Gene regulation in Saccharomyces cerevisiae

The Swi5p and Ace2p transcription factors are homologous proteins which bind to the same DNA sequences in vitro, but activate different gene targets in vivo, HO and CTS1, respectively. These proteins and their gene targets serve as a model for promoter specific gene activation in S. cerevisiae. This dissertation describes the identification of regions of both Swi5p and Ace2p that are required for promoter specific activation of HO and CTS1 through the use of chimeric fusions between the two proteins. In addition, the jointly regulated class of genes (SIC1, PCL9, PCL2, ASH1, CDC6, EGT2, and RME1) which are regulated by both Swi5p and Ace2p, was also analyzed using chimeric fusions between Swi5p and Ace2p. Those results suggest that specific regions of Swi5p and Ace2p are required for the activation of the jointly regulated genes. The Swi5p transcription factor interacts with the Pcl2p cyclin in the 2-hybrid assay. Swi5p is phosphorylated by the Pho85p Cdk which associates with the Pcl2p cyclin. Genetic interactions between Pho85p, Swi5p, and Ace2p are described as well as the effect of a pho85 mutation upon HO and CTS1 expression. The Ash1p protein is a transcription factor that represses the HO gene in daughter cells. Ash1p function is limited to daughters through the asymmetrical transport of ASH1 mRNA into daughter cells. Ash1p protein accumulates in both mother and daughter cells in a pho85 mutant strain. This is not a consequence of mRNA mislocalization as the mRNA is found specifically in daughter cells. The presence of Ash1p in mother and daughter cells leads to repression of HO expression in a pho85 mutant. We propose that Ash1p protein is stabilized in a pho85 mutant and that this stabilization causes the protein to be 'inherited' in maturing daughters, leading to the presence of Ash1p in both mother and daughter cells.