Yeast chemical genomics is a powerful tool that is being applied to drug discovery. This approach allows the study of compound-target relationships in the context of a living cell with a systems…

Yeast chemical genomics is a powerful tool that is being applied to drug discovery. This approach allows the study of compound-target relationships in the context of a living cell with a systems biology view that includes both the cell target and the interdependent networks that respond to chemical stress. The modulation of yeast colonies, especially the redox potential, allows study of different drug pathways. This application investigates microbial metabolic adaptation and cell differentiation to the multiple stimuli encountered in space flight environments, using the yeast saccharomyces cerevisiae as a model system. We have shown that microgravity activates a unique set of genes in yeast. Our previous work has largely defined yeast s cellular stress defense response to microgravity, but stress defense responses are only involved in the survival of yeast in liquid cultures. IT has recently been found that survival of plate-based yeast colonies is dependent on signaling mediated through ammonia which drives the multicellular yeast colony to differentiate into two populations, l or lower cells and u or upper cells. These two populations have different metabolic pathways and establish a cycle of nutrients that allows the u cells to continue to proliferate at the leading edge. Without ammonia signaling, the entire colony dies. Lack of convection in space is likely to dramatically impact signaling by ammonia, a volatile gas. We postulate that the flow of nutrient and ammonia signals will differ in microgravity where there is little to no convection. We hypothesize that yeast colony survival in microgravity depends on ammonia mediated metabolic adaptation and cell differentiation rather than on stress defense. To test our hypothesis, we will identify the mechanisms mediating colony stratification into u and l cells under multiple gravitational environments: microgravity emulated by rotating wall vessel and random positioning device, static one g cultures, and ultimately microgravity of space flight. Our experimental model will compare liquid and solid phase yeast cultures and make use of available mutant strains of yeast with deletion of the enzymes involved in ammonia signaling and the connected alternative metabolism activation, enzymes involved in stress responses, and the MSN4 promoter and SSA4 gene that we have previously identified as mediating response to microgravity. Relevant metabolic pathways will be studied using green fluorescent protein-linked reporter genes that can be analyzed by confocal microscopy of intact colonies, and flow cytometry of cells divided into l and u populations by sucrose density centrifugation. There are similarities between multicellular yeast colonies and tumors of clusters of mammalian cells in tissue culture. Thus insights into the effect of microgravity on cell to cell signaling may have far-reaching applications. Microgravity could provide a physiologically responsive test bed for drug pathway analysis at diverse redox states