Organization of the Saccharomyces cerevisiae actin gene UAS: functional significance of reiterated REB1 binding sites and AT-rich elements
The upstream activation sequence (UAS) in the Saccharomyces cerevisiae actin gene promoter contains three different motifs, specifically two AT-rich tracts, two binding sites for the yeast protein REB1, and an Mlul site. Synthetic UAS elements containing individual motifs, or combinations of them, were inserted in place of the natural UAS, and assayed using a lacZ reporter gene. The REB1 binding sites were found to be essential for, and sufficient to restore partial, UAS activity. AT-rich tracts alone were inactive. Multimerization of a REB1 binding site created a UAS that in galactose is more active, but in glucose less active, than a UAS having a single REB1 site with one AT-rich tract. In general, transcription during growth in galactose or glycerol/lactate responds more to multimerization of motifs. The results suggest that the natural actin promoter UAS retains activity on these alternative carbon sources because of reiteration of sequence elements within it; the additional elements appear to be redundant when cells are grown on glucose. The mlul site, which is present upstream of a number of yeast genes involved in DNA synthesis and confers cell cycle periodicity to those genes, contributes to the activity of the synthetic UAS elements, but not in a cell-cycle-dependent manner.