Functional Genomics of Weak Acid Stress Response in Yeast

This research project focusses on the mechanisms whereby baker`s yeast Saccharomyces cerevisiae develops resistance to weak organic acid food preservatives such as sorbate and benzoate. This phenomenon severely hampers large-scale food preservation, since resistance of spoilage yeasts to preservatives is a major obstacle preventing food industry from lowering preservative levels. In fact, about a quarter of world food production is lost to spoilage microbes upon storage, demonstrating the need for more efficient food preservation. These preservatives are also consumed by humans in substantial quantities every day. However, their safety cannot be taken for granted, especially since sorbate and benzoate are powerful pro-oxidants and mutagenic for mitochondrial genome. The implication of oxidative stress in the development of many cancers also implies the need to further improve the efficacy of food preservation. Our proposed work will entail screening of the entire yeast deletion strain collection to provide a catalogue of those genes required for low pH growth, for resistance to acetate, and for resistance to the more lipophilic sorbate or benzoate. We will dissect a novel signal transduction pathway required for stress induction of the Pdr12p ATP- binding cassette (ABC) efflux pump. We shall use state-of-the-art technologies such as whole genome DNA microarray analysis to identify the weak acid stress response transcriptome, and to characterize yeast genes whose expression is modulated upon stress. This will also define the functional overlap with other cellular signal transduction cascades. Yeast strains defective in weak acid adaptation will be subjected to a detailed molecular- genetic, biochemical, as well as physiological analysis through functional genomics and proteomic technologies. Furthermore, we shall dissect the mechanisms of War1p function, a novel transcriptional regulator required for sorbate resistance. Finally, we shall delineate in molecular terms the contributions of plasma membrane permeases and the tryptophane biosynthesis pathway on weak acid resistance. Our global approach shall identify all yeast genes necessary and sufficient for growth under conditions relevant to food preservation and weak acid stress. Taken together, this research proposal aims for a full genomic and genetic analysis of mechanisms of weak acid susceptibility and stress adaptation in the yeast Saccharomyces cerevisiae.

This research project focussed on the molecular mechanisms whereby the baker`s yeast Saccharomyces cerevisiae can develop resistance to weak organic acids such as the food preservatives sorbate and benzoate. The work provided a catalogue of all genes required for growth in the presence of weak organic acids, as well as those mediating resistance to lipophilic compounds similar to sorbate or benzoate. The project identified and dissected a novel signal transduction pathway required for stress induction of a membrane protein, Pdr12p, which is acting as an energy-consuming pump mediating detoxification of weak acids entering yeast cells. Pdr12 is also produced by cells at very high levels in the presence of stress agents and it is required for yeast cells to survive under these conditions. Furthermore, the project discovered and characterized a novel regulator protein acting as a gene switch for Pdr12. War1 is novel gen regulator required for sorbate resistance and weak acid stress adaptation. The global approach identified all yeast genes necessary and sufficient for growth under conditions relevant to food preservation and weak acid stress. Taken together, this research proposal provided a complete genomic and genetic analysis of mechanisms of weak acid susceptibility and stress adaptation in the yeast Saccharomyces cerevisiae.