-mediated, transcription of the GCY1 gene in yeast

Expression of the gene GCY1 in Saccharomyces cerevisiae is induced by about 25-fold in the presence of galactose as a result of activation by Ga14p. In contrast to other Ga14p-regulated genes, such as GAL1 or GAL10, GCY1 is transcribed at a relatively high basal level. We have analysed the basis of this behaviour and have found that, in addition to a UAS(GAL), a binding site for the general regulatory factor Reb1p is localized 100 bp upstream of the TATA sequence and about 140 bp 3' to the UAS(GAL). Reb1p binds to this site with low affinity. Reb1p, an abundant. multifunctional DNA-binding protein in S-east, acts as a weak transcriptional activator in the control regions of several genes encoding unrelated functions. The action of Reb1p is assumed to be strongly position dependent, In the control region of GCY1, Reb1p acts independently of position and stimulates basal expression of GCY1 about threefold, whereas Ga14p-mediated activation is not influenced significantly. Promoter-proximal insertion of an additional Reb1p recognition site enhances basal transcription only marginally, but can largely compensate for deletion of the natural Reb1p-binding site. Either an Abf1p- or a Rap1p-binding site can substitute for the Reb1p recognition sequence, indicating that these general regulatory factors fulfill related functions ill basal transcription, without affecting Gal fp-mediated activation. In addition to Reb1p, the sequence of the Ga14p-binding site influences basal transcription. This effect is independent of the Ga14 protein, as it operates in a gal4 mutant background as well. This finding suggests that the nucleotide sequence of the UAS(GAL) ill the GCY1 promoter has intrinsic properties, presumably st particular DNA structure, that influence basal transcription and act synergistically with Reb1p.