Insights into the molecular genetics of hexose transporter gene regulation in Saccharomyces cerevisiae

This work has investigated the glucose induction pathway of Saccharomyces cerevisiae, the major signaling pathway responsible for relaying a critical signal that glucose is present outside the cell. A member of this pathway known as Snf3 is the membrane associated low glucose sensor. Strains deleted for this gene are unable to grow via fermentation on low glucose due to the inability to activate the transcription of the high affinity hexose transporter genes. Identification of two spontaneous suppressors of a snf3Delta mutant resulted in the discovery that two members of the pathway, Mth1 and Rgt1, exhibit the rare genetic phenomenon of combined haploinsufficiency. Thorough investigation of this finding indicated that under repressing conditions, the levels of these two proteins are in a very tight balance with the number of binding sites that they must interact with to repress the genes that they regulate. This balance is likely an essential mechanism to maintain complete repression of these genes when glucose is not available, yet allow for quick induction of expression when glucose becomes available.;Analysis of Sks1, a multicopy suppressor of snf3Delta suggests that it may be a component of a signaling pathway that functions to regulate the HXT genes. In vitro kinase assays demonstrated that it is a functional kinase and can phosphorylate both of the homologous corepressor proteins Mth1 and Std1. Efforts to confirm in vivo significance were unsuccessful, however, a unique genetic screen revealed several components of other nutrient signaling pathways that must function for Sks1 to function as a suppressor.;Finally, a complete null mutation in the negative regulatory subunit of the protein kinase A Bcy1 was identified as a novel suppressor of the snf3Delta null mutant. This finding provides another example of the balance that exists at the promoters of the HXT genes along with the importance of the convergence of major nutrient signaling pathways. In combination, these results highlight the crosstalk and interdependency of these pathways and the importance of how the integration of these signals results in the expression of the appropriate HXT genes in response to multiple physiological signals.