In a search to identify new factors required for expression of SUC2 gene in Saccharomyces cerevisiae, we have partially purified a 27 kDa protein (p27) that bound both the DRSs of the HXK2 gene and the UASs of SUC2 gene. The amino terminal sequence of p27 identified the MED8 gene (open reading frame YBR193C), located in chromosome II of S. cerevisiae, as the gene coding for the protein. Disruption of this gene has demonstrated that is an essential gene for yeast growth. To determine whether the p27 protein represents the Med8 product, we expressed MED8 gene in E. coli and demonstrated that the heterologous synthesized protein specifically binds to both UASSUC2 and DRS2HXK2. This observation suggests that Med8 may be important for the coupling of the glucose repression pathway of SUC2 gene to the HXK2 gene expression. Med8 has been described as a mediator protein interacting with the CTD of the RNA polymerase II. Thus, the role of Med8 could be to act as coupling factor by linking activating and repressing transcription complexes to the RNA polymerase II holoenzyme transcriptional machinery.
Copyright 1999 Academic Press.