Factors Affecting Sugar Utilization and Fermentation Rates
In this grant year, the investigation of the genetic and environmental factors that influence fermentation rate was continued. A more detailed analysis of the effect of limitation of individual amino acids was conducted. This analysis revealed that yeast cells are particularly sensitive to lysine limitation, moderately sensitive to adenine and leucine levels, but fairly insensitive to tryptophan, histidine and uracil concentrations. The levels of an amino acid that support maximal growth rate seem to be lower than the concentrations required for maximal fermentation rate, for some but not all nitrogen sources. In addition to analysis of nitrogen levels and reduced fermentation rate, we also discovered that an imbalance of potassium to hydrogen ion concentration would lead to a stuck fermentation. Maximum fermentation rate is equivalent in low pH juices and in juice-like synthetic media of differing potassium ion concentrations. However, in low potassium juices stuck fermentations develop. Sugar transporters, the HXT proteins, are degraded under conditions leading to arrest of fermentation. Analysis of degradation of one of these transporters, the HXT2 protein, revealed dramatic instability. This protein is degraded rapidly upon cessation of growth and upon nutrient limitation. There are now 14 HXT genes known in Saccharomyces. Not all of these transporters is expressed during grape juice fermentation. We are still in the process of determining which ones are expressed under these conditions.