Genetic Analysis of the Ydr1-Bur6 Repressor Complex Reveals an Intricate Balance among Transcriptional Regulatory Proteins in Yeast
ABSTRACT A transcriptional repressor complex encoded by two essential genes,YDR1 and BUR6, was isolated fromSaccharomyces cerevisiae and shown to be the functional counterpart of the human repressor complex Dr1-DRAP1. To elucidate the mechanism of repression by this complex, altered forms of Ydr1 and Bur6 were studied in vitro and in vivo. Deletion of the C-terminal 41 amino acids of Ydr1 resulted in loss of repressor activity and a growth defect, suggesting that the C-terminal domain of Ydr1 functions as a potent transcriptional repressor. A screen for extragenic suppressors of a cold-sensitive ydr1 (ydr1 cs) mutant led to the identification of recessive mutations in theSIN4 gene, which encodes a component of the SRB-MED complex. The sin4 alleles suppressed not onlyydr1 cs mutations but alsobur6 cs mutations. In contrast, deletion of thegal11 gene, whose product is also a member of the SRB-MED complex, failed to suppress ydr1 cs andbur6 cs mutations, indicating that suppression is not due to general defects in the SRB-MED complex. Moreover, one of the sin4 alleles, but not the sin4 deletion, was found to specifically suppress the inviability of aydr1 deletion, demonstrating that the essential function of Ydr1 becomes dispensable in a sin4 mutant background. Biochemical analysis of the SRB-MED complex from the sin4suppressor strain revealed a structurally distinct form of the SRB-MED complex that lacks a subset of mediator subunits. These results define a delicate balance between positive and negative regulators of transcription operating through the Ydr1-Bur6 repressor complex.