Mutations that suppress the deletion of an upstream activating sequence in yeast: involvement of a protein kinase and histone H3 in repressing transcription in vivo.

Regulated transcription of most protein-encoding genes in Saccharomyces cerevisiae requires an upstream activating sequence (UAS); in the absence of UAS elements, little or no transcription occurs. In certain mutant strains, however, promoters that have been deleted for their UAS can direct significant levels of transcription, indicating that the remaining promoter elements (the basal promoter) are capable of directing higher levels of transcription, but they are normally represented in wild-type strains. To analyze this repression, we have selected for mutations that cause increased transcription of the SUC2 gene in the absence of its UAS. In addition to some previously studied genes, this selection has identified five genes that we have designated BUR1, BUR2, BUR3, BUR5 and BUR6 (for Bypass UAS Requirement). The bur mutations cause pleiotropic phenotypes, indicating that they affect transcription of many genes. Furthermore, some bur mutations suppress the requirement for the SNF5 trans-activator at both SUC2 and Ty. Additional analysis has demonstrated that BUR1 is identical to SGV1, which encodes a CDC28-related protein kinase. This result indicates that protein phosphorylation is important for repression of the SUC2 basal promoter as well as other aspects of transcription in vivo. Finally, BUR5 is identical to HHT1, encoding histone H3, further implicating chromatin structure as important for expression of SUC2.