Abstract 9724050 Gaber The expression of many genes in the yeast Saccharomyces cerevisiae, is regulated by the availability of glucose or other hexoses. For example, in the presence of low glucose concentrations some genes are transcriptionally induced whereas others are only induced when the cell is exposed to high concentrations of glucose. In addition, many genes, like those involved in the transport and metabolism of galactose, are transcriptionally repressed by glucose. The integration of both positive and negative transcriptional regulation in S. cerevisiae involves a complex, sometimes redundant network of signaling pathways. The identity of glucose signaling molecules remains largely unknown despite intensive efforts toward their discovery. Recently, a novel glucose signaling pathway was discovered, in which Snf3, a membrane protein with a resemblance to glucose transporters, was found to be required for the negative regulation of HXT6, a glucose transporter gene that undergoes transcriptional repression in response to moderate or high levels of glucose. This negative role of Snf3 on HXT6 expression in response to glucose is unique. Also unusual is the expression of HXT6 in the complete absence of glucose; all other glucose transporter genes require either low or high concentrations of glucose to induce expression. Moreover the expression of SNF3 in cells deleted for the functional HXT genes is insufficient to confer either growth on glucose or glucose uptake. Thus Snf3 may function more like a glucose receptor than a transporter and its principal role appears to the elicitation or maintenance of the appropriate signaling pathway that allows the cell to respond to the availability of glucose by inducing or repressing downstream target genes. One goal of the proposed research is to identify proteins that participate in the Snf3-dependent signaling pathway. A genetic characterization will determine the effects of known mutations on Sn f3 signaling. Genetic screens will also be carried out to identify mutations in genes that specifically affect this pathway. The genes identified through the mutant screens will be cloned and the proteins they encode will be determined. These results will enable the construction of molecular tools that are required to assess the relative positions and roles played by specific proteins in this pathway. This research will lead to the identification of the molecular mechanism involved in transducing the glucose signal, and lead to an understanding of how a membrane protein of the glucose transporter family can maintain a glucose availability signal that is transmitted to the nucleus where it can enhance or repress transcription of specific genes. %%% This research is examining how a membrane protein which is similar in sequence to glucose transporters, can negatively regulate glucose transporter genes that are repressed in response to moderate or high levels of glucose. The results of these studies in yeast will lead to our understanding of the mechanism by which cells can respond to different levels of glucose in their environment. ***
